planet - Maximum and minimum gas giant & ice giant densities

Here is a plot I generated in 5 minutes at the site exoplanets.org

To construct this I took planets discovered by the transit method and which had a $M sin i$ measured using radial velocities. I divided the $M sin i$ by the sine of the measured inclination angle (this is required to avoid using masses that have been estimated using an assumed mass-radius relation). The y-axis is density, which comes directly (and quite precisely) from transit measurements. Of course transiting planets are the only ones with radii and density measurements.

You have to do this to avoid some very uncertain values that are given for planetary masses that have been simply assumed from a theoretical mass-radius relationship.

As you can see, there is a wide spread (factor of three) in density at a given mass for hot Jupiter's (most of the transiting giant planets are hot Jupiters), but there is a strong correlation. The density is at a minimum for a few tenths of Jupiter mass, but then smaller planets (presumably rocky and icy, rather than gas giants) appear to show higher densities.

Remember, these are all transiting exoplanets, and therefore predominantly orbiting close to their parent stars. There could be biases and selection effects at work! For example although the cores of gas giants are governed by degeneracy pressure and this makes the theoretical mass-radius relationship quite flat, there is the perturbing effect of radiation from the parent star ("insolation") that can make some objects larger. Even beyond this there appears to be scatter that is difficult to understand.

EDIT: For an easy-to-use empirical formulation you could try the relationships proposed by the Planetary Habitability Laboratory.

amateur observing - Calculate Local Sidereal Time

A highly authoritative source is Explanatory Supplement to the Astronomical Almanac 3rd ed., Urban & Seidelmann, 2013.

Chapter 1 gives the introductory explanations, without all the refinements if you want millisecond accuracy. Page 12 explains that

local sidreal time = Greenwich sidereal time + east longitude

(This book assumes the reader is able to convert between angle notation and time notation whenever necessary.)

If accuracy of 0.1 second is sufficient, the formulas given by the US Naval Observatory should do the job. Note that for subsecond accuracy you will have to determine UT1, which can be up to 0.9 second different from the commonly available UTC. You will have to look up the difference each time you do the calculation because the difference cannot be predicted more than about 6 months into the future.

galaxy - What is the probability that there is life on other planets?

The probabilites are unknown at the moment (March 2014), since there is only one known planet (Earth) harboring life. This doesn't allow any meaningful probability estimates for the occurence of life, based on empirical data.

The overall formation of life is too complex to allow simulations based on current technology.
Although some intermediate steps can be simulated or performed by experiment, e.g. by the Miller-Urey experiment.

The Drake equation is a simplified attempt to decompose the probability into factors. Some of the factors aren't known yet. But there has been much progress in estimating the frequency of exoplanets, and the probability of planets to be habitable. From this news release one could estimate (order of magnitude), that there is one habitable planet around a Sun-like star per about 1,700 (about 12³) cubic light years in our region of the Milky Way ("... the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away...").

We don't know the probability for habitable planets to develop life, nor to be colonized. Probabilities of complex life forms are even more difficult to estimate (yet). More discussion about habitability of exoplanets on Wikipedia.

light - What is the ratio of cosmic microwave background radiation to normal radiation?

I would like to know the ratio of cosmic microwave background radiation to normal radiation in the universe. I am considering cosmic microwave background radiation to include the microwave, and any other radiation that is being emanated from near the "edge of the universe", while normal radiation is radiation emmitted by stars, nebulae, and other sources within the universe (excluding the cosmic microwave background). Since we are in a galaxy, I know that the normal radiation here far exceeds the cosmic microwave background, but I am interested in the "average" value over the universe. For example: total cosmic microwave background radation in the universe / total normal radiation in the universe.

the moon - Look up current positions and velocities of solar system objects

I am working on an orbital dynamics code and, for fun, I would like to model the Sun-Earth-Moon system with my code. I can look up the masses of each object just fine, as well as average distances and relative velocities (which I can also calculate). However, I would like to model the Sun-Earth-Moon system in great detail. I would like to know the positions and velocities of each object on any given day (today works just fine) so that I can represent the orbits of these objects with great accuracy, hopefully even reproducing their actual ellipticity. Where can I find values for:

• current Sun-Earth distance


• current Earth-Moon distance


• current Sun-Moon angular separation


• current velocity of Earth relative to Sun (or relative to Sun-Earth(-Moon?) center of mass)


• current velocity of Moon relative to Earth

and, maybe even

• current velocity of Sun relative to Sun-Earth(-Moon?) center of mass.

Is there any resource that provides these current values?

stellar evolution - What is the reason for high lithium concentrations in Sun-like stars?

In reading the European Space Organisation paper Lithium depletion in solar-like stars: no planet connection (Baumann et al. 2010), several conclusions based on their observations include:

• Lithium depletion in Sun-like stars is independent of having planets (an earlier theory), but rather is a function of the star's age. From the article:

For solar-like stars, the lithium vs age trends for planet-hosts
and stars where no planets have been found are statistically
identical. Thus, the presence of a planet does not influence
the observed surface lithium abundance.

However, they observed that

A number of solar-like stars with unusually high lithium
abundance for their age are present in the field.

and that these stars also have an anomalously low surface gravity.

What is the current theory as to why some Sun-like stars have a relatively high lithium abundance and low surface gravity?

Is it possible to break apart a neutron star?

It would appear theoretically possible (to some degree) through extreme applications of recycling to trigger mass shedding in pulsars.

Pulsars are rapidly spinning neutron stars, the fastest class of which are millisecond pulsars. The current belief is that they build up rotational speed through accretion, a process known as recycling. One study, Recycling Pulsars to Millisecond Periods in General Relativity (Cook, et al), explores the limitations of this process.

The following chart shows their results:

At the point where the dotted lines meet the two plots, you can see a reduction in mass at those energy levels. This is due to the angular velocity of the body creating instability which results in mass shedding - essentially mass at the equator of our neutron star being flung off the star due to the bodies angular velocity.

Unfortunately, this is not exactly an easy process.

The timescale to accrete the required rest mass, ~0.1 M_☉, at the Eddington limit, ~10^-8M_☉yr^-1, is ~10⁷ yr. This timescale is largely insensitive to the adopted nuclear equation of state. If other astrophysical considerations require a considerably shorter time scale, then the simple recycling scenario described here will have to be modified beyond the variations explored in this paper.

(Note however that the research here is actually attempting to avoid such instabilities, and they accomplish this by adding even more mass, such that the body can support even greater rotational velocity without encounter instability. Additionally, they're trying to create millisecond pulsars, but we don't need to do this as they exist naturally, so we could save ourselves a lot of time by (very carefully) approaching an existing millisecond pulsar)

I don't think this would exactly be breaking apart (despite Wikipedia's use of that exact verbiage to describe it), but it allows for the return of mass that was at one point in a neutron star. Of course, chances are our theoretical neutron star miners are very likely to be the ones who put that mass on the neutron star to begin with. On the other hand, this (hopefully) accomplishes the task without reducing the object to a quark star or black hole.

Cook, G. B.; Shapiro, S. L.; Teukolsky, S. A. (1994). "Recycling Pulsars to Millisecond Periods in General Relativity". Astrophysical Journal Letters 423: 117–120.

How big is the universe. How does it works?

There are features in the CMB, the distance and size of which can be derived from cosmological models (6-parameter Lambda CDM model). The expected size of the features can be predicted in terms of angles in the sky, depending on how curved large-scale space-time is. (The features are a statistical property of tiny temperature fluctuations in the CMB indicating the largest scale of causal connection - kind of sound waves - in the baryon-photon plasma before recombination 380,000 years after the Big Bang, called first acoustic peak of the CMB power spectrum. The angle should be about 1 degree for a flat space-time, in accordance with observation.) Things aren't quite straightforward, e.g. due to the way the universe has been expanding, therefore it's done with simulations.

Angles look larger in a positively curved space-time (e.g. a sphere) than in a flat space-time (e.g. a plane) than in a negatively curved space-time (e.g. a saddle).

The curvature of space-time follows the same principles as curvature of 2-dimensional surfaces. A triangle in an Euclidean plane has an angle sum of 180 degrees, while a triangle on a sphere has an angle sum of more than 180 degrees. Hence an object of a fixed size and a fixed distance looks larger (takes a larger angle) on a sphere than on a plane.

Here a sketch of a triangle on a sphere (may be interpreted as an observer looking to an object) and a triangle of the same size - meaning the same lengths of the sides in this case - in a plane:

(More on non-Euclidean geometry.)

If the observable universe is positively curved, it could be a fragment of the surface of a 3-sphere resp. 3+1-de Sitter space-time. A 3-sphere is finite. If the observable universe is flat (curvature 0), it could be a fragment of an infinite Euclidean space resp. 4-dimensional Minkowski space-time. That's plausible, but not the only possible extrapolation beyond the observable universe. An almost flat observable universe could e.g. be a fragment of a huge 3-spherical universe or of a huge 3-torus. Planck data don't indicate (in a significant way) that kind of structure outside the observable universe, but extrapolations become less valid the further we go beyond the observable universe.

According to the current (Lambda-CDM) model of the universe together with the best-fit parameters, the universe is flat within the limits of measurement. Hence it's plausible (by extrapolation), but not necessary, that the universe is either huge or even infinite.

star - Weight of a celestial body

I don't know if this is what most astronomers use, but it is certainly a method that could be used. It also makes for some interesting photos.

Gravitational lensing was one of the great predictions of Einstein's theory of general relativity, and was actually one of the first pieces of evidence for it - see the solar eclipse of 1919. Gravitational lensing is the bending of light by a massive object. Because mass and energy warp space-time, we know that if you can calculate how much the light is bent, you can calculate the mass of the body bending it. Wikipedia gives the following formula:

$$theta=frac{4GM}{rc^2}$$

where $M$ is mass, $r$ is the distance the object is from the light, and $G$ and $c$ are, of course, constants. So if you can measure how much light is bent by an object, you can calculate its mass. It's hard, and you would need good conditions for it to work, but it could work, nonetheless.

rotation - What is the evidence that galaxies rotate?

We have not discovered every galaxy in existence, nor have we watched each to see how it is spinning. Most of this answer will heavily be speculation. You have been warned!

We have three classifications of galaxy:

Spiral, Elliptical and Irregular.

Irregular galaxies are thought to be quite young, and predominantly gasses that are yet to combine.

Elliptical galaxies are thought to be like a disc, or elongated disk in shape.

Spiral galaxies are theorised to be the older of the three, and it is heavily theorised that they are spiral in shape due to a super massive black hole at their center.

We can see that it is thought elliptical galaxies will eventually become spiral galaxies

E classifications are elliptical. S classifications are spiral.

Therefore if we extrapolate the pattern and theories it could potentially be said that in the course of a galaxies life it will eventually become a spiral, orbiting a super massive black hole as more and more of its mass is drawn to the centre.

Ok so if it can be theorised that most galaxies will eventually spiral a black hole it leads us to believe that the rotation of a galaxy will then be because of the rotational direction of a black hole, right?

This NASA theorist says there may be such a thing as a backwards black hole, one that spins in the opposite direction to its accretion disks, which in this case I assume would be the galaxy.

Looking at observations of galaxies leads to most of them, spinning with the arms trailing, but there are cases where the arms are facing the direction of the spin!

Conclusion

So Yes, we have observed the spin of galaxies, no, not every galaxy rotates, irregular galaxies have no central point of mass of which to rotate about. As for if they all spin in the same direction, that is extremely hard to observe without more data points and further study.

star - Can solar luminosity & activity be predicted?

I will be specific to answering the part regarding the prediction of solar flares.

Solar flares are very volatile and dynamic phenomena associated to the breaking and reconnecting of magnetic field lines in the Sun's photosphere.

There is a phenomena associated with solar flares, called quasi-periodic pulsations, which sometimes precede a solar flaring event. As such, monitoring of such pulsations may give rise to a plausible prediction method. However, the accuracy and reliability is still to be debated.

A few good papers on QPPs:

Quasi-Periodic Pulsations in Solar Flares: new clues from the Fermi Gamma-Ray Burst Monitor

Time delays in quasi-periodic pulsations observed during the X2.2 solar flare on 2011 February 15

gravity - Stability of solar system

The wikipedia page you linked to tells you that the solar system is gravitationally "chaotic", in part because the mass of the sun is not fixed over time.

But even more simply than that, focusing just on the gravity (ignoring loss of stellar mass, etc.), the solar system is an N-body problem. We have 8 planets, a sun, and millions of asteroids, comets, and who knows how many individual particles gravitationally bound to our sun (plus ones that aren't and are just passing through the neighborhood, so to speak). When you have more than 2 bodies, the solutions to the N-body problem are unstable. What this means is that, say we describe the N-body problem with data $D$ (the "initial conditions", or a perfect description of the state of the system at some specific point in time). With a given complete data set the (Newtonian) gravitational evolution of the system is completely determined (but so difficult to do we can only approximate it). What instability means here is that if we have some other data set $D'$ that is only a little bit different from $D$, then the differences between the evolution from $D$ and $D'$ will become exponentially large over long enough time scales. So what may seem like minor differences now will result in radically different looking solar systems in the long run.

Since all of our observations can never give exact values, but only a range of values, there is necessarily a bit of uncertainty in what the exact gravitational state of our solar system is. We have very poor data on the exact asteroid and comet content of our solar system, and even planetary data has significant error margins. All of this means that there are lots of justifiable picks for the data $D$, each differing by a small amount from each. But due to the instability, eventually these data will produce radically different futures from each other. Currently we can only predict the solar system's evolution up to a few million years or so (the exact value stated can vary wildly depending on how you opt to define and compute the Lyapunov time). After that the evolutionary tracks become so disparate we can't really say we're predicting anything other than "it'll definitely do something".

One way or another, it is currently impossible for us to make any clear assertions about what the solar system will look like on a timescale of billions of years. Maybe all 8 planets will still be there; maybe their orbits will be very similar, but maybe they'll have much different orbits; maybe several planets will have been ejected from the solar system. At best we can observe a few things that lead certain objects to be most likely to undergo significant alteration. For example, Jupiter and Mercury appear to have a certain orbital resonance right now which could ultimately lead Mercury to undergo a significant orbit change. This may ultimately cause it to collide with another planet, or the sun, or be ejected from the solar system entirely. But maybe it won't. It's hard to say.

orbit - How does a gravity slingshot actually work?

The diagram is in the rest frame of the planet. Now suppose a spacecraft is slowing down in the frame of the solar system. A planet is nearby, so it now starts accelerating due to its gravity and gains speed. Now, this speed increase is added to some component of the speed of the planet's motion when it comes out on the other side (this added component can be changed by changing the angle from which it approaches the planet, in order to maximize the slingshot effect). Once out of the planet's influence, the spacecraft has the same velocity as before, plus a component of the planet's motion, which allows it to travel farther out. This is the slingshot effect.

Trying to look at this in another way, consider the angular momentum of the spacecraft. As long as it is only under sun's gravitational influence, its angular momentum cannot change. However, once it's under the influence of another planet, the two angular momenta - one w.r.t. the sun and one w.r.t. the planet (due to their relative motion) - add, and once out of the gravitational influence of the planet, their relative components can be adjusted (based on the angle of approach towards the planet and the angle at which it flies away after the slingshot) in order to increase the angular momentum w.r.t. the sun, which in turn puts it in a larger orbit, allowing it to travel farther away than before.

radio astronomy - Telescope in Sun's gravity lens focus - pointing, gain, distortions

A telescope located in the gravitational focus of the Sun can use the Sun as a magnifying lens. The focus begins 550 AU away, but maybe a 700 or 1000 AU distance is needed to get rid of disturbances from the Corona, and the focus extends practically indefinately. Here are some slides by Dr. Maccone who has promoted this idea he calls FOCAL:
http://www.spaceroutes.com/astrocon/AstroconVTalks/Maccone-AstroconV.pdf

I intend to ask about the technical design and feasibility of such a project in the Space Exploration SE. Here I rather ask about the scientific value and challenges.

POINTING: The magnification would occur only in the exact direction of the Sun. But since the Sun moves and the background objects magnified move, I suppose that the observed targets would change continuously. Would it even be practically possible to give the telescope a trajectory which keeps it aiming at for example Alpha Centauri? Would there most of the time be nothing in the right direction as the line between the telescope and the Sun sweeps across space, or would there alway be some star or galaxy in sight? Like CMB if nothing else.

GAIN: In the slides linked above, Maccone has calculated the expected gain to 114 dB for infrared wavelengths. How many times "magnification" does this mean? I don't think I understand the units here, I get a ridiculously large number. Can it be explained somewhat intuitively? Would a FOCAL mission be a unique revolution in astronomy, or could similar results be achieved by building an interferometer with interplanetary sized baselines here nearer to the Sun? How does the science value of a gravity lens compare to that of a wide baseline? Are they good for different tasks?

DISTORTIONS: Could the lensed signals be reconstructed thanks to our knowledge of the Sun and measurements of corona activity? If pointed towards a central part of the Milky Way, wouldn't signals come from multiple objects at the same time, some much further away than others? Would the gravity lens of the Sun have bigger problems with distortion than the intergalactic gravity lenses we know of today?

And finally, can any natural strong lensing inside the Milky Way be used today, for example using a globular cluster as a lens?

Data for Sun's Orbit - Astronomy

The planets generally kindof orbit around the sun, but a more precise statement is that the planets and the sun all orbit around a common barycenter.

Because the sun is so massive, this barycenter is very close to the sun. If we take the sun to be the origin, the barycenter travels in a complex, nonlinear trajectory (from Wikipedia):

Modeling this accurately is critically important to predicting celestial events. I wrote a simulator to predict the 2012 transit of Venus, based on estimates of orbits and the assumption that the sun is at the origin.

The error introduced by these assumptions was enough to miss the transit by a solar diameter.

This data must exist, since the transit actually did happen. Additionally, errors in the orbital data of the planets could have affected it. Where can I find data on where the planets and the sun are and what trajectories they're moving in (relative to the solar system's barycenter)?

the sun - What scientific evidence is there to support or refute the Iron Sun Hypothesis?

Even if the answer has already been accepted, more evidences can contribute to this thought-provoking topic.

I think, as it has been mentioned, eliosismology is a good way to map the interior of the star, but I am not able to argue about it.

Another disproof, comes from neutrinos.
Interior of Sun can be investigated by neutrinos detection, which do not interact passing by the outer shells, and can directly say where they come from, and how.
Standard solar theory predict neutrinos production by p-p chain.
This has been observed, even if the neutrino flux is about $1/3$ than the expected one, because of the neutrino oscillation.
Now, this theory become solid after Bruno Pontecorvo, but I do not think we have any other evidence of observed neutrino oscillation.

Furthermore, Earth's age is helping us. We know Earth is ${sim}4$ billion years old, that is roughly the age of the Solar System and the Sun itself.
From Niels Brendt website, we know that after such a long time, the white dwarf luminosity has become much fainter then Solar (where Solar is referred not only to the Sun, but to general G2 main sequence stars). Neutron stars cool even faster. And this is obviously not observed: if Manuel's conjecture was real, Sun should be much fainter now. To this, add that they say that their supposed SN happened something like 5 billion years ago...

As a final note, I would highlight that, not only this author has never been mentioned in literature, that is (as we know) synonym of poor quality, but he actually never published (at least since 10years or so) in any refereed paper, which is automatically translated, to me and I think to the community, as not science.

general relativity - Can A Black Hole Exist?

There is evidence that both black holes and their event horizons exist.

The primary evidence for stellar-mass black holes arises from observations of the dynamics of binary systems. What has been found, for at least 20 binary systems, is that the optically visible star has a dark companion, that is usually more massive, and more massive than can possibly be supported as a neutron star or any other degenerate configuration ($>3M_{odot}$ and mostly $>5M_{odot}$ - see Narayan & McClintock (2014) for an excellent, accessible review).

Supermassive black holes at the centres of galaxies can also be "weighed" using the dynamics of gas, or in the case of our Galaxy, actually watching the stars orbiting something that must have a mass of $4.4times10^{6}M_{odot}$, yet is remarkably compact and emits little or no radiation.

The evidence for event horizons is more circumstantial, but not absent. In any case, absence of evidence would not be evidence of absence - event horizons are difficult to prove; they are small and far away, with elusive observational signatures. The lack of an event horizon could only be accommodated by inventing something even more bizarre than a black hole.

The most compelling argument for an event horizon is found in sources that accrete from their surroundings, either in a binary system or at the centres of galaxies (see Narayan & McClintock 2008; read particularly section 4). In some circumstances, the accretion flow can become radiatively inefficient (dark), in which case one expects that if the accretion flow reaches a "surface", it will radiate its kinetic energy strongly as it thermalises on impact. On the other hand, a black hole event horizon can swallow such flows without trace. The prediction is, that in their low-states, transient black hole accretors will be much less luminous than their neutron star counterparts, and this is what is found (by a factor of 100).

Other evidence is that quiescent neutron star binaries show hot, thermal emission, presumed to be from the neutron star surface. None has been seen from the black hole binaries. Type I X-ray bursts are seen in neutron star binaries, caused by an accumulation of matter and subsequent nuclear ignition on their surface. The black hole candidates do not show these bursts. The accretion rate onto the supermassive black hole in the centre of the Galaxy should produce thermal radiation at any "surface" that would be quite visible in the infrared part of the spectrum. No such detection has been made.

All of these observations are best explained if the black hole has no surface and can simply make the accreted energy "disappear" inside its event horizon.
Narayan & McClintock conclude that these lines of evidence are "impervious to counterarguments that invoke strong gravity or exotic stars".

universe - How are Galaxy Super Clusters Generated

@AlexeyBobrick is correct. To add to his answer:

The scale of galaxy clusters are on the order of ~1Mpc (~3.14 Million light years in size), and are therefore much smaller than the cosmological horizon.

The cosmological principle is extraordinarily important for cosmologists, and makes the assumption about the universe's global properties (which turn out to be really good assumptions, so far):

1. Homogeneity on the scale of about 100 Mpc. This means that one would see very little change in density within a bubble of radiua 100 Mpc as you move said bubble around the universe.


2. Isotropy means that there is no preferred directions to look. No matter where you look, you should see roughly the same picture of the universe.

As for how these clusters are generated (addressing the title of your question), large N-body simulations are run over the age of the universe from different sets of initial conditions (this is where different models and assumptions come into it). These are dark matter only simulations and involve only the force of gravity. Some people are including electromagnetic interactions in their code, but it's far from being the norm, and are really only important on small scales, i.e. - inner regions of galaxies and clusters. These simulations can contain upwards of ~10 billion dark matter particles.

earth - Why does mars appear to retreat across the sky?

The effect is called apparent retrograde motion.

What happens is that Mars has a 'direction opposite to that of other bodies within its system as observed from a particular vantage point' when this loop occurs.

That's a bunch of words that don't mean a lot to me. A picture is worth a thousand clearer words:

^{(Imagine this turned sideways and you get the effect in your image)}

Basically, because Earth and Mars are orbiting the sun at different rates, our vantage point of Mars changes for each combination of points in the orbit of each planet.

On this scale, the background of stars is pretty much stationary - any apparent movement of the stars due to this effect is going to be negligible. Thus, the stars are our point of reference.

As our vantage point of Mars changes, it appears to shift directions on the stellar background, creating the effect you describe.

What is the distance that the Moon travels during one orbit around the Earth?

The Moon has an orbital eccentricity of 0.0549, so its path around the Earth is not perfectly circular and the distance between the Earth and the Moon will vary from the Earth's frame of reference (Perigee at 363,295 km and apogee at 405,503 km), see for example second animation explaining Lunar librations in this answer.

But its orbit can be said, in an oversimplified manner, to be periodic, with no significant apsidal precession (not really true, but somewhat irrelevant for my following musings here to be still close enough), so we can calculate its orbital length based on its quoted average orbital speed of 1.022 km/s and orbital period of 27.321582 days.

So, plugging our numbers in a calculator, $l = v * t$, we get the Moon's orbital length of 2,412,517.5 km (or 1,499,070 miles). Should be close enough. Source of all orbital elements of the Moon is Wikipedia on Moon.

spectroscopy - Any cheap (

Yes, with a piece of 1000 lines / mm of diffraction grating that costs a few dollars, and some odd bits and ends, you could see the Fraunhofer lines.

Here's a way to build it:

http://sci-toys.com/scitoys/scitoys/light/spectrograph/spectrograph.html

At the bottom there's a link to a place where you could purchase diffraction grating - but any grating around 1000 lines / mm is fine, and you could probably find it in other places too, if this store doesn't work for you for some reason.

You could try to use a digital camera and photograph the spectrum, instead of looking at it directly.

Beware, looking into the Sun poses a certain amount of danger. As long as you keep the slit pretty narrow, this spectrograph is safe. If the spectrum is too bright to look at comfortably, the slit is probably too wide and the instrument is not safe for direct viewing (but probably safe for a camera).

You could also use it to inspect other sources of light: LED lightbulbs, fluorescent lamps, open flames, etc. It's quite fascinating.

coordinate - how steady over time are right ascension and declination values?

I understand that astronomers use right ascension and declination for the position of stars. However both these values strike me as very anthropocentric and, more importantly, unreliable over time as I understand that the Earth wobbles a bit over time so the celestial equator shifts and I guess Sun's rotation around the galaxy and Milky Way's own trajectory should play a role in depreciating the accuracy of those values over time.

So my questions are:

[1] how steady are right ascension and declination values for distant stars over time?

[2] is there a more stable coordinate system or do we lack an absolutely fixed frame of reference?

universe - Big Bang / Big Crunch cycle?

I figured I might as well expand my earlier comment into an answer, as I've cobbled together some more information.

You're talking about the cyclic model, which states that the universe goes through cycles of Big Bangs and Big Crunches. We live in the middle of one of those cycles. Albert Einstein was one of the first to investigate it, but his efforts came to nothing.

The theory has been reincarnated, most recently in 2002 by Paul Steinhardt and Neil Turok. They propose that the cosmological constant has changed over time, which could make the accelerating expansion of the universe fit into the theory. Another good article on their idea can be found here. Roger Penrose has also come up with a theory - Conformal Cyclic Cosmology - along similar lines as Einstein's. You can use this as a starting point for more reading.

A related topic is the Big Bounce, which attempts to address the Big Bangs and Big Crunches.

Why does time get slow near a black hole?

Time does not pass slower near a black hole. Stay-Home-Sally far from a black hole observing Astro-Bob descending into a black hole would see Bob's time passing slower as Bob neared the event horizon, but Bob would not experience this. On the contrary, Bob, looking back at Sally, would see time pass faster for her as he neared the event horizon.

Who is right, or is time relative to the motion of the observers?

What is the projection of Earth's axis on the sphere with the Sun in center (see explanation)

Seems like it is the most convenient SE community for this question..

Imagine the sphere, witch center is the Sun and radius is, say, ~1.5 distance from Sun to earth (in arbitrary time, precise number doesn't matter much).
EDIT: the center of sphere is 'attached' to the Sun.

Then, imagine, the Earth's axis projection on that sphere in time. It leaves some trace.

What will be the picture? Will it repeat itself through some period? What will be the "scansion" for different time periods?

EDIT: as reply to comments I'll add, that my interest is purely out of curiosity, and degree of required accuracy is low. I don't want high precision here, just rough pattern. As I understand this could be done, using mentioned Milankovitch cycles, but I'm not much into mathematics. Though, if I'll not find answer here, I will be forced (by myself) to figure this out, when i'll get some spare time heh.

terminology - What is the name of that which exists beyond the Universe?

By definition, the capitalized word Universe denotes everything there is, so even if we one day discovered we're just a part of a Multiverse, all the parallel universes of it would still be parts of the Universe as a whole, where Multiverse would just describe its nature. Or, if some yet undiscovered regions of it would defy our current understanding of its physical laws and constants as we know them, all of it would still be a part of the whole Universe. So it really doesn't matter, if beyond the known universe, there are regions of honey, milk and chocolate biscuits and all of it is carried by a giant tortoise. All of it would be the Universe, the physical universe as we can observe, the honey, milk, chocolate biscuits and the tortoise. The lot. All of it. The whole shebang.

Notice that I use capitalisation here, i.e. you can have more than one universe, but they're all a part of the Universe. Without capitalisation, it's just any domain, a particular sphere in physical or metaphysical sense, and only a part of the whole Universe. Sadly, this capitalisation is often neglected or used inconsistently, as is often the case with earth vs the Earth (the top soil vs the planet), sun vs the Sun (any star with planets vs our Sol), moon vs the Moon (any natural satellite vs our Luna), even galaxy vs the Galaxy (any galaxy vs our Milky Way). For example, observable universe is a sphere, a region of space, within the Universe.

The beauty of this naming convention is, that we already know the name for (but not necessarily of) everything there is, even if we don't know or can agree on what all that encompasses, or what laws govern some regions of space, time, or some other, yet unknown existence of it. It is universally true regardless of anyone's beliefs, even if they choose to call all of it by other names or attribute this existence to a sentient being, super being, or God. We're all a part of everything there is - the Universe.

the sun - Is Sun a part of a binary system?

There is no observational evidence that the sun is a member of a binary (trinary, or more) star system, where "star" means an object that is at least ~80 times the mass of jupiter and emits energy/light via standard hydrogen fusion.

Some evidence that people point to is that the majority of stars in the Galaxy (perhaps 60% or so) are binary. However, that does not mean that we are a binary system, just that we are among those which are not.

There is the Nemesis star idea which was a hypothetical binary companion to the sun to account for periodic mass extinctions roughy every 26 million years (as in, the two stars would therefore be on a 26 million year period, so the companion is a few light-years away). There are two problems with that: First, that a 2010 reanalysis of the data showed that it was too periodic over the past 500 million years, during which time we've gone around the galaxy twice, and so our hypothetical binary orbits should have been perturbed and not be perfect (though others say that the precision of the geologic timescale isn't good enough to say this). But, second, we can see stars out to a few light years -- we can see small, faint stars out to thousands of light years. We have all-sky infrared surveys that should be able to pick up the faintest even star-like objects to at least a few light-years, and yet ... nothing.

The other problem is what LDC3 pointed out in their answer: We should see some systematic motion of our own star in orbit around the common center of mass of the hypothetical binary. This would not be a yearly wobble, but rather it would be a slow motion of the entire sky on top of the other >1 year motions that we see. We now have very accurate astronomical records dating back at least a century of star positions, especially close stars. Even if we were on a 26 million year orbit - and especially one much shorter as some people claim - we should see effectively that our star is making a small arc, a part of a circle as it orbits the center of mass of it and the binary. We don't. There does not seem to be any systematic signal in the motions of stars that require the binary star model.

So, to summarize: A binary companion simply lacks any observational data; if it existed, we should not only have been able to see it by now, but we should also observe not only the stars in our sky show a systematic motion due to our orbit around it, but we should again see THAT binary star also move very quickly, relative to other stars, as it orbits the common center of mass.

amateur observing - What is the object in this photo?

Your guess was correct. It is the Andromeda Galaxy, M31.

Here is a map of the part of the sky near zenith at the place and time you provided: Sky map for Taganrog, Russia on 11/23/2013 5:00:00 PM UTC. Even the rotation is small. The sky map is rotated approximately 30° counter clockwise relative to the photo. You were approximately facing south when taking it.

On your picture there is a part of the Andromeda constellation. I will use only a small bottom part of your picture (below the imaginary horizontal line going through M31) to identify the constellation.

• The bright star to the left of M31, very close to the edge is μ Andromedae (marked 37 μ And on the sky map). It is the star between Mirach (out of photo) and M31.


• On the bottom of the right part of the photograph you see three stars on a imaginary vertical line: ψ, κ and ι Andromedae.


• Between ψ and κ, to the right there is λ Andromedae (marked 16 λ And and connected with μ on the sky map).

The bottom of the three stars in the upper-left corner of your photo is φ Andromedae (connected by the vertical line with 37 μ And on the sky map).

The 1 ο And on the sky map is not on your photo.

What do we know about the lifecycle of the Milky Way (or any other spiral galaxy)?

If we look at Newtonian physics, and how galaxies will interact, a central black hole should just be considered as a massive, dense object.

The Milky Way does not fall into its own central black hole, it orbits it - as physics tells us it should.

When we get closer to Andromeda, the gravitational influence of Andromeda will act more strongly on us, and when we get really close, individual masses within each galaxy will have dramatic effects on each other, but as with any such system, the two black holes will not suck everything in.

If the two galaxies end up coalescing - which is not a given - the orbits of the stars and the black holes will be very complex. For billions of years the 2 black holes will orbit each other, getting closer as they shed energy, but during that time the stars around them will suffer many effects, including:

• some will be expelled


• some will hit the black holes


• some will go nova


• and so on

The best way to understand what will happen is to avoid thinking too deeply about black holes being weird, and treat them as dense masses. For most purposes this will help you model galactic collisions.

Have a look at this NASA simulation of the collision between the Milky Way and Andromeda:

Could Venus be a source of Earth's water?

No idea is stupid per se. But in order to place an answer to your question, please consider Venus as a comet. It once had, in that image, a coma (a tail) pointing outwards from the sun, made of evaporated water pushed away by solar wind. How frequently is the Earth exactly inside that coma?

As a comparison, when the Earth strikes a real comet coma we see meteor showers that last no more than one day. So if we consider that Earth may go through Venus imaginary coma once a year, we have an upper limit of 1/365 of Venus' water captured by Earth. This, without taking into effect that Venus had not have a real coma, that evaporation and expelletion of water from Venus was not a so directional process, and surely other factors I can not think about just now.

Besides, do not consider comets the main source of water on Earth. You need to count also with the water vapour generated on volcanoes and that generated on acid-base reactions among Earth's rocks.

Does anyone know why three of Jupiter's largest moons orbit in 1:2:4 resonance?

When the Galilean moons formed, they weren't in resonance with each other. All of them were in slightly smaller orbits than they are now. Over time after their formation, Io's orbit slowly moved outward due to tides from Jupiter. This is the same effect that is causing our moon to slowly move away from the Earth (at about the same rate your fingernails grow). It goes like this. The Moon's gravity causes tides to form in Earth's oceans. This bulge of water gets carried forward with Earth's rotation, because the Earth is spinning faster than the Moon is orbiting. Because the moon is still attracting the bulge, it causes a drag on the Earth, slowing its spin. At the same time, the bulge is attracting the moon, causing it to go faster in its orbit. As the moon speeds up, its orbit gets bigger. So essentially, the Earth's spin energy is getting transferred into the Moon's orbital energy. The same thing happens with Jupiter and Io, with a bulge in Jupiter's atmosphere causing Io's orbit to get bigger.

As Io's orbit expanded, its 'year' got longer, until it approached a 2/1 resonance with Europa. Once they reached resonance, they got 'locked in', their mutual gravity acting on each other reinforced it. Io was still raising tides on Jupiter, though, and its orbit was still trying to expand. As Io's orbit kept expanding, it gave a gravitational kick to Callisto on each pass, expanding both of their orbits until Callisto reached a 2/1 resonance with Ganymede. This is where the inner 3 Galilean moons got their resonance. The orbits are still expanding, but much more slowly because with each one you add on, it gets harder to transfer the energy. Given enough time, all 4 Galilean moons would probably reach resonance, although the sun will die before that happens.

I may have some of the minor details wrong here, but this is the story as I understand it.

launch - Why are spacecraft not air-launched from airplanes

Lots of good stuff on this topic in Wikipedia.:
Air Launch to Orbit

Air Launch

A typical rocket spends the first few seconds going straight up (almost) to get out of the atmosphere. After that it spends almost all of its time accelerating to orbital velocity. Thus getting out of the atmosphere, while hard (rocket is heaviest at this point) is really only a very small portion of the process.

The mass you would need to carry to a higher altitude is so large as to exceed the capacity of the largest aircraft ever built, let alone a balloon.

Consider the case of StratoLaunch. They intend to build a carrier aircraft composed of parts from 2 Boeing 747 (in the top 3 of the biggest aircraft commercially available. A380, B747, and C5 Galaxy are probably the biggest). It will be the largest airplane (in mass, wing length, etc) flying if it succeeds.

Even then, it can only carry a scaled down version of a Falcon (The original payload was to be a SpaceX Falcon 9 but with 5 engines instead of 9 and a concomitant reduction fuel/oxidizer load and thus mass). So the biggest aircraft yet to be built could only carry a smaller version of a medium size booster.

The primary benefit of air launch is not extra mass, but rather launch constraints. If you launch from a fixed site, you have limited launch options to differing orbits, and inclinations. An aircraft can in theory fly to wherever is convenient to hit the right orbital parameters. (Assuming there is a big enough runway for something the size of Stratolaunch vehicle within flight range fully loaded).

Currently, there is the example of Virgin Galactic's SpaceShip Two that uses the White Knight carrier vehicle, whose payload to actual orbit (LauncherOne if they actually build it) would be in the 100 kilogram range.

Pegasus, launched beneath a Lockheed L-1011 (3 engine airplane), maxs out around 500-1000 kilos to orbit.

There was a European proposal to launch an orbital payload (in the 200Kilo range) from the top of an Airbus A300.

The scaling up, that would be required just does not work.

the sun - Did the Babylonians believe in the Heliocentric version?

It's mentioned that the Babylonians decreed that there were exactly 360 days in a year. This statement was based upon the utilization of 360 as a standard (based on the sexagesimal system) i.e., they divided the circle into 360 equal parts.
Although, there is a mention about the Sun making one full circle across the sky in one year (as estimated by them), but, how were they able to measure it? And did they believe in the Heliocentric version of the solar system?

universe - Where is all the antimatter?

To answer your second question first: Yes, antimatter does exist in the same space as matter. In fact, the universe creates antimatter (and an equal amount of matter) every day as a matter of course in events like lightning strikes and supernovae, and even in certain nuclear decays. Humans create it in particle accelerators for research and for commercial/medical applications such as Positron Emission Tomography. The thing is, when we create antimatter, we also create an equal amount of matter.

In the hot flash of energy after the Big Bang, particle-antiparticle pairs were popping into existence and annihilating each other constantly. There were almost exactly equal amounts. For some reason, though, for every 100 trillion (10^11) particles of antimatter, there were 100 trillion and one particles of matter. In the ensuing few minutes, all the antimatter and all but that tiny fraction of matter annihilated each other and turned back into energy. Everything we can see today, all the galaxies, stars, and planets, are made up of that tiny amount of matter that was left over. Particle physicists still aren't sure why there was this tiny imbalance in the amount of matter and antimatter, because all interactions we've seen so far produce equal amounts of both. This is one question particle colliders like the Large Hadron Collider are attempting to answer.

inclination - iteration to cover the whole sky with right ascension, declination, angle

I am sure I get parts of the terminology wrong but if anyone can shed some light in the following:

I understand that for a given right ascension (RA) and declination (DEC), one has defined a ray (half-line) in the sky starting from the center of the earth towards infinity.

Now, if I also provide a angle, say d degrees around that ray, I have essentially defined a cone in the sky. My question is the following:

Which steps (RA_step and DEC_step) should I use in the following loop, as a function of d to ensure that I cover the entire sky and don't leave any patches anywhere?.

for (RA = 0 ; RA <= 360 ; RA += RA_step)
    for (DEC = -90 ; DEC <= 90 ; DEC += DEC_step)
         examine-cone(RA, DEC, d)

Is it safer to be near a star or a black hole?

the candidate star is an "average" 5 solar mass star, and the black
hole is a 5 solar mass black hole

Then their gravity is identical. Black holes don't have magic powers. A 5 M☉ star and a 5 M☉ black hole exert the same attraction from the same distance. The only difference is that the black hole would be much, MUCH smaller (about 30 km diameter in this case), so you could get a lot closer to the center, which is where the extreme gravity happens. But at cosmic distances they are the same.

Hawking radiation from a 5 M☉ black hole is negligible, likely too small to measure - its temperature is about 10^-8 Kelvin.

If it has an accretion disk, that might be a problem, but usually it only generates two relativistic jets at the poles - if you're not hit by the jets, you're fine.

The overall thermal glow from the accretion disk of a 5 M☉ black hole might be intense, but it can't last very long. It's not a continuous burn like a star, unless it's a much bigger black hole with a huge accretion disk and plenty of source material nearby to feed it - which is not the case here.

Your conclusion is technically correct. Gravitationally they are the same, but the star has the added problem of generating extra heat. Not that it would matter anyway, because both would completely disrupt orbits in the solar system, and the Earth is going to be flung out into space and would freeze anyway. It's just that the star would bake us first, and THEN we would freeze solid.

Or we would settle into a highly elliptical orbit around either the Sun or the invader body, which will have us alternately baking to death and freezing to death.

There is also the very tiny chance that, due to orbital disruptions, we would collide with something else, either with the invader, or with the Sun, or with another planet. This would mean a quicker death and might be overall "preferable".

Regardless, having a 5 M☉ body entering the system is not a good scenario for life on Earth.

http://xaonon.dyndns.org/hawking/

distances - Diameter of any galaxy

Given the angular diameter $a$ in radians and the distance $d$ in Mpc, you can get the actual diameter $D$ from:
$$D = dtan{a}$$

Using the small angle approximation, you get:
$$D = da$$

$a$ is in radians, so to get the distance in Mpc from the angular diameter in arc seconds you'd need to convert the angle in arc seconds to the angle in radians: $a = frac{2pi A}{360times3600}$, where the factor $3600$ is used to convert arc seconds to degrees, and $frac{2pi}{360}$ to convert from degrees to radians:

To get the diameter in kpc:
$$D = 1000times d frac{2pi A}{360times3600}$$
$$D = d frac{pi A}{648}$$
$$D approx frac{dA}{206} $$

where $d$ is in $textrm{Mpc}$, $D$ is in $textrm{kpc}$, and $A$ is in $textrm{arcsec}$.

Here $D$ is the diameter for round objects (even for disk galaxies seen at an angle). If the object is not round then this would normally be the maximum diameter.

[EDIT (see answer and comments from HDE 226868)] For irregular galaxies you may also need to have more information (viewing angle) to find the real maximum diameter of the galaxy. But that information is (I think) only available for galaxies in the local group.

gravity - A day in earth, a thousand years somewhere else

Assuming you mean a day in terms of an Earth day in both cases, the phenomenon I believe you're referring to is called time dilation, which affects the speed at which time is experienced by one body relative to another. This rate is affected by velocity and gravity, each of which can cause the bending of spacetime that results in the discrepancy between objects. Both gravity and velocity slow one's time as they increase.

The situation you describe is probably not naturally occurring, however is not outside the realm of reality. As referenced by Wiki from Calder's book Magic Universe, Calder claims that an acceleration of a constant 1G (what we feel on Earth all the time) would result in the affects of time dilation allowing you to traverse the entirety of the known universe in a single human lifetime (for the traveler). Conversely, during your trip, the rest of the universe would be aging 'normally'. There are plenty of technical problems with the feasibility of achieving this, but is conceptually possible.

I'm not particularly qualified to explain the equations behind it, so I'll leave that to our more capable members. Coincidentally, time dilation is the excuse I give when asked why I walk so quickly.

which pulsar has the longest spin period so far?

Vela X-1 is an example of an accretion-powered pulsar. These emit pulsed emission because they are accreting material onto their magnetic poles. If the magnetic poles and rotation axis are misaligned this results in pulsed X-ray emission. The power for the pulsar comes from the infalling material.

The classic P-Pdot diagram you show is for radio pulsars, or rather rotationally-powered pulsars. These are objects that are pulsators by virtue of accelerating charged particles from their magnetic poles out along the field lines. These accelerated relativistic particles emit synchrotron and curvature radiation that is beamed and intensified. The energy ultimately comes from the rotational kinetic energy of the pulsar.

In other words, other than the fact that they both involve neutron stars, these are completely different phenomena.

In terms of radio pulsars, your diagram looks reasonably up to date. I think the longest period object on your diagram is PSR J2144-3933, which has a period of 8.51 seconds (Young, Manchester & Johnston 1999).

Your diagram has a line marked as "graveyard". I believe this is a locus defined by Bhattacharya et al. (1992) and has the form
$$P = 2.42times 10^{-6} B^{1/2} s,$$
where the pulsar magnetic field $B$ is in units of Gauss (typical pulsar values would be $B=10^{10}-10^{13}$ Gauss (as also marked on your diagram). The theory behind this "death line" is discussed by Ruderman & Sutherland (1975). Briefly it arises from the requirement of a minimum potential difference to be generated such that accelerated particles produce energetic enough radiation to stimulate the production of further electron/positron pairs. If the magnetic field strength falls or the rotation period gets too long, then this mechanism fails and the pulsar is quenched.

PSR J2144-3933 appears to lie beyond this death line, but there are other ideas and models of how this death line may arise (of which I am not very familiar, but see for example Zhang et al. 2000).

Note that millisecond pulsars are thought to recycled accretion-powered pulsars. That is that they gain sufficient angular momentum from an accretion process that they spin up to become radio pulsars again.

Radio-quiet pulsars are thought to be rotationally powered but where the radio beam is narrower than say the beaming of gamma rays in which they are detected.

Magnetars are something different again. Their power comes from the decay of extraordinarily strong magnetic fields.
The soft gamma ray repeaters and anomalous X-ray pulsars fall in this category.

So you can see that the word "pulsar" might include all sorts of different types of objects and physics.

Viewing a solar eclipse through a leafy tree

Viewing my Facebook feed today, my local news station posted regarding a solar eclipse taking place today:

Note the line about using a leafy tree as a filter:

Scientists say NOT to directly look at the sun (if skies are clearing where you are), but looking at the eclipse through a leafy tree creates a natural filter!

I've heard a lot of people recommend viewing a solar eclipse by looking at the projection of the eclipse caused by light passing between leaves in a tree, in this case looking at the ground or a wall, where ever the light hits after passing through the tree. However, the Facebook posting here seems to recommend looking up at the solar eclipse through the tree.

I suspect that they have misunderstood the projection method (or inadequately explained), and their advice is not only wrong, but potentially dangerous.

Is this method recommendable? Should it be considered dangerous?

Follow-up

Almost immediately upon having seen the posting I had commented on it noting my belief that such advice could be dangerous, including a more clear explanation of how viewing using trees should be done. The staff member who originally posted the status later took note of this and edited the status to less ambiguously explain viewing the projection. Unfortunately, this was done after the solar eclipse had concluded. Hopefully no damage was done.

history - How many constellations in the Zodiac?

There are thirteen modern constellations in the Zodiac. In modern astronomy, a constellation is a specific area of the celestial sphere as defined by the International Astronomical Union. In total, there are 88 constellations.

Astronomy and Astrology are not the same thing. Astronomy is a science while Astrology is not. As such, I'll restrict myself to the historical and modern constellations of the Zodiac.

According to Encylopedia Brittanica

Zodiac (is) a belt around the heavens extending 9° on either side of the ecliptic, the plane of the earth’s orbit and of the sun’s apparent annual path.

In historical astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude that are centered upon the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. Historically, each of these divisions were called signs and named after a constellation: Sagittarius, Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, and Scorpius.

In 1930, the International Astronomical Union defined the boundaries between the various constellations, under Eugène Delporte , who,

... drew his boundaries along vertical lines of right ascension and horizontal parallels of declination. One governing principle was that all variable stars with an established designation would remain in that constellation, as requested by the IAU’s Variable Stars committee.

"Constellations ecliptic equirectangular plot" by Cmglee, Timwi, NASA - Own work, http://svs.gsfc.nasa.gov/vis/a000000/a003500/a003572. Licensed under Public Domain via Commons.

As a result, the path of the ecliptic now officially passes through thirteen constellations: the twelve traditional 'zodiac constellations' and Ophiuchus (which was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy), the bottom part of which interjects between Scorpio and Sagittarius.

Source: journeytothestars.wordpress.com

So, the 13 constellations of the Zodiac are Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius and Ophiuchus.

As seen from the (first) figure, the ecliptic also touches the edge of the constellation Cetus, though it in not usually included in the Zodiacal constellations.

Note about Precession: Because the Earth in inclined (by $23.45^{circ}$), it rotates like a top. This is called precession, which results in a shift in the position of the constellations relative to us on Earth. The result is that the 'Signs of the Zodiac' are off by about one month.

solar system - Could there be another planet between Mercury and the Sun?

No, that is not possible. There are quite a few capable telescopes studying the Sun since some decades. Already Galileo stared at the Sun until he got blind. Don't you think a planet would've been detected if it passed by in images like these? But Sun grazing comets are pretty common visitors in the corona.

It is a bit weird that a couple of percent of exoplanetary systems have hot Jupiters. They probably didn't form there, but migrated. And still stay there somehow.

space - Did Big Bang sound as loud as we think?

"Was it as loud as we think" is difficult to answer, since it's opinion-based. But since sound is nothing but longitudal oscillations in a gaseous medium, Big Bang was not at all silent.

If the Universe were completely homogeneous, it would stay like that. But primordial quantum fluctuations ensured that space was a tiny bit more dense in some places, and a tiny bit more dilute in other places.

Gravitation then ensured that the overdensities attracted matter and grew in size, until the pressure thus built up resisted further compression, and waves traveled outward from these overdensities. They then contracted and expanded again a few times, until 380,000 years after Big Bang when the photons decoupled from the gas, relieving the gas of its pressure, and freezing the waves in (comoving) space.

This phenomenon is called baryonic acoustic oscillations (BAOs). You may also be interested in my answer to the question "What is the speed of sound in space?"

However, humans wouldn't be able to hear it, since the wavelength (at decoupling) were roughly half a million lightyears, and the corresponding frequency thus orders of magnitues below the human threshold of ~20 Hz. Due to the expansion of space, the frequency increases as we go back in time, and scaling by a factor of $sim10^{26}$, it is possible to get a notion of what the early Universe sounded like. This has been done e.g. by John Cramer from U. of Washington on the basis of WMAP's observations of the cosmic microwave background which hold information about the BAOs.

You can hear it here. It doesn't really sound nice, though.

What is the temperature of an accretion disc surrounding a supermassive black hole?

It depends on the distance from the central body. This gives the temperature $T$ at a given point as a function of the distance from that point to the center ($R$):
$$T(R)=left[frac{3GM dot{M}}{8 pi sigma R^3} left(1-sqrt{frac{R_{text{inner}}}{R}} right) right]^{frac{1}{4}}$$
where $G$, $pi$, and $sigma$ are the familiar constants, $M$ is the mass of the central body (and $dot{M}$ is the rate of accretion onto the body), and $R_{text{inner}}$ is the inner radius of the disk - possibly (if the object is a black hole) the Schwarzschild radius $R_s$, in which case we can simplify this a little more. So the temperature in the accretion disk is far from constant.

Whether or not there is plasma depends on the exact nature of the disk, the central object and the region around it. For example, a supermassive black hole may have different matter in its disk than that of a stellar-mass black hole. I should think, though, that black holes in binary systems accreting mass from a companion should have plasma in their accretion disks, and supermassive black holes might also have plasma from nearby stars.

Black Hole / Hawking Radiation: Why only capture anti-particle?

I thought that the anti-particle was annihilating with "normal" mass inside the black hole? No?

No. First, both particles and anti-particles have "normal" mass (should they have mass in the first place) and "normal" (positive) energy. The distinction between them is either a matter of convention or a question of which type is more common in the universe. Furthermore, for typical-massed blacked holes, the bulk of Hawking radiation would be made of photons, which properly speaking do not even have anti-particles, though one could also say that they are their own anti-particle.

Shouldn't both particle and anti-particle have equal chance to be the one to fall in, or just manage to escape?

Yes, and uncharged ones do. A smaller black hole would radiate both neutrinos and anti-neutrinos, assuming all neutrinos are massive (otherwise, all black holes would do it already), and a sufficiently small (and thus sufficiently hot) one would radiate both electrons and positrons. Very roughly, a black hole will radiate non-negligible amounts of massive particles when the temperature of the black hole is on the order of the particle mass or greater, in natural units.

It seems that there should be an equal chance of either the particle, or the anti-particle, would be captured while the other "ejected."

Correct, with a minor exception that if a hot black hole has electric charge, it is more likely to radiate particles of the same sign of charge.

So it seems that the black hole should be somewhat steady-state as far as mass change with respect to virtual particles.

If either a particle or an anti-particle falls into a black hole, its mass will go up. It doesn't matter. Fundamentally, the "reason" for Hawking radiation is that the vacuum state in quantum field theory is a state of lowest energy, but different observers can disagree about which state is the vacuum. Thus, since particles are fluctuation on top of the vacuum, they can disagree about whether or not there are particles.

I don't think there is a good way to repair the "antiparticle falls in" story except some roundabout appeal to energy conservation: if the escaping particle is real and have positive energy, the one that fell in must have negative energy, and would therefore decrease the mass of the black hole. Unfortunately, that only shows what must happen for the situation to be consistent, not that it does actually happen.

Although with some knowledge of general relativity, one can motivate this slightly further--e.g., for the Schwarzschild black hole, there is energy conservation given by a Killing vector field, which goes from timelike to spacelike at the horizon--so what an external observer considers time/energy would be space/momentum inside the black hole, and momentum is allowed to be negative.

gravity - If Mars orbited the Earth how distant would it have to be to cause the same tides?

If it were possible to replace the Moon with Mars, how distant would it have to be to essentially create the same oceanic tides as the Moon currently does? Mars seems to be roughly 3 times the mass of the Moon, so does that mean it should be 300% the distance?

Furthermore, would the increased distance cause any issues with Earth's orbit around the Sun or potentially cause some sort of situation like Charon and Pluto where the centre of mass is between them?

gravity - Maximum Amplitude of a Lissajous Orbiting Object in a L4 or L5 Position

I stumbled on The Lagrangian points during some wikipedia reading. After looking at the gravity contours, I naturally come to the conclusion that the L4 & L5 should have a wave pattern and then found the Lissajous orbit page. It states:

Orbits about Lagrangian points L4 and L5 are dynamically stable in theory so long as the ratio of the masses of the two main objects is greater than about 25, meaning the natural dynamics keep the [third object] in the vicinity of the Lagrangian point even when slightly perturbed from equilibrium.

After reading it I started to wonder if there is a maximum possible amplitude (height of the peaks and troughs relative to the orbital plane of the second object) of the pattern?

Also, if there theoretically is none, for cases where it is extremely large, say larger than the 2 times the radius of the second orbiting object, what would the respective object weight ratios have to be to keep such a perturbed orbit stable for any realistic period of time?

FYI, I'm a computer scientist who loves reading about physical cosmology but can be kinda a noob sometimes. Please forgive me if I'm asking for the wrong parameters.

Edit: Here is an animation of 2010 TK7, Earth's first trojan asteroid, showing the wave pattern I'm referring to. Recall that my question is referring to the height of the peaks and troughs relative to Earth's orbital plane. Since the video is a top-down view, the peaks and troughs are going into and out of the screen.

space - Is the expansion accelerating or Decelerating?

The light we see now is not a direct indication of how the galaxy was moving at the time the light was emitted. In the cosmological frame, the galaxies aren't moving (on average, at least); rather, space between them is expanding. The rate at which it is expanding is the Hubble parameter.

If cosmic expansion is homogeneous and isotropic, then all distances on the cosmological scale should be affected in the same manner, proportionally, unless other forces are involved. Thus the distance between some galaxy and us is affected in the same proportion as the wavelength of the light:
$$frac{D_text{now}}{D_text{then}} = frac{lambda_text{now}}{lambda_text{then}}text{.}$$
The light we see now is a direct indication of the scale factor, i.e. by what factor the distance has grown since the time of emission. We can also consider distance $D$ as a function of cosmological time $t$ and define a recession velocity as the rate at which it is changing:
$$v_text{r} equiv frac{mathrm{d}D}{mathrm{d}t} = underbrace{left[{dot{D}}/{D}right]}_{H(t)}Dtext{.}$$
Therefore, the recession velocity now is given by the Hubble parameter $H(t)$ now.

neutrinos - Nuetrino interaction with plasma and electromagnetism

Following question enter link description here

After watching the Thunderbolt Project on Youtube I have a very very very fresh perspective on the universe - the electric universe. From the electric comet theory that pretty much proves water forms at the comet, and comets are not frozen water.

In the solar wind all that hydrogen missing an electron and/or free electrons, fuse/react with oxygen-silicate elements of long orbit bodies that have been long long long holding opposite --- charges +++ inside and at the outside surfaces. When the rocky body nears a sun it gains plasma discharged from the sun, it arch's like a plasma torch, and water is formed, stealing the oxygen long held in the rock formation. A comet is an asteroid that has just had a long long long time to build up a charge.

So with that and deep space showing dancing archs of electric plasma, ie space lightning, across the universe being as, or more influential as a force/energy, than gravity even, and with Einstein's admission that he was missing something in a unified theory, my question is: while neutrinos interact weekly with quote unquote solid matter, in a magnetic electrified universe are neutrinos subject to capture/interaction in Electro-magnetic currents or where voltage potential and capacitance is built up ? A?

I understand particle accelerators speed up sub atomic particles using electromagnetism but does it also translate that due to speed of atomic particles they are more likely to react with neutrinos than as a solid in out environment? B?

Space expansion in layman terms

Basically, if two particles are placed with no other interaction between them, the distance between them will increase.

Imagine living on the surface of a balloon which is being blown up. Your size stays fixed, because you're more or less rigid, but items not attached to you will move further away. Your ruler, another rigid body, stays fixed in size (though it may bend to accommodate the new curvature — this isn't so important). But two rulers (which are not attached to each other) move further away.

...also, if everything, including all our measurement devices expands at the same speed, how can we determine the fact it's expanding? :D

On the outset this seems true, however there are other forces at play here. Our measurement devices are held together by electromagnetic interactions, and the strength of these will not change. So the measurement device will hold itself together.

Imagine two faraway atoms. When space expands, the distance between the two atoms increases. However, the size¹ of the atom does not — this is determined by electrostatic equilibrium (and quantum mechanical considerations), and this remains unaffected. Even if the atom was stretched, it would rebound.

This scales up to measurement devices, so they don't get distorted either. Indeed, the expansion of space only really makes sense when you look at galaxies — these are pretty far away (when not in the same supercluster) and they don't have any interactions maintaining an equilibrium distance between them.

^{1. Whatever closest analog we have to "size" for atoms; eg the area which contains 99% of the charge density; or the nth Bohr radius.}

star - Is the Sun homogeneous?

No it does not have the same composition everywhere. In the core hydrogen is fused into helium, so the fraction of hydrogen (denoted by $X$, between 0 and 1) decreases while the fraction of helium ($Y$) increases as time goes by. There is not much exchange of matter between core and envelope so the envelope will essentially have the same constitution as when the Sun formed.

In other stars the convection zone extends into the core, and for these stars there will be more exchange of the different elements within the star.

Evolved stars (e.g. red giant stars, horizontal branch stars) often have multiple shells where different nuclear fusion processes occur. An Asymptotic branch star, for instance, has a carbon-oxygen core surrounded by a helium burning shell, surrounded by a inert helium shell, surrounded by a hydrogen burning shell, surrounded by a very large envelope consisting mostly of hydrogen.

For non-degenerate matter density depends on the pressure. The deeper you descend into the star, the higher the pressure and therefore the higher the density.

galaxy - Milky Way Formation

Great link in your answer, @LCD3. In addition, I'm going to cover some things about the continuing evolution of the Milky Way:

The Milky Way is currently merging with one or more of its satellite galaxies (I say "one or more" because several objects in its vicinity, such as the Virgo Stellar Stream, may or may not be galaxies). The major merger is with the Sagittarius Dwarf Galaxy, which undergoing a 100 million year+ merger with the Milky Way. It sill eventually be completely torn apart, and become part of the Milky Way.

There is also evidence that this has happened in the past (see this paper) and it could happen in the future. The Milky Way has a lot of satellite galaxies, and even though they are far away, closer satellite galaxies could have been consumed in the past. The eventual merger with Andromeda will also add to the Milky Way's growth, as the two become a new galaxy.

---

Other sources:

List of Milky Way satellite galaxies

Sagittarius Dwarf Spheroidal Galaxy

Note: I am looking for more non-Wikipedia sources, and I will update this answer as soon as I can.

numpy - Python troubles - Stack Overflow

I've been trying to throw together a python program that will align, crop and create an RGB image from HST and VLA .fits data. Unfortunately I've run into a bit of a problem with it continually opening a past file that does not exist in the folder and neither is it opening in the code itself. I've googled and googled and haven't found anything like it, so perhaps it's just common sense to most, but I can't figure it out. Here's the error message:

You can see at the top that the program I'm running has the filename rgbhstvla.py. I'm not sure what the error message means. Here's the python program as well:

import pyfits
import numpy as np
import pylab as py
import img_scale
from pyraf import iraf as ir

fits.open('3c68.fits', readonly)

j_img = pyfits.getdata('230UVIS.fits')
h_img = pyfits.getdata('230IR.fits')
k_img = pyfits.getdata('5GHZ.fits')

jmin,jmax = j_img.mean()+0.75*j_img.std(),j_img.mean()+5*j_img.std()
hmin,hmax = h_img.mean()+0.75*h_img.std(),h_img.mean()+5*h_img.std()
kmin,kmax = k_img.mean()+0.75*k_img.std(),k_img.mean()+5*k_img.std()

img = numpy.zeros((1024,1024,3))
img[:,:,0] = img_scale.asinh(j_img,scale_min=jmin,scale_max=jmax)
img[:,:,1] = img_scale.asinh(h_img,scale_min=hmin,scale_max=hmax)
img[:,:,2] = img_scale.asinh(k_img,scale_min=kmin,scale_max=kmax)

pylab.clf()
pylab.imshow(img)
pylab.show()

(I'm still working on the program since I'm new to python, tips here would be nice as well but they're mostly unnecessary as I'm sure I'll figure it out eventually).

solar system - Galactic Habitable Zone

The zone, you mean, in galaxies would be very unstable - huge changes in electromagnetic flux (something like explosions) because of more dynamic changes of the environment there (than in the outer rim of galaxy: black hole, jets, etc...) - so there is considered lower probability of life in the center of galaxies (or Galaxy). But in fact I think we have to few information about galaxies to seriously think about probability of life on free planets close to the center of the Galaxy (the more other galaxies).

solar system - What would happen if we stepped on the Sun?

Well first thing's first: You would disintegrate. At the temperature of the Sun, most of the molecules that make up our bodies could not even survive, that is why we would not only fry and die, we would really disintegrate (all the molecules breaking apart, leaving only loose atoms).

Let's now pretend heat doesn't exist. This is what would happen. First, we must remember that there is no solid surface to the Sun just like there is no surface to Uranus.

As soon as you reached the Sun itself, you would sink. The sun's density is less than 1 — and 1 is the density of water. So you would be sucked inside kind of. Except that there are convection currents. If we happen to be just above one, we might be kept close to the surface.

But, eventually, the eddies would entrain us sideways until we came to a "downdraft" that would take us downwards.

Anyway, let's talk about gamma rays now. Gamma rays are a form of light.

So even though heat is out of the way, all of these light rays are now trying to tear up whatever remaining molecules we have. This could trigger nuclear reactions.

Eventually, you (not me, I'm out of there) would cycle between two layers, in a convection cell moving you up and down, above and below the depth where the density of the gas is the same as the density of your body.

So I guess there's one sure thing about all of this.

You're going to need some sunscreen.

the sun - How was the core temperature of the Sun estimated?

Hydrodynamic models of the Sun allow one method of estimating its internal properties. To do this, the Mass, radius, surface temperature, and total luminosity (radiative energy emitted)/s of the Sun must be known (determined observationally). Making several assumptions, e.g., that the Sun behaves as a fluid and that local thermodynamic equilibrium applies, the stellar equations of state can be used. Numerical methods are applied to these equations to determine the internal properties of the Sun, such as its central temperature.

A great example for how to work this problem your self can be found in the undergraduate text, 'An Introduction to Modern Astrophysics' by Carroll and Ostlie (Section 10.5). The FORTRAN code to run your own stellar model is included in Appendix H.

A comprehensive review paper on how stars of different masses evolve internally (e.g., with respect to T, P, etc.) that is worth reading is:
http://adsabs.harvard.edu/abs/1967ARA%26A...5..571I

A very interesting historical overview of the development of the Standard Solar Model:
http://arxiv.org/abs/astro-ph/0209080

This (admittedly dry) paper gives you a good idea of how well the 'standard' solar models estimate the internal properties of the Sun using helioseismology and neutrino measurements to help tie down their boundary conditions:
http://adsabs.harvard.edu/abs/1997PhRvL..78..171B
The answer is that they match incredibly well (>0.2% error)

These were the least technical (but still academically published) references I could find.

Here is a comprehensive page on the state-of-the-art in solar modelling and measuring the internal Sun using Helioseismology:
http://www.sns.ias.edu/~jnb/Papers/Preprints/solarmodels.html
(highly technical)

spectroscopy - How are molecules detected in space?

This is a broad question. The energy levels occupied by molecules consist of a mixture of rotational and vibrational energy states. The values of the energies of these states are characteristic of particular molecules - they can be measured in the laboratory or occasionally need to be calculated if the molecules can only be formed in conditions found in space. A molecule may make a transition from a higher to a lower energy level. When it does so, the energy difference is emitted as a photon with a frequency corresponding to the energy level difference: $nu = (E_2 - E_1)/h$.

Generally speaking, the differences in energy between these quantum states is smaller than of electronic transitions in atoms. This means that the photons emitted or absorbed, corresponding to these transitions are in the infrared, microwave or even radio part of the spectrum. What one measures is intensity as a function of frequency. If one is looking at a warm gas, then a pattern of emission lines will be seen at characteristic frequencies depending on what molecules are present.

In the case you mention, the observations were conducted with the Atacama Large Millimetre Array - ALMA, a microwave telescope. The molecules exist in a warm gas (a few hundred Kelvin) which excites certain transitions in the molecules and the emission of mm-wave (100s of GHz) radiation. This is then received by the ALMA instruments.

If you are looking for a treatise on mm-wave astronomical techniques, this is not the place. ALMA is able to detect microwaves with a high angular resolution, but also able to separate the microwaves out into narrow frequency windows of tens of kHz (i.e. it has good spectral resolution too).

the moon - Identification of Lunar mare (maria) from a taken picture

So your image looks more like this, with the red line being the terminator. The illuminated side in your image is the left side of the reference image, turned upside down. I think you've gotten it backwards, the mare that you've labeled "1" is the Sea of Tranquility, and the one you've labeled "2" is the Sea of Serenity.

Expansion of the Universe - Astronomy

The current explanation is that "space" is continuously created in the intergalactic void, "pushing" the galaxies apart. From what i understood ( i am also ignorant ), the current rate at which "space" is created is expressed by the Hubble's constant 67.15 ± 1.2 km/s/Mpc ): for every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometers per second, which means that the distance between 2 points that are 1 megaparsec apart grows with 67km every second; between 2 points that are 2 megaparsecs apart with 134km/s, and so on so the relative speed between 2 points that are 5000 Mpc apart would be 335000 km/s which is more than the speed of light. In fact, two objects at this distance would not move, but 335000km worth of "extra space" would be "created" between them every second.

However in the last week there was an article depicting the shape of the universe, and it is hard for me to understand how it got to look like this : http://ifreepress.com/wp-content/uploads/2014/09/laniakea.png

http://en.wikipedia.org/wiki/Laniakea_Supercluster

distances - How many light years away is Earth from the closest outer edge of the black hole at the center of the Milky Way?

As @Py-ser already said, there is the very clear information about that, although his link correctly is here.

As we can read there, the distance of this is around 25000 light years, with a precision of 1400 light years.

Normally, the size of most galaxies is around some ten thousands of light years. Compared to the black holes, their size (the diameter of their event horizon) is some kilometers (stellar black holes) or some astronomical units (some millions - hundred millions of kilometers).

Even the size of the event horizon of the greatest known black hole is around the size of the solar system, which is around some thousandth of light years.

You don't need to ask, "how far we are from the nearest edge of Sagittarius A*", you can simply ask "how far we are from Sagittarius A".