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33 an estimate of the Sun’s internal temperature, Tvir ∼ 4 × 106 K. 8×10 ergs−1 . ) The above estimate can also be used to argue that, based on its observed luminosity, the Sun must be composed primarily of ionized hydrogen. If the Sun were composed of, say, ionized carbon, the mean particle mass would be m ¯ ≈ 12mH /7 ≈ 2mH , rather than mH /2. 33 would then give a virial temperature that is 4 times as high, resulting in a luminosity prediction in Eq. 52 that is too high by two orders of magnitude.

Basicastro4 18 October 26, 2006 CHAPTER 2 absorbed, and therefore depleted, from the light emerging from the photosphere of the star in the direction of a distant observer. The same atom or molecule, which will be excited to a higher energy level by absorbing a photon, can eventually decay radiatively and re-emit a photon of the same energy. However, the re-emitted photon will have a random direction, which will generally be different from the original direction toward the observer. Furthermore, the atom can undergo collisional de-excitation, in which it transfers its excitation energy to the other particles in the gas.

Stars more massive than about 8M , after passing through the giant stage (where they are called red or blue supergiants because of their large luminosities – these are the stars near the top edge of the H-R diagram), undergo a runaway process of gravitational core collapse that ends (at least in some cases) in supernova explosion. The stellar remnants in these cases are neutron stars and black holes. Compared to white dwarfs, neutron stars are even hotter (by an order of magnitude) and more compact (by three orders of magnitude in radius), and hence less luminous (by two orders of magnitude).

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