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85

Summary There's a 1 in 500 billion chance you're standing under a star outside the Milky Way, a 1 in 3.3 billion chance you're standing under a Milky Way star, and a 1 in 184 thousand chance you're standing under the Sun right now. Big, fat, stinking, Warning! I did my best to keep my math straight, but this is all stuff I just came up with. I make no ...


41

The dark lines are colder areas at the edge of the convection cells, where the cooled down plasma sinks towards the inside of the Sun. The yellow parts are where the plasma rises to the surface. Each yellow spot (which is actually the size of a country) is called a granule, and this web-like appearance is called granulation. In the outer part of the Sun (...


22

Usernumber's explanation of the light and dark regions is correct, but there is more detail to be added about granulation on other stars. Granulation is expected on other stars with surface convection zones, but the properties and timescales of the granulation can be quite different. On the Sun, the granules appear and disappear in timescales of 10-30 ...


18

When a galaxy recedes from us, the light we see from it is redshifted. For galaxies at cosmological distances, this redshift is fundamentally different from a Doppler shift; whereas the latter is due to a velocity difference between the emitter and the receiver, a cosmological redshift is due to photons traveling through an expanding space$^\dagger$. Hence, ...


15

There are scenarios where the effects of planet accretion on a sun-like star would be very significant indeed, at least in the short term. Whilst the amount of mass accreted by the star would be a tiny perturbation, the amount of accreted energy and/or angular momentum may not be. Scenario 1: The scenario where a hot Jupiter just drops into the star from a ...


15

It is possible in a Trojan configuration: In the place of the "Planet" on the image, also a small star could exist. The third star would be at $L_4$ or at $L_5$. This configuration could be made stable. However, as this link shows, In unnormalized units, this criterion becomes $$\frac{m_2}{m_1+ m_2} < 0.0385$$ We thus conclude that the $L_4$...


15

In short: no one knows for sure, but currently it looks that the probability is 1. Longer: On our current understanding, the Universe is probably infinite in space. This depends on the recent WMAP satellite results, which have shown a zero curvature of the Universe below measurement precision. The other two options were a positive curvature (thus, we would ...


13

Yes, a star can exist that's entirely hydrogen. Hydrogen is the fuel that makes stars happen unless they're very large. A star made entirely of hydrogen, so long as it was massive enough, would be very similar to the stars we see. The "metalicity" which refers to non hydrogen-helium elements has some effect on the star's rate of fusion, density and ...


12

No, it cannot. A black hole is something vastly different from a star. It's vastly different from anything else. It's not a thing, really, but more like a portion of very distorted spacetime. Nothing escapes from it simply because there is no way out - spacetime is distorted in such a way that all trajectories lead to the center. Now, there is a mechanism ...


10

It won't make much difference. The sun has a mass that is 1000 times more than Jupiter. Adding Jupiter to the sun will change the mass from $1.989\times10^{30}$ to $1.991\times10^{30}$kg Adding more mass to the sun will slightly reduce the life of the sun. The greater mass will cause the core to compress and heat up, increasing the rate of nuclear fusion. ...


9

Systems of three stars can exist, but a system of three stars in a triangle is unstable and won't exist in reality. There are configurations of three stars that are stable, for example, two stars in a close orbit about their common centre of gravity, and a third star in a distant orbit. Planets can exist in such a system, they could orbit around the distant ...


9

Stars are formed from the thin gas in space, and the thin gas in space is made of roughly 3/4 Hydrogen and 1/4 Helium (+trace amounts of other elements). There's no way to get a star made of 100% Oxygen as there is no way to gather enough oxygen in one place to get sufficient mass to make a star. So all real stars will be made mostly of H and He, with small ...


8

Yes, it is easily possible, but it depends critically on how massive the black hole is. A simple calculation will suffice. If we take a Newtonian approximation for the tidal acceleration across a star of radius $R_*$ as it reaches the event horizon $$ a_{\rm tidal} \sim 2\frac{GM_{\rm BH}R_*}{r_s^3}\ ,$$ where $r_s = 2GM_{\rm BH}/c^2$ is the Schwarzschild ...


8

Red shift is used for measuring the distance to very distant stars (galaxies mostly, in fact). The secret is to use spectral lines. Specific elements when very hot emit light at very specific colours and you can spot the pattern of those colours when the whole pattern has been shifted towards the red and see how far it has shifted. For closer stars, there ...


8

An answer to your question is contained within What is the largest hydrogen-burning star? The hottest observed main sequence stars are of type O3V, with photospheric temperatures of about 50,000 K. However, it is indeed possible that hotter main sequence stars may exist in the present-day universe, but have simply evolved into Wolf-Rayet stars (and lost a ...


8

Almost certainly Jupiter, if the following are true: The title of the image: Inked20190904_194204.LI.jpg gives the accurate date and time of September 4 2019, 19:42:04. The orientation of the Moon in the image is such that the illuminated portion is on the right side of the image. The circled object is a planet or star, and brighter than any other nearby ...


8

Processes energetic enough to accelerate a star to 99% of the speed of light are not known. A close interaction and ejection from a multiple black hole system could perhaps provide enough energy, but such close interaction with black holes would probably rip the star apart before it could reach such excessive speeds. Hypervelocity stars are known. They have ...


8

We would have $$\tau = d/0.99c - d/c,$$ of warning, where $d$ is the distance to the star when we first detect its light and $c$ is the speed of light. Would we see it - yes indeed. Since $$\lambda_{\rm obs} = \lambda_0 \left( \frac{1 - v/c}{1+ v/c}\right)^{1/2},$$ the light from the star would be blueshifted by a factor of 14, so for a solar type star, ...


7

To clearify: In your question you distinguish between a star forming a neutron star when its core exceeds the Chandrasekhar limit and a star undergoing a Type II supernova. These are, however, the same events. To summarize: Generally an isolated star can die in one of basically four ways. With increasing probability with increasing mass these outcomes are:...


7

Here is another plot of a Hertzsprung Russell diagram (luminosity versus temperature), but this time based on theoretical models. (The plot is from D. Prialnik 2000, An Introduction to the theory of stellar structure and evolution). Note that the zero age main sequence is well behaved in this plot. Luminosity and temperature are related by smoothly changing ...


7

Stars turn into Red Giants not because they're running out of fuel, but because they're accumulating material they can't use for fusion (yet) in the core. The star isn't so much dying of starvation as it is wallowing in its own muck. Red giants form when the fusion is no longer taking place in the centre of the star, but instead in a shell around the centre....


7

Formation of pure hydrogen stars is not likely to have happened in this universe, because Big Bang nucleosynthesis resulted in the production of helium and lithium (plus the unstable isotopes tritium and beryllium-7, which decay into helium-3 and lithium-7 respectively) in addition to hydrogen. It is therefore likely that the initial star-forming ...


6

A Black Hole (BH) is an object of General Relativity (GR), not of Newtonian physics, so the answer involves both. First Newton: As another answer notes, the acceleration due to gravity depends on the mass of the body divided by the square of the distance from it. At a given distance (say, ten million miles) the acceleration due to gravity is simply ...


6

What can you tell from a picture of some stars? At the very least, you need a recognisable asterism If the exposure is too low or the light pollution too high to identify unambiguously an asterism of three or more stars, you're simply not going to be able to tell much from the photo. An asterism on its own can only tell you where the picture wasn't shot. ...


6

Does "overhead" mean over the center of your head, or over some part of your head? If we assume the latter, it changes the problem! I don't want to recapitulate all MichaelS's lovely work above, so I'll do a quick back-of-the-envelope calculation borrowing from his numbers. The area of a human head as viewed from above (or below) is, umm, let's see, ...


6

You seem to be confusing the simple mathematical relationship between radius and surface area, and the more complex relationship between mass and size. If you double the radius of a sphere the surface area quadruples. This is pure maths, and is not particular to stars. The volume is multiplied by 8. But in a star, increasing the amount of matter by a ...


6

It's important to realize that binary stars form much differently than planets do. Assuming that both stars form in situ (i.e. excluding scenarios where one is captured from outside the system), there are several main ways for a binary star system to form from a molecular cloud. The most widely-accepted model at the moment is the fragmentation hypothesis, ...


6

Yes, but not very likely. The closest orbit that does not require constant expenditure of energy to maintain it is the prograde equatorial ISCO. For a Kerr black hole the time dilation factor on this orbit is $$\frac{dt}{d\tau}\approx \frac{2^{4/3}}{\sqrt{3}(1-a/M)^{1/3}},$$ which at the astrophysically likely Thorne limit $a = 0.998M$ gives a dilation of ...


6

I'll add to @usernumber's answer some graphics. Unfortunately we can't yet "has YouTubes" for some reason so I'll just add the links. There are two videos of the Sun linked in Phil Plait's Bad Astronomy article DKIST first light high-resolution video of solar granules DKIST First light video of solar granulation (wide angle). Here are the same kind of ...


5

The closest service to what you are describing is the SIMBAD Astronomical Database from the Université de Strasbourg/CNRS. At the time I write this post, it contains 10.8M objects and 35.5M identifiers. It does not have a single CSV you can download with this information (to the best of my knowledge, and I've asked), but there is an API and TAP service ...


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