# Tag Info

10

Gravity is only important insofar that it is capable of compressing the material to high densities. Whether that material is capable of solidifying depends on the competition between Coulombic potential energy and the thermal energy of the particles. The former increases with density, the latter increases with temperature. A dense plasma can still be a gas ...

6

The difference between a brown dwarf and a star is not a sharp boundary. A brown dwarf is simply a ball of gas where the (small) fusion rate is incapable of providing a significant fraction of the luminosity (which is mainly provided by gravitational contraction). A star will contract and reach a minimum luminosity, whilst a brown dwarf's luminosity will ...

4

These stars are currently moving away from us, in the case of Epsilon Eridani, at 15.5 km/s. In 31500 years they will be further away than they are now, the distance can be calculated by applying Pythagoras' theorem to the distances and velocities you mention. It turns out that Eps Eri will be about 12.2 light years from Earth, and Lutyen 726-8 will be 12.6 ...

3

Both the radiation and gravity follow an inverse square law, so there is no "horizon" at which gravity would overcome light radiation, they both get weaker at the same rate. Radiation pressure affects smaller objects disproportionately. This is one factor in the formation of comet tails: dust and gas is pushed away from the heavier nucleus, which is ...

3

While I'm lacking the formulae, the applicable surface area of a body would have to be great enough, and it's density low enough that it's gravity remains below that threshold. So a smaller body is much more likely. If you think about it, most dust from near the sun gets pushed out by the pressure. There wouldn't be a specific point where you would get the ...

2

Your intuition is largely correct: the key is that the proto-bulge region had a deep enough potential well so that the supernovas couldn't expel the remaining gas, and so new stars could form out of the gas (enriched by the supernova ejecta) in a continuing cycle. In the low-mass, isolated protogalactic clouds which probably contributed to the halo, the ...

2

Stars really are immensely far, however it's a common misconception that the stars that you can see are millions of light years away. Most of the visible stars are a few tens to a few hundreds of light years away. However it is possible that there are stars that have exploded in a supernova: Eta Carinae looks to be approaching the end, and is 7500 light ...

2

If your question is why the star starts moving up the red giant branch, it's in essence because of the behaviour of the surface opacity and the development of a substantial convective envelope in order to meet the surface boundary condition. It's basically the Hayashi track in reverse. We can say this because if you create a model of a star and ...

2

The EUVE catalog gives you a count-rate in a certain EUV band. To convert this into a flux, you need a flux conversion factor to go from counts per second to energy flux at the Earth per second. This conversion factor, also often known as an "ECF", depends on the intrinsic spectrum of the source and also how much absorption (known as the hydrogen column ...

1

The test to see whether degeneracy pressure is going to be significant is to compare $kT$ with the Fermi energy $E_F$ The Fermi energy is the energy level up to which all energy states would be occupied in a completely degenerate fermion gas. It is given by (for non-relativistic conditions)  E_F = \frac{h^2}{2m}\left(\frac{3}{8\pi}\right)^{2/3} ...

1

So then why would gas giant cores be electron degenerate, but not become hot enough for nuclear fusion? Degeneracy isn't an on/off switch. It's a quantum mechanical aspect of pressure that is always present, just as is thermal pressure. A substance is highly degenerate if degeneracy pressure completely dominates over thermal pressure, non-degenerate if ...

1

If you're looking at Table 2 "EUVE Deep Survey" from the Second EUVE Source Catalog, the cheap way out would be to compare the star of interest to other stars of similar spectral type in that table, correcting for apparent magnitude. To compare to the Sun, part 4 "Flux Calibration" and Figure 3 "Deep-survey effective area vs. wavelength models" may help. ...

1

Earth has orbited the galaxy 16 or more times since the planet formed. Given that even stars in a single open cluster usually have somewhat different velocities relative to galactic center, it's highly doubtful that we can see stars made from the exact same starting material as earth. As for the supernova that delivered the compression wave to our stellar ...

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