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12

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 ...


8

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

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, ...


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 ...


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....


6

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

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 ...


4

Proxima Centauri can't explode as supernova as it is about 2 orders of magnitude too small and light. But if a supernova were to happen from that distance? How bright would it be? You can use the formula: $$\text{Apparent magnitude} = \text{Absolute magnitude}+5log_{10}(distance)-5$$ where the distance is in parsecs. Proxima centauri is 1.3 parsecs ...


4

In the comments of the other answer, the question came up whether decay of the orbit would limit the time amount of time dilation. The answer is of course yes. But by how much? This question can be answered using some modern results for the modelling extreme mass -ratio binaries in the limit of extremal spins. The answer will depend crucially on ratio of ...


3

Yes, it is possible for stars to exist that contain basically no hydrogen and helium - they are abundant and they are called white dwarfs. Rarer examples are the sdB and sdO stars that are composed almost entirely of helium. However, white dwarf "stars" are not undergoing fusion processes - they are inert. This illustrates fundamental physics. There is ...


3

Nothing will happen. Neither star is massive enough to become a supernova and their velocities relative to the Earth mean that we will almost certainly be hundereds of light years away from them when they become red giants. A 1 km/s velocity difference over 100 million years leads to a distance difference of about 300 light years. Both Procyon and Sirius ...


3

Life on Earth will not be adversely affected when nearby stars become red giants. Certainly, things will get very messy inside those systems: red giants throw out a lot of gas and dust. Red giants are a lot more luminous than the Sun because they have such a huge surface area, but they don't emit large amounts of dangerous radiation, like X-rays or ...


3

Such surveys are designed to detect an excess of emission over that expected from a stellar photosphere. The emission from a circumstellar disk is often modelled as a blackbody at lower temperature than the stellar photosphere. A hotter blackbody with the same emitting area emits more flux than a cooler blackbody at all wavelengths. Therefore, if you had a ...


3

According to Ginsburg, Loeb & Wegner (2012) "Hypervelocity planets and transits around hypervelocity stars", planet-hosting hypervelocity stars could form from the disruption of a stellar binary by a massive black hole, and the probability that the planet remains bound to the hypervelocity star is fairly high for suitable initial conditions. In their ...


1

The HR diagram is a plot of luminosity (or a proxy) versus temperature (or a proxy). $B-V$ is a proxy related to temperature, therefore to plot your point on a 2-d plot you obviously need the other axis information - the luminosity, or its proxy the absolute magnitude. This in turn needs an apparent magnitude and a distance to the star. It is also possible ...


1

The "radius" of the Sun is basically where the optical depth to radiation at any particular wavelength is about 1 (or some times 2/3 is used). This is known as the photosphere and is from where the light that we measure comes from. The "thickness" of the photosphere - i.e. the depth over which the optical depth changes from being negligible to very large is ...


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