Are the viewable nebulae only in our galaxy? Where are all the nebulae located?

Yes, a simple question that should be obvious but apparently is not to me.

To be more explicit, how far are all these objects? Are they mostly in our galaxy?

I'm not suggesting that the Milky Way galaxy is the only galaxy that has nebulae, but that the ones we have pictures of are only within the Milky Way...if that is indeed the case.

This may also apply to stars themselves. What is the radius in ly of discernibility of (most) stars? Is it beyond the distance to the center of our galaxy (where even in principle individual stars might be discernible but there's too much of a mess to differentiate)?

1) The nebulae are spread out over the universe in many different galaxies. Nebulae called extra-galactic nebulae are nebulae outside of our galaxy. They aren't common, but we can see them.

2) Most of the ones we can see, such as the Helix nebula or the Orion nebula, are in our galaxy, but the ones outside of our galaxy are just more harder to see - there is a same dispersion of nebulae in our galaxy than in other galaxies.

3) Credit to @AtmosphericPrisonEscape (thanks!) - many nebulae outside of our galaxy are too dim for us to see. I assume that since the Tarantula Nebula is so massive and prominent, it is able to be visible from such a far distance.

For instance, NGC 2070, or the Tarantula nebula, located in the LMC. (Link: Here). The LMC is rather close to us, and the nebula is around 160,000 light years away.

I really can't find other major nebulae that are outside of our galaxy, but there are a lot of research papers on them, so they are visible - just not as prominent as the ones in our galaxy.

Some research papers with extra galactic nebulae include: Here, Here, Here, and there are many more online.

I don't understand the 2nd part of your question with the coordinates, but I hope this at least helps!


Could the planets in the Solar System have been captured? Ignoring any current beliefs, is it possible that the planets could have been captured by the Sun's gravitational force after drifting through space while retaining their axis of spin and speed?

Notably, Venus uniquely has retrograde rotation; how would this have come about based on current theories?

I'll address the second question first, Venus uniquely has retrograde rotation; how would this have come about based on current theories? The answer is that there are a number of hypotheses related to Venus's rotation. See the related question What is the current accepted theory as to why Venus has a slow retrograde rotation?. None of them involve capture of a rogue planet. Note that this also applies to Uranus; it too has an odd orientation.

So why not? Note that I am now addressing your first question, is it possible that the planets could have been captured by the Sun's gravitational force after drifting through space while retaining their axis of spin and speed?

The answer is simple. Capture is extremely unlikely (and that's putting it mildly), and it is not needed to explain the odd orientations of Venus and Uranus.


What are the "lines" in a constellation or asterism called? Is there a technical term for the "lines" in a constellation or asterism? Alternatively, is there an astronomy related coloquialism or any informal term that refers specifically to these lines, and not to the constellation itself?

To illustrate:

In the two examples below, what is the (most?) correct way to describe all of the green lines in the two images.

Argo Navis, an asterism and former constellation.

Source: Wikipedia
The constellation of Draco.

Source: Wikipedia

Further Background/Details:

I was doing some astronomy related browsing and noticed the some of the representations of the same constellation had differences between different sources. The majority were very similar, others had more significant differences.

Version 1

Version 2

Version 3

Version 1 has significantly more lines included in it than Versions 2 and 3.
Version 1 has the line between η Cygni and β Cygni, appears to go via φ Cygni.
Version 3 has the line between η Cygni and β Cygni, goes via a star to the upper right of χ Cygni on the charts.

I want to search for more examples of the different ways the lines in the constellations have been drawn, and all my google fu has failed to produce any useful search terms. Consequently I'm hoping to find there is a term that describes the lines themselves which should be more helpful at finding  material focused on the ways these lines have been drawn.

My understanding is that the stick-figure lines (in terms of applying them to every constellation) are a fairly modern invention - before that the individual stars were not (generally speaking) linked together directly but were instead highlighting important areas of each constellation figure. The constellation figures were fully 2D and potentially quite detailed, and there was not always a one to one correspondence between stars and details. In this sense one is free to modify the existing stick figure patterns to suit one's needs. Many of the stars visible to the naked eye have individual names (sometimes more than one name in fact) and these names usually mean/meant something about the constellation figure they are/were in. It's not always possible to follow these names when redrawing the figures (the constellations we use today are not all ancient and star names remain from figures we no longer use), but usually you can accommodate most of them. This is one of my main problems with H.A.Rey's versions - they often pay no attention at all to what the star names mean and you end up with a star named for a tail now marking the figure's mouth. :)


What was "space" like before big bang? I have a simple question which I think about often but have no answer.

If The Big Bang is true, than if the whole space was just a point 14 billion years ago, then what was around that point, some other kind of "space" or nothing? In latter case how do we define nothing.

This is a common question that comes up - one I've (at least tried to) answer a few times.

Let's try to understand what the Big Bang is. The Big Bang Theory is a model, and scientists came up with it when they observed the universe expanding - and thought that since the universe is now expanding it must have been smaller in the past. Running the clock backwards eventually means all of the universe was squeezed into a "singularity". Our physical models of the universe break down at this point - so we are not too sure of what the universe was like at the Big Bang.

Notice that I said the universe was squeezed into a singularity - not the stuff inside the universe. This means space-time was also compressed into a single point. The universe is not expanding out into empty space - it is space-time itself that is expanding. Hence, there was no "before" the Big Bang - time (as well as space) was created when the Big Bang happened. There is no outside the universe! This means there is no outside the Big Bang!

You can't go back in time and watch the Big Bang - since there is no universe or space (or time) outside the singularity. This can be very confusing, but I hope this helps!

For a few places to look for more info:

https://en.wikipedia.org/wiki/Planck_epoch

https://en.wikipedia.org/wiki/Big_Bang#Singularity


How to identify component of velocity of a star from its red shift? When a telescope observes a distant star, all it sees is the light being emitted by the star. Observing the wavelength of this light, we determine the velocity of the star. If the wavelength is red-shifted (longer) then the star is moving away from us, and if the wavelength is blue-shifted (shorter) then the star is moving towards us.

But how do we know that the wavelength that we see is not the red shifted wavelength but the original wavelength emitted by the star? Do we already know what wavelength of light the star is supposed to emit (if yes, how?)

Also, using this method we should be able to identify only the radial component of velocity. Because the tangential component of velocity of the star would have no effect on the wavelength that we observe? So how do we determine the actual velocity (including the proper direction) of the star?

The first part of this question is already answered at How Do we know about redshift?

Regarding the second part, measuring tangential velocity is much harder, and indeed impossible for stars that are far away. Basically the method is to observe the position of the star very accurately compared to very distant stars which are assumed not to be moving and then observe it again a few years later. After adjusting for known effects like aberration and parallax, what remains is "proper motion".

The most sensitive current telescope for this kind of measurement is the Gaia mission, which is able to measure the movemnt of stars up to a few thousand light years away with reasonable accuracy.


Why do we define the Milky Way's rotation in a left-hand coordinate system? When we talk about Milky Way kinematics, we define it as rotating clockwise from top.  In the rest of physics, rotating objects are defined as rotating counterclockwise from top (following the right-hand rule).  Why do we do the opposite when talking about the Milky Way?

The problem, as far as I can tell, is that the North and South Galactic Poles were defined well before we had any idea of which way the Galaxy rotated. It turned out that the Galaxy's angular momentum vector (in the standard right-hand sense) actually points in the same direction as the previously defined South Galactic Pole. Since there's a separate convention for having "north" point "up", you end up with the backwards rotation definition.

William Herschel seems to have started this back in the late 18th Century; I assume his idea was that whichever Galactic pole was in the North Celestial Hemisphere should be called the "northern" pole. (No one back then had any clear sense that the Galaxy necessarily rotated at all.) By the time the actual direction of Galactic rotation was figured out in the 20th Century, the traditional orientation was too well entrenched to be worth upending.


Is this a real picture of the moon? I found this picture and I am really wondering if it is possible for the moon to get as close to the earth as this picture shows?

( Super moon above Rio De Janeiro)

I don't know whether this  picture is a fake, but you could get this sort of image without cheating, if it were taken a long way away from Rio, and the magnification cranked up.  In other words, get far enough away from Rio that its angular diameter is about 1 minute, and take the picture from there.


Stuck with SDSS data I want to do project on astronomical dataset. Regarding that project, I try SDSS who give public dataset but it is quite complicated for me. I gone through public dataset and downloaded some .FITS file one is ~3.2mb and other one is ~3 GB but I don't know how to deal with this file and generate images or other files from .FITS files.

Please help me, this is my Master's project in computer science.

You should go to the site https://fits.gsfc.nasa.gov/ where you can read about the FITS data format. This site also has utilities for examining and viewing the data. Most of your concerns about this data should be answered by the documentation on this web site.


How bright was Scholz when is passed near the Sun 70,000 years ago? I just read that a star called Scholz passed only 0.8 years-light away from us 70,000 years ago.  It is a red dwarf. I don´t know how bright it was in the night sky. Does someone know how bright it was?

I just found the answer to my question at this place:
http://astronomynow.com/2015/02/18/suns-close-encounter-with-scholzsstar/

Here is the answer:

  Currently, Scholz’s Star is a small, dim red dwarf in the
  constellation of Monoceros, about 20 light-years away. However, at the
  closest point in its flyby of the solar system, Scholz’s Star would
  have been a 10th magnitude star — about 50 times fainter than can
  normally be seen with the naked eye at night. It is magnetically
  active, however, which can cause stars to “flare” and briefly become
  thousands of times brighter. So it is possible that Scholz’s Star may
  have been visible to the naked eye by our ancestors 70,000 years ago
  for minutes or hours at a time during rare flaring events. The star is
  part of a binary star system: a low-mass red dwarf star (with mass
  about 8% that of the Sun) and a “brown dwarf” companion (with mass
  about 6% that of the Sun). Brown dwarfs are considered “failed stars;”
  their masses are too low to fuse hydrogen in their cores like a
  “star,” but they are still much more massive than gas giant planets
  like Jupiter.


How many supernovae events are on their way from within the Milky Way? The Milky Way has a diameter of around 100.000 light years with the Solar system placed some 25,000 light years from the center.
If we assume a supernova rate of 1 per century, and also assume that supernovae are distributed somewhat uniformly in our galaxy, then it seems to me that light and neutrinos from hundreds maybe even thousands of these events are propagating through the Milky Way towards us at this very moment.
Can this be estimated more accurately? Does the supernovae distribution even affect this estimate, given the age of the galaxy?

The supernova rate is only estimated/known to a factor of 2-3, so high levels of accuracy are not possible or warranted.
If we assume high mass stars are born uniformly in a disc of radius  $r$  light years, then you can work out a "surface density rate" of supernovae of $1/100 \pi r^2$ per square light year, per year (adopting your rate of one per century over the Galaxy).
Then, consider a thin annulus of radius $x$ and width $dx$ around the Earth. The number of supernovae that explode per year will be
$$dN = \frac{2\pi x}{100 \pi r^2} dx$$
and the number that have exploded in the last $x$ years, who's light is yet to reach is just this multiplied by another factor of $x$.
If the Sun was at the centre of the Galaxy the calculation is then simple
$$N = \frac{1}{50r^2}\int_{0}^{r} x^2\ dx = \frac{r}{150},$$
with $r$ measured in light years. If $r\sim 30000$ light years (I think 50,000 is a bit big), then the number is 200.
Unfortunately, that isn't the geometry. Instead of being a circular annulus around the Sun, you have to work with an annulus that is truncated where it reaches the "edge" of the Galactic disc. I may add to this answer later, but I doubt this will change the number above by very much.


an exercise from Spherical Astronomy by Smart I want to solve this exercise from Spherical Astronomy by Smart:

Prove that, in latitude 45°, the interval between the moment at which a star's azimuth is 90° east and the moment of setting is constant.

I can't understand it, it says prove that it is constant for a star, or it is constant for every star with different declinations?

Thanks.

Constant for every star with different declination's.

Though you would have to assume the the declination is such that it can have an azimuth of 90 each, not all stars will. Depending on your latitude some stars will never get to an azimuth of 90, they will either be always be further north or south.


Why don't storms on gas giants move to the poles, like hurricanes on Earth do? On Earth, hurricanes usually move from the equator to the poles. But if you look at a picture of Jupiter, you'd see that the Great Red Spot isn't anywhere near the poles. So the question is: why don't storms on gas giants move to the poles like storms on Earth do?

The answer is the Coriolis effect, on Earth this produces cells within which storms move, converging towards the cell boundaries as you can see below.

Jupiter however spins much faster that the Earth which produces a stronger Coriolis effect and thus more cells. This is the reason why there are so many different coloured bands on Jupiter (see image below). So storms like the big red spot don't have as far to move laterally within these cells compared to hurricanes on Earth (in comparison with the planets size).


How exactly does inflation convert random gravity fluctuations into coherent gravitational waves? In the course of this very enjoyable press announcement, it is mentioned that inflation can create gravity waves by amplifying gravity fluctuations.

I do not properly understand this statement. I always thought that quantum fluctuations (fluctuation of the metric in this case?) should occur pretty randomly whereas waves are rather coherent motions.

So I understand somehow that inflation can amplify quantum fluctuations, but I don't see how it can convert these random "microscopic" processes into "macroscopically" observable coherent phenomena called gravitational waves.
For example, how are the sources of these gravitational waves distributed? Does each point in space-time behave as some kind of a point source? And what is observed at a specific point as the superposition of all these excitations?

In addition, in the same video it is said that not all inflation models produce gravitational waves, the ones that have them are favoured by the data now, etc.

When inflation occurs the stretching of the gravitational field is dependent on the perturbations within that field originally. These perturbations are caused by quantum fluctuations. This leads to fluctuations in the magnitude of the inflation itself at a given point in space, causing gravitational waves due to the gravitational potential between neighboring points. The waves are fed by the inflationary energy, but are seeded by these quantum fluctuations.

This site gives a good explanation, as well as this particular quote:

  Gravitational-wave perturbations are different. They are not modulated
  by some unknown potential; they are produced by inflation, and we
  observe them directly. In straightforward models of inflation, the
  amplitude of the gravitational waves is directly proportional to the
  inflationary energy scale.


How to manually locate a certain declination in the sky for a given location? (no sextant available) Consider this example. I am standing in a flat surface, at latitude X, looking to the equator, in a random day of the year. (For simplicity, let's assume X > 23.5º)
I want to find the position of the sun at noon on the winter solstice (that is, the highest point in the sky of the sun, the day which this reaches the lowest altitude), location which has a declination of 23.5º.
We know the sextant does exactly that. But say I don't have a sextant. I want to find an alternative method to do the same.
If my latitude is X, then such position of the sun is X-23.5º above the horizon (looking North). I could then divide this result by 90, and get a proportion of the sky between the horizon and my zenit where that point is. But this is too rough.
Perhaps one way would be to have a long stick and a measuring tape. For a given value of X-23.5º there must be some relationship between my eye, the vertical height of the stick and my horizontal distance to the stick which allows me to find exactly the position in the sky.
Or perhaps another method?

Jacob's staff is probably all you need.  It is a pole with a cross arm. By positioning the arm at a certain point on the pole and sighting down the pole one can get a position on the sky.  A skilled user can achieve sub-degree accuracy with such a tool.

The position of the cross beam is easy enough to calculate by trigonometry, or by careful construction of triangles.
$\tan(\theta/2)= c/2l$
Where $\theta$ is the angle above the horizon, $l$ is the position of the crossbar from the eye and $c$ is the length of the crossbar.


What causes objects to become tidally locked? I'm trying to write a gravity simulation (suns planets etc), and was hoping tidal locking could be one feature demonstrated.

Using a simple equation for gravity has produced some interesting results, but (unless its emergent behaviour) I see nothing that would encourage tidal locking. But, after some reading it appears tidal locking is quite common, planets and their satellites, planets and suns, suns and other suns (binary stars).

Is it a result of the formation stage of these objects, or is it somehow a function of the equation of gravity?

Tidal locking occurs because the planet deforms the satellite into an oval, with long axis pointing towards the planet. If the satellite is rotating the long axis will move away from being pointing towards the planet, and the gravity of the planet will tend to pull it back, slowing the rotation until one face is permanently facing the planet. Tidal locking isn't a result of the formation processes, but a consequence of satellites not being perfectly rigid.

In order to model the effects of tides on the orbits and rotation periods of satellites you need to know several important pieces of information.

First you obviously need to know the size of the planet and the satellite (both in terms of mass and radius) the shape of the orbit and the rotation rate of both planet and satellite. For many objects, these values are well known.

Next, and this is the tricky bit, you need to know how the satellite and planet will be deformed by the other's gravity, and how much tidal heating will occur. These are the so-called "love number" (after Augustus Love) and the dissipation function, Q.

It is hard to estimate these. For the Earth Moon system the ratio k/Q is known to be 0.0011. (but the Earth is a poor model for other planets, which don't have a substantial ocean, or a liquid core)

For other planets the value of Q varies between 10 and 10000, with larger values for the gas giants, and k can be estimated from the rigidity of the bodies.

A simple gravitation model is not able to capture the subtleties of the gravitational interaction between two mutually deforming bodies, indeed for most simulations, the planets are modelled as points, or at most as spheres, and this is good enough for all but the highest precision calculations.

Tidal locking takes a long time (by human standards) but a relatively short time compared with the age of the solar system. The time taken is very strongly dependent (order 6) on the radius of the orbit.

Direct simulation would be more or less impossible: the deformations are too small, and the time scale of locking is too large. It would be possible (though difficult) to model tidal locking in a simulation with unrealistic values for the rigidity of the satellite, and the size of the planet (think jelly world, orbiting a (Newtonian) black hole) so the deformation is greater and the locking time shorter. However modelling the  elastic deformation of a body under gravity is far from trivial.


Will the Sun rotate slower in red giant form? When the Sun runs out of hydrogen to fuse it will grow bigger in size and my question is does it rotate more slowly, like a spinning ice skater extending their arms to reduce angular velocity? What consequences will accompany with a slower dynamo (big ball of plasmas), such as more frequent solar flares or perhaps more dark spots?

Indeed conservation of angular momentum dictates that in a single star like the the Sun, rotation should be much slower when it becomes a red giant. This is because at the present time the Sun does not rotate at vastly different rates with depth, thus when it expands, the moment of inertia increases drastically and convection in the outer envelope will ensure that slow rotation is enforced throughout most of the star.

The consequence for magnetic activity will be that the Sun will become magnetically inactive (in relative terms - magnetic activity will not be absent, but its signatures will be much reduced when expressed as a fraction of the stellar luminosity) as a red giant, because dynamo-generated magnetic activity is strongly correlated with rotation rate.

In general it is found that giant stars are magnetically inactive and slowly rotating, but there are exceptions. If a star considerably more massive ($\sim 3-5M_{\odot}$) than the Sun evolves to become a giant it may have preserved a considerable amount of angular momentum, since such stars are not magnetically active on the main sequence and are unable to lose their angular momentum through a magnetised wind in the same way that a solar-type star does. Secondly, it is possible that some rapidly rotating giants (e.g. the FK Com stars) could be the result of mergers in binary systems or possibly even giant planet engulfment. In which case the angular momentum is from the binary system and these stars can be very magnetically active. Finally it is also found that giants in close binary systems (the RS CVn stars) can be fast-rotating and magnetically active because tidal locking in the binary system enforces rapid rotation. The FK Com and RS CVn stars manifest many signs of extreme magnetic activity - hot X-ray coronae, chromospheres, coverage of large fractions of the surface by cool starspots. We thus deduce that the difference between these and the vast majority of magnetically-inactive giants is their rapid rotation.


What effects do other planets have on the solar system? For instance, Jupiter is the biggest planet in our solar system. Due to it's gravitational mass it can deflect meteorites and protect Earth from the Oort cloud.  While these are positives, the negative effects are that Jupiter can draw comets in from the Kuiper belt which may have catastrophic consequences.

Do other planets have a similar effect on the solar system?

The gravitational pull of all the planets and the sun, and the rest of the galaxy and the universe, all play a part, but gravitational effects fall off with distance.

For Earth's orbit, the Sun is far and away the single biggest influence. Jupiter perturbs our orbit slightly, but with it or without it we have a simple elliptical orbit round the sun. We can measure the perturbation in our orbit from all the planets, but the effects are minor.

When it comes to gravitational effects on small bodies passing one of the planets, however, the effects can be dramatic - an asteroid passing close to Jupiter will be moved far from its original path. The same would be true when passing close to Saturn or in fact any planet - just to a lesser degree.

So while you can't say that Jupiter protects Earth from anything, especially not the Oort Cloud, you also can't say that it draws objects in. Anything out in the Kuiper belt or Oort cloud will be orbiting the barycentre of the solar system (which is within the Sun) and will keep doing that unless knocked out by something  like a collision with another object, or the pull of other Kuiper belt objects like Pluto, which can give a much greater tug.


How might WFIRST help us better understand dark energy? In the video linked in the Space.com article What Would It Mean for Astronomers If the WFIRST Space Telescope Is Killed?, (available in YouTube as WFIRST: The Best of Both Worlds) after 02:00 Caltech cosmologist Yun Wang says:

  WFIRST will allow us to potentially make ground-breaking discoveries, finding out what dark energy is. So this will tell us if dark energy is an unknown form of energy, or if its a modification of general relativity.

Without going too deep in to GR, how might that happen?

Question: What kinds of WFIRST observations  and interpretations would be used to potentially address the nature of dark energy?

Based on comments, I'll reclassify the following as an afterthought:

Is this in any way related to news about a recent observational non-exclusion of Erik Verlinde's theories (the guy who's "Trying to kill dark matter"), discussed in Margot Brouwer's 2016 video Verlindes new theory first tested with weak gravitational lensing about the VLT Survey Telescope kilo-degree survey results (see also Cosmos Magazine's New theory of ‘dark gravity’ passes first test, but Einstein’s still on top, or am I mixing apples and oranges?

The WFIRST's study of dark energy is pretty straight forward.   It's no different than the previous ways of studying dark energy.   It's not studied directly, cause we can't see it or re-create it, it's studied by the effect it has on the relative motion of distant galaxies.

WFIRST will get much more accurate readings on the relative velocity of galaxies both relative to each other in their neighborhood and relative to us, this will get a significantly more accurate estimate for dark energy than was previously possible and questions can be answered such as, is it speeding up.

NASA's WFIRST Web page discusses this in more detail

  The High Latitude Spectroscopic Survey will measure accurate distances
  and positions of a very large number of galaxies. By measuring the
  changes in the distribution of galaxies, the evolution of dark energy
  over time can be determined. The High Latitude Survey will measure the
  growth of large structure of the universe, testing theory of
  Einstein's General Relativity.

  Type Ia Supernovae (SNe) Survey uses type Ia SNe as "standard candles"
  to measure absolute distances. Patches of the sky are monitored to
  discover new supernovae and measure their light curves and spectra.
  Measuring the distance to and redshift of the SNe provides another
  means of measuring the evolution of dark energy over time, providing a
  cross-check with the high latitude surveys.

  High Latitude Imaging Survey will measure the shapes and distances of
  a very large number of galaxies and galaxy clusters. The shapes of
  very distant galaxies are distorted by the bending of light as it
  passes more nearby mass concentrations. These distortions are measured
  and used to infer the three-dimensional mass distribution in the
  Universe. This survey will determine both the evolution of dark energy
  over time as well as provide another independent measurement of the
  growth of large structure of the universe.

Dr. Yun Wang's comments appear to be written for a Science-TV show and/or youtube video, and video like those are designed for a general audience so I wouldn't analyze that particular sentences meaning too closely.

As for Erik Verlinde's . . . ideas.   I'm not a fan of his proposals, but the answer is mostly the same.   WFIRST will get more accurate readings and better readings should help get more specific answers.


Remains of the progenitor of the Sun It has been argued that it is not possible to trace back the remains of the progenitor star of the Sun (sometimes called Coatlicue), which, being estimated in the mass range of 30 $M_{\odot}$, should have left a black hole after its explosion as a supernova, nor is it possible to find the sister stars formed from the compression of the same nebula:

https://astronomy.stackexchange.com/a/10729

https://phys.org/news/2012-08-solar-genealogy-revealed-meteorites.html

because "[The Sun] has completed many Galactic orbits [since its formation]. Not only that, but it could have migrated in orbital radius as well." and "These twin stars did not influence its development nor that of its planets [and] are now dispersed in the Galaxy and cannot be identified".

Why did the supernova's remains and the sister stars not remain gravitationally bound together? Why did they not share the same angular momentum of revolution around the Galaxy?

Why did these massive bodies formed from the same nebula not become a bound system among them and with the supernova's remains, like the Sun and its planets in the Solar system?

Most stars are born in groups or clusters of anywhere between 100 and millions of stars. Most of these groups and clusters do not survive more than about 10 million years or so and the fraction that survive decreases further with time.

There are two basic reasons. First, it seems that most young clusters and associations may be born in an unbound state. Second, those that are gravitationally bound can become unbound through the expulsion of gas or through tidal interactions with the Galaxy, spiral arms and giant molecular fields. A supernova is one event that might be a culpable agent in expelling mass from a young cluster and consequently unbinding it.

It is therefore highly unlikely that a 4.5 billion year old star will be found in a cluster. There are very few examples of clusters at this age or older.

As an aside, the theory that the Sun was born from the debris of just one or even a few stars is totally false and easily disproved by looking at the isotopic composition of dust grains formed in other stars before the solar system was born. These can be found inside meteorites and demonstrate that the solar system contains debris from many, many stars; of both high and relatively low masses.

See How can there be 1,000 stellar ancestors before our Sun? and Parent stars of our Sun - Where are its remains?

Edit: It seems that you are just asking why we can't see siblings of the Sun, including this massive star that may have been involved in the formation of the solar system.

First - it isn't clear that the Sun was born in a cluster from material seeded by a supernova from another star in that cluster. There is a timing problem. The massive star that exploded would have to form more than 10 million years before the Sun. Clusters we see today do not seem to have such an extended star formation history.

Second - even if that weren't the case, a supernova very frequently gives rise to a "kick". The velocity dispersion of pulsars (recent supernova remnants) is exceedingly large (100s of km/s). Many remnants would escape the Galaxy and there is no chance of predicting where they would be in billions of years.

Thirdly - even if the first and second problems were not applicable, as I mentioned above, even bound clusters do disperse. Once free of any binding potential stars can undergo a process called "disc heating" - the action of encounters with spiral density waves and giant molecular clouds gradually increases their velocity dispersions (changing their kinetic energies and exerting torques that can change their angular momentum wrt the Galactic centre). This allows the solar siblings to go their separate ways. A Galactic orbit at the solar radius is more than 200 million years. A velocity difference of just 1 km/s (typical in a cluster even before it unbinds or is heated) could result in a separation by 200 pc in that time. The Sun has executed more than 20 such orbits.


How would the solar system look in a Geocentric model? If I fix the Earth as the center of the solar system, what would this look like? Are there any good images? How complex is this system?

Only for Sun, Earth and Mars, but here's an interactive Flash that illustrates it graphically. Third link from the top "Part I, Equivalence of Hypotheses (Flash)" Note that the planets don't move as you move the mouse over the image, only the circles (the theory) change.
http://science.larouchepac.com/kepler/newastronomy/part1/MegaEquivalence.swf for the animation. More generally: http://science.larouchepac.com/kepler/newastronomy/
(And I don't know or care who that LaRouge Pacman is, some rich guy for some reason financed good explanations of what Kepler did, that's what I know about it and that's all very good)

If you move the mouse pointer to the upper left you get the heliocentric system. To the upper right you get the geocentric system. The point is to illustrate how Kepler proved that Ptolemy, Copernicus and Tycho all had the same mathematical model for the solar system. They just used different frames of reference to it, just choosing different fixpoints from where to view it.

Some dots in the illustration represent important points in that ancient model. Some of them are stand-ins for the foci of an ellipse in a model made out of perfect circles. And the Sun was actually not the center of the Copernican model, a point near the Sun was. According to Copernicus the Sun was just a decoration which randomly happened to hang around near the center. Kepler was the first to come up with the thought that maybe there is a relationship between the geometry and the physical existence of stuff. An idea later developed to the theory of gravity by Newton and Einstein, who just added to Kepler's initial formula for describing the movements of the planets.


What is the average time between a nebula formation and a neutron star? I have a star project, and I couldn't find anything on the amount of time it takes from nebula to neutron star.

How long does it take, from a nebula, to form a neutron star?

Short answer: It depends on the progenitor mass, but somewhere between 5 million and 30 million years.

I presume you are talking about the sum of the star formation timescale and the time it takes for the star to evolve through the various phases of its life and then explode as a supernova.

As LaserYeti has pointed out, the formation timescale is extremely short compared to the rest of a massive star's life. In fact, for the massive stars that will eventually yield supernovae, there really isn't a pre-main sequence phase at all.

The lifetime of the star then almost entirely depends on its mass, and to a lesser extent on its rotation and metallicity. The types of star that produce supernovae and neutron stars are probably in the range 9 to 40 solar-masses. Less massive progenitors produce white dwarfs, more massive progenitors probably produce black holes and there may not even be a supernova. Stars at 9 solar masses are about 30 times more commonly born than 40 solar-metallicity stars. To find their lifetimes we must appeal to theoretical models.

Schaller et al. (1992) provide a well-respected set of "vanilla" calculations for non-rotating, solar-metallicity stars. Their table 45, summing up the H-burning and He-burning stages (subsequent evolution happens comparatively instantaneously), suggests the evolutionary lifetimes range from about 30 million years for 9 solar-mass star, to as short as 5 million years for a 40 solar-mass star. These would be the numbers I would use, though rapid rotation can increase these lifetimes by perhaps 30%. Metallicity is a smaller effect.


Do galaxies change shape and size over time? Galaxies come in different shapes and sizes. What factors determine the shape and size of a galaxy, and how can these change over time?

The size of a galaxy is dependent on how the matter was distributed in the early universe, and how it collapsed under its own gravity to form clumps of (dark) matter that the galaxies would form around.

The very earliest galaxies are irregular, but as matter fell towards them, and they developed a consistent orbital direction they formed into a disc. Gravity waves in the disc then show up as spiral arms. Large galaxies that are actively forming stars tend to have this structure. Small galaxies may never reach the point of forming a disc and remain irregular.

As galaxies collide and interact, the orbits of the stars are disrupted. The result of the merger of two large spiral galaxies is often a galaxy in which the stars orbit in all directions, which looks like an elliptical galaxy. Star formation in elliptical galaxies is often very low. Elliptical galaxies are the most evolved form of galaxies.

The time scale of this evolution is slow. It takes billions of years for galaxies to evolve from one form to another.

The details of the process of galaxy formation and evolution are still uncertain. The exact nature of dark matter will be important in understanding how galaxies form. The role (if any) that black holes play in galaxy formation is uncertain.


What is the naming convention for craters on Mercury? The two largest craters on Mercury are names Caloris (meaning "hot") and Rembrandt (for the painter). So there doesn't seem to be a pattern.
By convention, craters on Mars are named for people who studied the planet (like Cassini) or wrote about it (like Bradbury).
Is there a similar naming convention for the craters at the surface of Mercury?

A quick search returns that wikipedia has an extensive list how different things are named on various celestial bodies.
For Mercury's craters it quotes "Famous deceased artists, musicians, painters, authors". As source of its wisdom it gives the IAU working group on planetary system nomenclature as authoritive source for them - the source which decides on official naming.


How was the distance from the Earth to the Sun determined How was the Earth to sun distance measured. How do we know how far from the Earth the Sun actually is.

There are a bunch of ways.  The simplest is triangulation.

Consider a transit of Mercury or Venus, when the planet gets between the Earth and the Sun. If you observe the planet's path across the face Sun from two different places, measuring the chords across the Sun's disk and the timings, using no more than trigonometry, you can get an accurate distance to the Sun.

Here's a good discussion of the method on the Physics SE

(It's worth noting that it is easy to measure the distance to, say, Mars in terms of the ratio of the distance to Mars compared with the Earth-Sun distance, than it is to measure the Earth-Sun distance in miles.  Hence the Astronomical Unit.  We accurately knew many astronomical distances in AU before we knew the length of the AU equally accurately.)


Why does the precession of the earth affect other planets? In the Wikipedia article on numerical integration of planetary motion it states:

  ...the flattening of the Earth causes precession, which causes the
  axial tilt to change, which affects the long-term movements of all
  planets.

Why would precession affect the motion of the other planets?

Why would precession affect the motion of the other planets?

First things first: That's an unreferenced portion of a wikipedia article.

That said, a perfectly spherical body acts exactly like a point mass in Newtonian mechanics. A non-spherical body does not. The Earth's equatorial bulge has a significant effect on satellites in low Earth orbit. Sun synchronous orbits would be impossible without that bulge. For example, a sun synchronous orbit about Venus with its very slow rotation rate and hence very small equatorial bulge is not possible (reference).

The Earth's equatorial bulge adds a quadrupole (inverse quartic, or $1/r^4$) term to the inverse square law relation for point masses, and this inverse quartic term depends not only on distance but also on latitude. The added term is
$$3 J_2 \frac {GM_\oplus a^2}{r^4}\left(\frac 3 2 \cos^2 \lambda - 1\right)$$
where $J_2$ is the Earth's second dynamic form and $\lambda$ is the geocentric latitude.

Since this term drops off as $1/r^4$, it becomes very small beyond the Moon's orbit. But it still exists beyond the Moon's orbit. However, I've yet to see a paper that does account for (or says we need to account for) Earth's $J_2$ effect on the other planets.


Can you explain the pattern of Hill sphere sizes of the objects of the Solar system? I found this image on calculations of Hill sphere for planets/dwarf planets of the Solar system.

 From http://en.wikipedia.org/wiki/File:Hill_sphere_of_the_planets.png

I found it interesting that variation of Hill sphere is intuitive for the first five planets, as the variation is similar to variation of mass/radius of those planets. Mercury has the smallest Hill sphere, Venus/Earth/Mars quite similar, and a giant leap from Mars to Jupiter.

But, Saturn has its Hill sphere bigger than Jupiter, even it is smaller than Jupiter. and this anomaly continues to Uranus and Neptune: They have progressively larger Hill spheres.

And Hill spheres of Pluto and Eris are quite larger than Mercury, Venus, Earth, and Mars.

This was quite surprising for me. Could someone explain why this—for the lack of a better word—anomalies are there?

Hill sphere is the region of space around a satellite where the satellite wins the gravitational tug-of-war with its primary.

If the mass of the primary object is $M$, mass of the satellite is $m$, semi-major axis of satellite is $a$, and eccentricity of the orbit of the satellite is $e$, then the radius $r$ of the Hill sphere for satellite is given by:

$$
r \approx a (1-e) \sqrt[3]{\frac{m}{3 M}}
$$

Note that this formula does not take into account the other objects in the  vicinity.

The anomaly pointed out in the question is not really an anomaly. The contributing factor for the surprise values is the semi-major axis of the planets ($a$).

Take Jupiter and Saturn for example: Saturn has only around $30\%$ of the mass of Jupiter, and if the two gas giants had the same semi-major axis, this mass reduction will make Saturn's Hill sphere around $68\%$ than that of Jupiter. But Saturn is around $84\%$ farther from Sun than Jupiter. This is just enough to make Saturn's Hill sphere slightly larger than that of Jupiter.

Thinking along the same lines, we can also explain why Uranus, Neptune, Pluto, and Eris have surprisingly large Hill spheres.


Metal lines in Lyman-alpha forest I need to find metal lines in the Lyman-alpha forest of quasar spectra, without the red side of lyman alpha emission line.
Can you guide me how to do this?

I'm a bit unsure what you mean: Your spectrum doesn't include the red side of the quasar (seems weird), your Ly$\alpha$ line doesn't have a red side (even weirder), or you're only interested in the metal lines on the blue side?
Anyway, metals on the blue side are somewhat difficult: At high redshifts, they're blended with the $\mathrm{Ly}\alpha$ forest (or erased by the Gunn-Peterson trough), while at low redshifts they're observed in the UV which is absorbed in Earth's atmosphere.
So, you really need to know where to look. But if you know the position of the quasar's Ly$\alpha$ line, then you can look e.g. for

Fe II + Fe III at 1123 Å
O VI doublet at 1032/1037 Å (the strongest one, but unfortunately very close to Ly$\beta$
Semi-forbidden C III] at 988 Å (close to Ly$\gamma$)
O III at 831 Å
Ne VIII and O IV at 772 Å

where all wavelengths are given here in the rest frame.
Except for the O VI line, they're all rather weak. But if you have many spectra, you can try to stack them so that the noise and the Ly$\alpha$ forest lines cancel out, enhancing the metal lines. This has been done e.g. by Lusso et al. (2015).


Detecting habitable planets From my understanding we detect planets by measuring dips in light intensity from the star the habitable planet is orbiting when it passes by it.
There are 2 things I don’t understand about this method:

Planets in a solar system tend to orbit their star in one disc like plane. Can we only detect planets if this plane is in-alignment with our sensors here on earth/ space. I assume if someone looked at our sun from the ‘bottom’ they would never see a planet cross it. If so this would exclude a big portion of the stars we are looking at?
Our orbit is 1 year. I assume the orbits of habitable plants can vary and having a short/ close to 1 year orbit is not a criteria for habitability. Even so we have to measure 1 dip every year or 10. How can we tell that this a planet with a regular yearly orbit or just anything passing between the star and us. So maybe a better question to ask for number 2 is what can we find out from these dips?

It isn't the only way, Aaron. See the Wikipedia Methods of detecting exoplanets article. The first method listed is radial velocity. That's where we measure the Doppler shift of the star's "wobble".

The second method is transit photography. And as you say it has a drawback. In fact there's more than one:

"This method has two major disadvantages. First, planetary transits are observable only when the planet's orbit happens to be perfectly aligned from the astronomers' vantage point. The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit (in small stars, the radius of the planet is also an important factor). About 10% of planets with small orbits have such an alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a Sun-sized star at 1 AU, the probability of a random alignment producing a transit is 0.47%. Therefore, the method cannot guarantee that any particular star is not a host to planets...

The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the Kepler mission could be as high as 40% in single-planet systems..."


Assuming proton decay is false, what's the ultimate fate of a neutron star or white dwarf? A still unproven theory that protons can decay, and have a halflife of $10^{30}$ years or so, meaning eventually all matter will dissolve because their constituent protons and therefore neutrons will decay. But this is yet to be proven. So assuming this is false, what would be the ultimate fate of a neutron star or white dwarf? What would become of it in $10^{50}$, $10^{100}$ years?

This is a classic question in physical eschatology, seeing what happens if we extrapolate current understanding of astrophysics forward. The classic papers are (Dyson 1979) and (Adams & Laughlin 1997).
Obviously, over very long timescales white dwarfs cool down, crystallize. and become "black dwarfs". This is fairly well-established from observation and modelling, although the final stages have not been studied much.
If weakly interacting dark matter can be captured, dense objects would acquire internal halos: in this case, if the dark matter is a mix of particles and antiparticles there would be some annihilation, heating the object for a long time. If it is non-annihilating in principle it could build up until the object imploded into a neutron star or black hole. This is highly dependent on the dark matter model, so this should be regarded as conjectural.
However, it is fairly well established that galaxies dissolve due to gravitational interactions over long timescales, and this will dump such objects into the central black hole before they plausibly could undergo collapse, or eject them into intergalactic space where they would no longer acquire dark matter.
The fate of intergalactic black dwarfs and neutron stars without proton decay depends on what other modes of decay and change are possible. In white dwarfs pycnonuclear fusion would continue until all fusible elements had fused. Dyson estimated the timescale until everything is iron to $10^{1500}$ years, although there are environmental effects in white dwarfs that likely speed things up. This can actually make heavier white dwarfs (above 1.2 solar masses) collapse into supernovas on a timescale of $10^{1100}$ years (Caplan 2020).
Dyson noted that over timescales of $10^{65}$ years matter behaves like a quantum fluid due to tunneling. But this doesn't change structure of remaining objects much. A more important issue may be tunneling into black hole states where a small part of the object tunnels together to form a small black hole that evaporates. Adams and Laughlin estimate timescales of $10^{45}$ years for neutron stars and $10^{336}$ years for white dwarfs to evaporate this way.
Even if this does not happen, there is an argument to be made that thermodynamic fluctuations eventually dissolve bound objects since this minimizes the Gibbs free energy $E-TS$: at a finite temperature (which is the standard assumption for accelerating expansion in $\Lambda$CDM) if there is enough space the entropy $S$ can be maximized by separating particles despite some binding energy: tunnelling will eventually dissolve everything. This is similar to the discussion about Herzfeld's paradox of the spontaneous ionization of hydrogen atoms. The mere presence of other matter "outside the lab" normally stabilizes bound systems but in the very far future isolated systems become destabilized. This assumes that (1) temperatures will stay finite (i.e. our understanding of horizon radiation and continued accelerated expansion are right), (2) there are no other limits on dissociation, (3) the Gibbs energy argument is valid in this context. All three can be debated.
Overall, the trend seems to be that entropy maximization will tend to dissolve objects into isolated particles while gravity either just keeps them together or causes implosion into black holes through some pathway, followed by evaporation.


Will stars live longer if they consume more hydrogen? I heard that stars would live longer if they consumed more hydrogen and material from their 'environment'. Is that true? For example, if the sun consumed all of the gas giants, would it live a little bit longer?

Adding mass of similar composition to the Sun would effectively turn the Sun into a higher-mass main-sequence star.  That would not increase the gravitational pressure, it would lower it, because it would cause the Sun to expand to lower density.  It would also cause the Sun to fuse hydrogen faster, to the point that it would actually reduce the lifetime of the Sun, not increase it.  All this can be seen by consulting models of higher-mass main-sequence stars.

If it seems counterintuitive that adding mass would weaken the gravity, this is because the core must self-regulate its fusion rate to match the rate that light is escaping, and simply adding mass would upset that balance unless the Sun expanded to recover it.  Note that main-sequence star models do not particularly care about the history of how the mass got there, they are simply the only equilibrium models that have the appropriate mass and composition and are undergoing core fusion.


Companion Sun the cause of comet impacts with Earth On television, I heard a scientist who believes that our Sun has a companion (brown dwarf) Sun, that is responsible for comets destroying life here on Earth every 26 million years. He proposed the theory that when this companion Sun travels through the Ort Cloud, every 26 million years, it brings billions of Comets with it into our Solar System and inevitably one crashes into the Earth. Any thoughts on this?

Ahh, I see you've met our friend, the hypothetical star known as Nemesis. I don't know which scientist(s) you're referring to, but the idea goes back quite a while. The original theory was created by Raup and Sepkoski way back in 1984. Their paper analyzed mass extinctions in the past and concluded that there was a pattern. Note, though, that the paper does not specifically say that a star orbiting the Sun is the cause; it merely favors general impacts of "extraterrestrial" origin.

However, two more teams analyzed the data and posited that there was a celestial body responsible. Whitmire and Jackson and Davis, Hut, and Muller (Regrettably, you must pay to see the papers, which I have not done) suggested that the Sun had a companion in the far reaches of the solar system, and it was perturbing comets and other bodies, which came towards Earth.

Honestly, I (and quite a few others) don't think there's much truth to the idea. Why? Because we haven't found any evidence of its existence. As Wikipedia explains,

  Searches for Nemesis in the infrared are important because cooler stars shine in infrared light. The University of California's Leuschner Observatory failed to discover Nemesis by 1986. The Infrared Astronomical Satellite (IRAS) failed to discover Nemesis in the 1980s. The 2MASS astronomical survey, which ran from 1997 to 2001, failed to detect a star, or brown dwarf, in the Solar System.

and, later on,

  In particular, if Nemesis is a red dwarf star or a brown dwarf, the WISE mission (an infrared sky survey that covered most of our solar neighborhood in movement-verifying parallax measurements) was expected to be able to find it. WISE can detect 150 kelvin brown dwarfs out to 10 light-years. But the closer a brown dwarf is the easier it is to detect. Preliminary results of the WISE survey were released on April 14, 2011. On March 14, 2012, the entire catalog of the WISE mission was released. In 2014 WISE data ruled out a Saturn or larger-sized body in the Oort cloud out to ten thousand AU.

In fact,

  According to NASA, "recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals, and thus, the Nemesis hypothesis is no longer needed." And, indeed, a recent sky survey by NASA's WISE mission found no star or brown dwarf orbiting the Sun.

NASA's conclusions are laid out in this page, which talks about how WISE found no brown dwarf in the Oort Cloud . . . but found other brown dwarfs further out.


Is axial tilt critical for life? According to this explanation at the Georgia State University website,

  Axis Tilt is Critical for Life

  . . .

  The Earth's spin axis is tilted 23.5° with respect to the ecliptic, giving moderate seasons and preventing temperature extremes anywhere on the planet. . . . Not only is the Earth's angle of tilt close to the optimum value, it also seems to have been essentially constant. That is crucial for the development of advanced life.

Is this claim correct?

Why does it matter if there are some areas of a planet with extreme temperatures, as long as there are other spots on the planet that are not extreme?
Why are "moderate" seasons required for life to exist?
If humans can live at the equator on Earth where there is the least amount of tilt, why would an exoplanet with less tilt or no tilt be necessarily non-inhabitable?
Even if humans could not live on a planet without axial tilt, are there no other forms of known "advanced" life that can? We know that extremophiles exist, such as tardigrades' ability to survive in the vacuum of space.  What is the most "advanced life" that could live on a planet without axial tilt?

I agree with David Hammen.   Hyperphysics is mostly a very good site but they dropped the ball on that page IMHO.   Hope you don't mind a partially speculative answer, but here goes:

  Why does it matter if there are some areas of a planet with extreme
  temperatures, as long as there are other spots on the planet that are
  not extreme?

It shouldn't matter if part of the planet is uninhabitable.   There are deserts on Earth which are all but uninhabitable but that doesn't effect life elsewhere.   Prior to 5.3 million years ago the Mediterranean sea evaporated and that entire region could have had a salty basin and been hugely hot, but I've not read of life on Earth having any problem with that.   Source

  Why are "moderate" seasons required for life to exist?

There's a boatload we don't know about the evolution of life on other planets but seems perhaps universally true that life adapts, so I find it difficult to believe that moderate seasons are necessary.    Very extreme changes could be difficult, but change can force adaptation.

  If humans can live at the equator on Earth where there is the least
  amount of tilt, why would an exoplanet with less tilt or no tilt be
  necessarily non-inhabitable?

The tilt is planet wide but the lowest variation happens near the equator, but animals that thrive near the poles adapt by hibernation or migration and smaller stuff can be frozen and then come back to life, so, I don't agree with the article on this point.

  Even if humans could not live on a planet without axial tilt, are
  there no other forms of known "advanced" life that can? We know that
  extremophiles exist, such as tardigrades' ability to survive in the
  vacuum of space. What is the most "advanced life" that could live on a
  planet without axial tilt?

One of the interesting historical facts of life on Earth, at least to me, is how long it took what we might consider advanced life to develop.   One celled life in various forms was around for over 3 billion years but the first fossils are about 650 million years old.   It took life a very long time on earth to get from too small to see to large enough to leave a footprint . . . but, I digress.

I agree 100%, one celled life or Tardegrades could live on a planet with no tilt or 90 degree tilt.  Easy.   Ocean life in general should be fine cause oceans are more adaptive.  Evaporation keeps ocean surfaces colder than land gets during peak heat and while a completely frozen over ocean isn't great for life, cold oceans hold more oxygen and CO2 which can be good for life.   Oceans also circulate as an effective means of temperature moderation and fish don't really care how windy it is or how much or little it rains.   The tilt question, I think, is really just about life on land.

Land life could be more vulnerable to high wind, extreme temperature shifts, droughts or floods, which could be driven by greater axial tilt, but I find it hard to believe that Axial Tilt is the be-all and end all.   Day length and year length are key factors too.

One point I agree with the article on, is that a close to 90 degree tilt might not be ideal with one part of the planet always facing the sun and the other part never facing it but outside of extreme tilts, I don't see why it would be a big deal.

A thick cloud cover, for example, reduces seasonal changes.   There's a number of factors.


Could two stars collide without a resultant explosion? Are there any scenarios in which two stars could collide, and simply fuse without triggering a supernova. Maybe a mid-sized star slowly spiraling in together with a smaller star? It seems like any time stars collide it's incredibly violent. It seems that one star always gets torn to pieces and accretes matter onto the other, or ends in a red nova. Is there any way this process could be calmer?

Are there any scenarios in which two stars could collide, and simply fuse without triggering a supernova

As said in the comments, binary evolution of stars is very uncertain, but there exist numerous codes (i.e. MESA, STARTRACK, BSE, etc...) which incorporate LOTS of physical processes over the course of millions of years (and if they evolve to black holes billions of years) of binary evolution. Ono Pols has a nice textbook free on his website here.

Stellar binaries usually experience at least a couple mass-transfer (MT) phases after the binary components fill their Roche lobes. To put it simply, MT is either stable or it is unstable, but either way it results in the donor star losing it's envelope so that it's core emerges (which will later turn into a compact object). In stable MT, the donor star transfers some mass to its companion and thusly loses rotational angular momentum (it's spin decreases) while the accretor's spin increases. In unstable mass transfer, the mass transfer rate is too much for the accretor to handle so that the binary is engulfed by the envelope, this is also called a Common Envelope (CE). The CE results in either 1) the envelope is ejected from the binary resulting in a drastic decrease in the binary separation, or 2) the binary merges leaving a single star which accretes the leftover envelope.

So, I think the case 2) is an answer to your question. Keep in mind that CE is very very uncertain, as Postnov explains, and it is a relatively quick process (less than 1 million years). It's worth noting that case 1) of CE is crucial for forming compact binaries that have small separations so that they merge within the lifetime of the universe (so we can measure they're gravitational waves).

For your nutrition, here in Postnov's review on compact binary evolution the section on CE is lovely (section 3.6 on page 35).

Of course, if you're willing to include black hole binaries in your question, then their merging due to the emission of gravitational waves is certainly a calmer scenario.


Reversal of magnetic poles of earth When the magnetic poles of the earth get reversed, does the earth keep on revolving in the same direction or does it start revolving in the opposite direction?

The Earth will keep revolving in the same direction during a geomagnetic reversal.

There have been a few articles about this phenomenon published during 2018:

National Geographic - No, We're Not All Doomed by Earth's Magnetic Field Flip

Science Daily - Earth's magnetic field is not about to reverse

The Conversation - The Earth’s magnetic field reverses more often – now we know why


Why do astronomers not use past tense when discussing observations of the universe Describing a galaxy 70 million light years away, I consider it incorrect to state for example "it has an active black-hole at its center". It did 70 million years ago, who knows what is out there now. I could understand using the present tense for objects within the solar system and even nearby (less than say 100 ly) but not for such distant objects.

Is there some convention to use present-tense for all observations, or am I being a pedant?

You're being a pedant and are not necessarily correct!

The present tense could be clearly defined as what is experienced at the time of observation. To write that it had a black hole 70 million years ago would (pedantically) only be correct at one instant of time, not correct when the reader read the sentence and even ignoring that pedantry, not necessarily true for gravitationally lensed light where the light has not travelled along a straight path!


How can you verify the theories presented on TV shows like "How the Universe Works" is current and/or valid? There are many TV shows such as "How the Universe Works" that explain theories about the origin of the universe such as the Big-Bang Theory.

What resources can I access to verify the scientific thinking on the origin of the universe theories presented on such TV shows?

Use other, more scientific sources.

You have no way to test them by some home measurements - science was tuned to pass even more sophisticated tests.

Popular media is often not very exact. They try to open the results to the public. For example, to find the geometry of the Kerr Black Holes was a decade long work for the best mathematicians of the time. The result is a relative simple formula, which is understable with not far more than high school math. Most in the popular media about the wormholes and white holes is based on it, or at least strongly related to it.

So, to check it, best if you try to find the formula and try to learn, what does it mean. It still doesn't need to understand the decade long work behind that, and also this is amazing - but only after you've understood it.


How often do supernovae occur? Type Ia supernova are used as standard candles. But they also are transient events. This means that to determine the distance of a galaxy using supernovae, you have to wait for one to occur. How often do type Ia supernovae typically occur in a galaxy ?

There is a lot of scope to provide a very detailed answer here. The rate depends on what sort of a galaxy you are considering and when, what its star formation rate is (or was) and what its total stellar mass is.

A good reference is the Annual Review of Astrophysics article by Maoz et al. (2019). This says that for a Sbc galaxy like the Milky Way, the specific rate ( the rate per unit of stellar mass in the galaxy) of type Ia supernovae is $10^{-13}$ per year per solar mass of stars. For a Milky Way stellar mass of $\sim 6\times 10^{10} M_\odot$, this means a rate of 0.006 per year, or one type Ia SNe every $\sim 200$ years.

In the local universe this rate scales as about $M^{-0.5}$, where $M$ is the galaxy mass (valid over about 5 orders of magnitude for $M>10^7 M_\odot$; Brown et al. 2018). So the specific rate of type Ia supernovae is larger in smaller galaxies.

The rate also decays after a galaxy is formed, or after a burst of star formation as something like $t^{-1}$.


Binary Star Initial Speed I am writing a 2D simulation for binary stars orbiting each other. Currently, the initial position of the stars are on opposite sides of the screen with the left star having a velocity up the page and the right star having an equivalent velocity down the page.
The question I have is there a formula for calculating what that initial speed should be so that the stars remain in a stable orbit? Optimally, the formula would take into account the mass of the stars (which are equivalent), the distance apart, and G.
My apologies if this question would be better directed elsewhere or if I overlooked the answer in my own research.
Thank you for your help.

You will always get a "stable" orbit if the stars have less than escape velocity relative to each other. (unless you are modelling the stars as having non-zero radii so they can collide)  The stars will enter into elliptical orbits around a barycentre.
But I guess you want a circular orbit. For a circular orbit the speed $v$ is given by
$$v^2=GM/r$$
where r is the orbit radius (from the centre of mass), G is Newtons gravitational constant, and M is the reduced mass
$$M= \frac{m_1m_2}{m_1+m_2}$$
for stars of mass $m_1$ and $m_2$.
You will find it convenient to use astronomical units (not SI units), so distance in AU, time in years, mass in "solar masses"  In these units $G=(2\pi)^{2}$ and you avoid difficulties with very large and very small values. (in these units the speed of the Earth is $2\pi$ AU/year which makes sense)


If the whole observable Universe is as big as planet Earth, how big would Earth be? If the whole observable Universe is as big as planet Earth, how big would Earth be?

The radius of the observable Universe is 41.5billion light years (ref).
A light year is $9.5\cdot10^{12}$ km.
The radius of the Earth is 6371 km.

So the answer is $6371 km \cdot \frac{6371 km}{41.5\cdot10^9\cdot9.5\cdot10^{12}km} = 1.03\cdot10^{-16}km = \underline{\underline{1.03\cdot10^{-13}m}}$.

The size of the atoms are $\approx 10^{-10}m$.


Are Tharsis Montes and Hellas Basin a result of the same event? Looking at Mars map:

the highest, biggest mountains and their associated highlands are essentially antipodes of the deepest, huge basin. Roughly 180 degrees away, similar latitude except opposite sides of equator... this immediately brings to mind the idea that an asteroid impacted the area of Hellas Basin, and the shockwave traveled clear across the planet causing a bulge of Tharsis Rise, and causing multiple volcanoes to sprout there.

Is there anything to this impression, or am I entirely wrong?

Are Tharsis Montes and Hellas Basin a result of the same event?

You were not the first to have seen that the Hellas Basin and parts of the Tharsis Rise are roughly antipodal (Peterson 1978, Williams and Greeley 1994). The Tharsis Montes themselves are far too close to the equator to be considered antipodal to the Hellas Basin. The shield volcano Alba Mons however is almost exactly antipodal to the Hellas Basin.

This shield volcano is a bit to the north of the Tharsis Rise proper, and well to the north of the Tharsis Montes. Both Peterson and Williams suggested that the Hellas Basin impact might well have triggered the vulcanism that resulted in Alba Mons.

There's an issue with this hypothesis, which is that the apparent ages do not align. Improved remote observations of Mars has provided a tool for estimating ages of Mars features: crater density. Based on crater density, the Hellas Basin appears to be very old, at least 3.8 billion years old (Carr 2010). Alba Mons, while also quite old, isn't that old. It formed at least 200 million years after the Hellas Basin impact and perhaps as much as a billion years after (Ivanov 2006).

Bottom line: If the huge impact that formed the Hellas Basin did have any antipodal effects, those effects are buried under the magma that formed Alba Mons several hundred million years later.

References:

Carr, Michael H., and James W. Head III. "Geologic history of Mars." Earth and Planetary Science Letters 294.3-4 (2010): 185-203.

Ivanov, Mikhail A., and James W. Head. "Alba Patera, Mars: Topography, structure, and evolution of a unique late Hesperian–early Amazonian shield volcano." Journal of Geophysical Research: Planets 111.E9 (2006).

Peterson, J. E. "Antipodal effects of major basin-forming impacts on Mars." Lunar and Planetary Science Conference. Vol. 9. 1978.

Williams, David A., and Ronald Greeley. "Assessment of antipodal-impact terrains on Mars." Icarus 110.2 (1994): 196-202.


Which planet or star is this? The picture was taken with a smartphone camera at Latitude 48.860045 | Longitude: 2.366202 the 31th of July at 01h07min15s Paris time. On the left is obviously the Moon, and the lower red circle points the point of Latitude: 48.852886 | Longitude: 2.369167. (It's the angel of liberty, place de la Bastille, Paris.)

What does the upper circle point to ?

At the beginning I thought it was Venus, but as far as I remember, it disappears quite early in the evening. I don't think it is Mars : the light reflection on it isn't red enough.

I think it is Mars.  Although it's difficult to tell, just looking at the image, but if it were Saturn then you would also be able to see Mars.  Mars is much brighter than Saturn at the moment, so if Saturn were visible, Mars would be even brighter.  The fact that there is only one bright object (other than the moon) is, I think, conclusive evidence that this must be Mars.

Looking over the city, the large brick building on the Rue St Gille is visible, and the object is visible directly in line with the phone mast on that building. That gives a direction just East of South. Checking with Stellarium at the given time shows Mars at exactly that position, at an azimuth of 171.


Any cheap (<100 euro) tools for observing the lines in solar spectrum? I am now reading the book ''The story of Helium and the Birth of Astrophysics''.

I really want to observe the lines (dark or bright) in the solar spectrum.

Any easily available tools for an amateur?

You need a diffraction grating in order to split the sunlight into a spectrum. It turns out even a normal CD or DVD because of the pits that they have to encode data work very well as high resolution diffraction gratings. You could make a simple enclosure with a CD and a slit like this:

http://coolcosmos.ipac.caltech.edu/cosmic_games/spectra/makeGrating.htm

This should allow you to observe the sun's spectrum, although I would also recommend that you be very careful when observing the sun directly. Preferably use a very narrow slit--this will both make it safer and make the absorption lines much clearer.


Why is Gaia operating around Earth orbit? Why not send it to Neptune's orbit? Gaia is an astrometry spacecraft that's currently operating around the Sun-Earth L2 Lagrangian point. Question: why here? Why not the Sun-Neptune L2 Lagrangian point? By orbiting the Sun at a larger distance, it should be able to get more accurate parallax measurements.

Only reason I can think of is cost. I'm not familiar with estimating how expensive space probes cost, but Wikipedia says Gaia cost ~\$1 billion and this is comparable to the cost of the Voyager program, which also cost about ~\$1 billion. Of course Gaia's instruments should be more sophisticated than Voyager's, but there were also two Voyager probes, not one.

Well, you thought about the spatial aspect of a parallax measurement, but not about the temporal one.

Gaia's intention is to measure 3D positions as well as 3D velocities. For the distance, you need accurate parallactic measurement, which come in with your orbital period.
For a typical Gaia-star with several measurement per year, you'll get 5 values of the parallax after 5 years of time, which you then average. If you'd send Gaia towards Neptune (besides the fact that no one has ever sent an orbiter, to say nothing of a L2 mission that far out) that has a period of 168 years, then after 5 years you'd get... 5/168 th of one paralactic measurement.

It simply couldn't achieve its science goals if put around the L2 behind Neptune. Also no one on this planet has any experience in putting something into a outer system L2 point. This is different than putting it into Earth's L2, because reaching the L2 around one of the giants has vast and very precise $\Delta v$ requirements. This would be a massive technological leap, and things don't work that way in space. Small, incremental technological steps are required in an anyways unfriendly environment, to make sure everything works properly and no millions of dollars have been wasted.
Compare that to Gaia's predecessor, the Hipparcos satellite, which was parked in geostationary orbit.

Now you could still say, why not use Jupiter hypothetically anyways. Well, the orbital period there is still 11 years, and Jupiter's L2 still suffers from the intense radiation environment that is provided by Jupiter's magnetosphere. This would lead to rapid degradation of the CCDs used for scanning across the sky.


Can I find exoplanets without a telescope? I would like to know please whether or not I can look for exoplanets without a telescope.

No. With a probability nearly equal 1 you will not find any exoplanet with naked eye from Earth, even under perfect conditions like no light-pollution. The human eye simply does not have enough resolution to observe any of the closest exoplanets directly.
Neither of the methods of detection an exoplanet work without any additional tool (such as a telescope for any part of the spectrum).


Why are Bok globules so cold? I have recently read a bit a out Bok globules. One thing, that is repeatedly emphasised is that they are among the coldest objects in space, with a temperature of ~3 K.

Why is that so? Naively, I would expect the coldest objects to be far out in the super voids. Also, I would expect these globules to have retained heat from the gravitational collapse. After all, the kinetic energy of their free fall should be dissipated, shouldn't it?

Maybe someone can elaborate.

Most of my information is sourced from here:
https://apod.nasa.gov/apod/ap990511.html

To summarize the source, Bok globules are extremely cold because of their composition -- they're generally just clouds of pre-stellar material which block out all light. This makes the interior of Bok globules "shielded" from interstellar radiation that would otherwise heat them up.

Also, you claim that the globules should have retained some heat from gravitational collapse. However, since the globules don't encounter much friction in space, and there aren't many major collisions that produce changes in kinetic energy, there isn't much energy to dissipate. After all, even Bok globules aren't dense objects by any stretch of the imagination.


How much of the Sun's disk must be covered for a visible shadow to be cast? At first it seems like the answer is obvious, $100$% or more, that's what a total solar eclipse is after all. But after seeing satellite images of the June 21st 2020 annular eclipse, the shadow cast by the Moon on Earth was indistinguishable from the one cast during a total eclipse despite the Sun not being completely covered. Only difference is that there was no umbra, but that doesn't matter.

But then again, during an annular eclipse the Moon is covering about $99$% of the Sun's disk, and that's a lot. Surely if less of the Sun's disk was covered then there would be no shadow visible. But then I learned Phobos, which covers ≈$25$% of the Sun's disk as seen from Mars' equator, casts a very clear shadow despite covering only a quarter of the Sun's disk.

  What exactly is the cutoff point for a visible shadow to be cast then? Venus only covers $0.108$% of the Sun's disk during a transit, and thus no visible shadow is cast on Earth. If Venus was larger, say the size of Jupiter, it would cover $14.52$% of the Sun's disk. Would that be enough for a visible shadow to be cast on Earth's surface? Granted, it wouldn't be an umbral shadow, but that's irrelevant.

The answer for your question is quite complicated. Basically, the cutoff point you are saying about depends on the amount of light, which is reflected from the certain surface experiencing the eclipse. Another factor is the sensitivity of your eye, which seems to be individual...
http://www.mkrgeo-blog.com/the-role-of-contrast-in-ability-of-human-vision/
This cutoff is nothing like the contrast difference between two separate areas, where one of them is less illuminated.
Since the albedo of the Earth is various, we shouldn't expect the fixed border between shadowed area being visible. The biggest albedo feature clouds, snow, and glacial areas, and consequently the deserts.
The satellite view is usually perfect for emphasizing the shadowed area. This is driven by the huge amount of reflected light, which comes from the outside, where Sun shines in 100%. As we are moving down to the ground, the visible cutoff changes. The phenomenon, which could explain this situation is the light transition.
The light transition defines the border between non-shadow and half-shadow surface, and next the half-shadow and full shadow surface.
This is better described here:
http://www.mkrgeo-blog.com/what-is-a-light-transition-what-examples-of-it-can-we-see/
where you have got the experiment on a white background, which reflects the light perfectly. An analog situation happens during every solar eclipse seen from the space, even the partial one. As our sight is higher from the surface, we are able to see a bigger area. Obviously our shadowed area becomes smaller, which yields eventually the regions from outside the penumbra, fully illuminated.
The details of this situation have been described here:
http://www.mkrgeo-blog.com/how-the-lunar-shadow-looks-like-during-the-solar-eclipse/

Above you can see the shadow produced by the partial phase of the solar eclipse in 2015. As you can see, it's almost the same deep as you have shown in your question. This is where I am heading to in order to make a good explanation. The image of a partial solar eclipse was taken in specific conditions, where the sun was low above the horizon. hence your penumbra remains the fuzzy oval, bringing more reflected sunlight from the outside. This is why it looks darker.
Even the ISS doesn't show the real shadow cutoff, which you are asking about. The most expressed prenumbra-affected regions are to be spotted via some satellite imagery like sat24.com or so.
See the below examples from June 21, at the eclipse beginning in Africa.

this is the most expressed penumbra, as you can see. Next, if you really wish to know roughly what minimum phase of the eclipse causes the penumbra visible, you can:
1. Draw the visible line of the penumbra cutoff, which you can spot by your eyes
2. Pick up the location just behind this line
3. Check the eclipse circumstances in this area (knowing the time provided) by some astronomical software like Stellarium, Cartes du Ciel, etc.
Remember, that this visible shadowed border will be different for the other circumstances! Imagine, that you are standing inside the umbra during the totality. In this case, your eye can even distinguish the areas brighter and darker inside the shadow! On the other hand, the prenumbral regions will appear as fully illuminated. Threading this way, the cutoff will be marked by the border of the umbra only.
In terms of the photos, as you said. It covers only 25% of the sun but the shadow is visible well. This is what I explained above. Look at the shadow itself, it is very small compared to the image frame (the area of sight). As a result, a vast majority of the surrounding area is fully illuminated, therefore reflects possibly the biggest amount of light outshining the small shadowed region effectively.


What would be the required conditions for a lunar eclipse to occur only once on a precise annual basis? As I am aware, lunar eclipses can take place several times a year on Earth. However, I am curious whether it would be possible for this event to only occur once annually on a specific date (and preferably on the same face of the planet) if the moon's angular orbit around Earth were to be altered from 5°.

To put it bluntly:
How would the components causing a lunar eclipse need to be altered for a singular 'blood moon' to be able to appear reliably every year on say the 25th of September? If it is at all possible.
Additionally, would these alterations cause any other significant effects between the Earth and Moon?

A lunar eclipse occurs when a full moon occurs at the same time as a lunar node (when the moon crosses the ecliptic - the plane of the Earth's orbit around the sun)

The time between full moons is 29.53... days (although it varies slightly due to the moon's orbit not being circular) This is the synodic month. The time between nodes is 27.21... days. This is the draconic month (because of the dragon that eats the sun!) It is shorter than the time between full moons because of the curved motion of the Earth around the sun, and because the gravity of the sun distorts and rotates the orbit of the moon.

Now on Earth, 223 synodic months is almost the same as 242 draconic months. And so an eclipse will recur every 223 months, or 6585 days.

Now you want an eclipse to recur every year. For that you would need the length of the synodic month, and the draconic month to be an exact fraction of a year. If you change the size of the orbit of the moon you will vary both the length of the synodic and draconic month. If you change the size of the Earths orbit you can change the difference between a synodic, and draconic month, but you also change the length of the year.

So you end up with a juggling game: You have two variables to change, the Earth-Moon distance and the Sun-Earth distance. And you get three values: Synodic, Draconic month and length of a year. You aim to get both of the months an exact factor of the year. It's probably possible to achieve this, but you may end up freezing or baking the Earth. You have also changed the calendar (so "September" may not exist)

Then you need to consider the perturbations of other planets and so with time the length of the month will vary, and your three-way co-incidence will break eventually.


Why is it impossible for a Quasi Star to exist these days? I've heard about quasi stars a while ago, but I've just recently decided to go more in-depth about the characteristics and facts about them. I learned that they gain their energy through radiation when matter falls into the black hole in the core. I may be wrong about this part, though. I also learned that they can't exist these days because the universe's gases are polluted with metals. Why does that stop quasi stars from existing?
Edit request from comment: A quasi star is a hypothetical star that existed in the early ages of the universe. It got its energy from a black hole at its core, unlike normal stars that get energy through nuclear fusion.
https://medium.com/predict/quasi-stars-black-holes-at-the-core-of-the-universes-largest-stars-38bfbc4e1b95
If you would like more specifications, please tell me

This is a highly uncertain prospect, since the physics of the system itself relies on processes that are themselves uncertain to varying degrees, e.g., stellar nuclear evolution of very high-mass stars, accretion of gas onto compact objects, etc...
A relevant paper, cited by the wiki article for quasistars, is the work by Begelman $\it{et~al.}$ (2007). They show that newly formed quasistars should have surface temperatures higher than 10,000 K (9,700 °C). As a quasistar cools over time to a limiting temperature of 4,000 K (3,730 °C), for metal-free (Population III) opacities, at and below which hydrostatic equilibrium is no longer sustainable, the end of the quasistar's life leaves behind an intermediate-mass black hole.
Such quasistars would have a short lifespan, at most ~ 7 Myrs, during which the core black-hole would grow into an intermediate-mass black hole which are suspected to be the progenitors of current-epoch supermassive black holes. This implies that quasistars are transient, and so observing them directly would be very difficult. However, it is possible that we could observe indirect effects of their existence on their environments, for example this is how we infer the existence of common envelope evolution, or on the evolution of intermediate-mass black holes.


Can the Sun contain degenerate matter? Degenerate matter (neutronium) is hypothesized to be very dense and, at a certain amount, unstable - in the sense of collapsing on itself and causing fusion. The result would be a massive fusion detonation. Such a detonation could cause the Sun to lose its photosphere and cook the inner Solar System with a wave of radiation. This event is described enjoyably in Robert Sawyer’s book, "The Oppenheimer Alternative". The discovery of the neutron core, however, is made by identifying a transient rise in products of the CNO fusion cycle which is hypothesized to require 20 million Kelvin, whereas the Sun’s core temperature is ‘only’ 15 million Kelvin. These fusion products are detected by solar spectroscopy.
Can such a neutron core exist? If it did exist, how could the by products of the CNO cycle be identified before the actual explosion? I mean, if the neutronium degenerates, fuses, and raises the Sun's temperature to allow CNO fusion, then isn't the explosion going to happen before any of the photons from the explosion have time to heat the Sun up enough to make CNO products detectable in the spectra?
EDIT: Fortunately there are many people here who are smarter than me and still kind enough to show it nicely. I will try to clarify the facts of the novel without spoilers (Rather like describing the phenomena of Red Matter in Star Trek (2009) without spoiling the plot)

Edward Teller presents three spectra of the sun taken in 1929,1938 and 1945 at a colloquium in Los Alamos
Fermi remarks that the second spectrum is not of our sun but of an F class star because of the strong carbon absorption lines
Teller infers that something happened to our sun around 1938 to heat it up a little to spark off C-N-O fusion
Oppenheimer relates von Neumann's weather data that the Earth was indeed statistically warmer during that 'period'. He goes on to say that the problem is not with the spectrograph plates from Bethe nor the math done by Teller. Rather, Oppenheimer says that the sun is indeed having a 'problem'
Oppenheimer recalls publications from Zwicky and Landau that hypothesized a neutron core to our sun.
Oppenheimer recalls a paper he wrote with Robert Serber that refuted Landau's work with calculations that a neutron core greater than 0.1 solar masses would be unstable.
Neutron core instability would manifest as a hotter sun
Oppenheimer starts to say that such instability is transient and Teller interrupts him to blurt out that the unstable neutronium would be ejected from the sun
Teller goes on to say that a neutron core would be formed by an implosion, then compares it to the process that happens in the atomic bomb,FatMan, where an explosive array is used to implode a plutonium core. The inevitable result he states is an explosion (It is unclear if he means fission or perhaps his own demon, fusion)
Hans Bethe then calculates that based on the expected size of a neutron core, the known size of the sun, and the opacity of the sun that the outwardly exploding degenerate matter would hit the photosphere in 90 years.

At this point, I will let the other really knowledgeable people here comment on these points and their veracity. However, I am still haunted by the idea that a small really heavy object (microscopic black hole, white dwarf, etc) could be captured by our sun and then precipitate instability.

The answer to whether a normal star can contain a core of degenerate neutrons is a yes. Thorne & Zytkow 1977 produced numerically models where a neutron star becomes embedded in the center of a massive giant or supergiant star, surrounded by a large gaseous envelope. In this scenario, the main source of energy becomes not nuclear fusion but instead gravitational contraction from matter flowing from the inner envelope onto the outer core. The energy production ratios are
$$L_{\text{nuc}}/L\approx0.04,\quad L_{\text{grav}}/L=0.96$$
for stars with $M_{\text{tot}}\leq10M_{\odot}$, as above this, convective envelopes form. The models, regardless of mass, predict some degree of shell burning, with hydrogen-, helium-, and carbon- burning layers outside the core. In the inner regions, temperatures are orders of magnitude higher than that required for the CNO cycle, which sets in at around 15 million Kelvin, actually (not 20 million K) and dominates over the p-p chain at about 17 million K.

Thorne & Zytkow did find that for $M_{\text{tot}}<2M_{\odot}$, the envelopes were unstable against radial adiabatic pulsations, implying that it's likely not possible to extend the analysis to the case of the Sun - at that mass, the object is quite vulnerable to instabilities. That said, I am fairly confident that we would be able to tell if there was a degenerate object in the solar core; unlike the red giant or supergiant case, there is no large envelope to hide it, and the mass of the neutron star would at least be comparable to or (much more likely) exceed the mass of the Sun itself.

Some points on the stability of neutron degenerate matter: The problem is actually with small quantities, not large quantities. Small amounts are incapable of remaining bound by their own gravity; the pressures involved are simply too high, and the solar core is nowhere near high-pressure enough to maintain stability. Optimistically, you'd need somewhere in the $0.1M_{\odot}\text{-}0.2M_{\odot}$ range at minimum for stability, though I would be surprised if a mass of degenerate matter this low was produced naturally - a neutron star of a more typical mass would likely have to lose mass somehow.


Planetary Declination and Axial Tilt Is it a coincidence that Earth's axial tilt and maximum declination are both approximately 23 degrees?

Mechanically speaking, do planets physically cross the Solar equator, or is declination due to the axial tilt angle? The later is difficult to visualize given variety of planetary axial tilts i.e Mercury 0, Jupiter 4, Earth and Saturn 23 degrees approximately.

I'll try and give a concise example. At JPL's Horizons you can download data of RA and DEC for all planets and objects placing either Earth or the Sun at the center. This will calculate the Declination of a planet geocentrically or heliocentrically.

The calculations suggest all planets will obtain a maximum declination North and South between 23° and 28° (i.e Mercury reaches 28° when viewed from the Earth). Earth's axial tilt (obliquity) at 23.45° matches its maximum declination N/S.

However, Mercury's Axial tilt is 0° and Jupiter 3.13° so if declination is the angle between the ecliptic plane and the celestial equator how do Mercury and Jupiter obtain maximum declination of 23° and 28° N/S?

I'll appreciate any help clarifying this.

A planet's apparent position in the sky is determined by orbital geometry alone.
The tilt of its axis only affects its orientation as we see it in a telescope.

It's easier to think of the solar system in terms of ecliptic coordinates.
The Sun's geocentric latitude is always near 0°, as is the Earth's heliocentric latitude.
A planet's orbital inclination is measured relative to the ecliptic, and its maximum excursion in latitude is related to that inclination.
Each planet crosses the ecliptic northward at its ascending node and southward at its descending node.

For example, Saturn's orbit has a semimajor axis of 9.6 au and is inclined 2.5° to the ecliptic.
Its heliocentric latitude runs that far north or south of the ecliptic over its 29.5 year period.
Earth's orbit brings us 1 au closer at opposition, so Saturn's geocentric ecliptic latitude can reach ±2.8°.

Mercury's orbit has a semimajor axis of 0.4 au and is inclined 7°, so its heliocentric ecliptic latitude range is ±7°.
At inferior conjunction ~0.6 au away, its geocentric ecliptic latitude range is about ±5°.

The transformation from ecliptic to equatorial coordinates is a 3D rotation by the 23.4° obliquity, not a simple sum.
Also a planet's orbit and the Earth's axis may be tilted in different directions.
Saturn's ascending node is 114° longitude east of the J2000 vernal equinox; its orbital inclination and our axial tilt are that far out of phase.
A JPL HORIZONS ephemeris for 29 years of Saturn oppositions and conjunctions gives geocentric declinations in a range of ±22.6°.

Saturn's 26.7° axial tilt relative to its orbital plane does not affect its declination at all.
The effect we do see is that at different points in its orbit, its rings appear open to the north, open the south, or edge-on and slanted east or west, as shown here.

Since you mentioned the solar equator, it's inclined 7.2° to the ecliptic.
The planets do cross it, but their declinations are not related to it either.