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.