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In my astronomy class I learned that a only low-mass stars (< 0.5 Msun) will contract, and then become degenerate, until it is a white dwarf. However, we also learned about the Chandrasekhar limit, which states white dwarfs have a maximum mass of 1.4 Msun.

If the starting star has less than 0.5 Msun, how does the resulting white-dwarf reach up to 1.4 Msun?

Am I perhaps wrong in my assumption that the mass of the resulting white-dwarf depends on the mass of the original star?

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  • $\begingroup$ Mechanisms like this one can take a low-mass white dwarf over the Chandrasekhar limit. This is one of the main runways up to Type Ia supernovae. $\endgroup$ – Emilio Pisanty Mar 14 '16 at 15:04
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You were taught wrong. Stars of up to about 8 solar masses will end up as white dwarfs. But it is only their cores that become degenerate and end up as the white dwarf. The rest of the envelope is lost during the giant phase due to a dense wind.

There is a non-linear, but probably monotonic, relation between the initial progenitor and final white dwarf masses (see below, from Kalirai 2013) - the Sun will likely end as a 0.5 solar mass white dwarf, but in normal stellar evolution, degenerate white dwarf stars can only be produced up to about 1.25 solar masses by the most massive progenitors. Any more massive than this and it is likely that the core does not become degenerate before igniting and burning through the heavier nuclear fuels. The most massive, probably single, white dwarf known is "WD 33" in the cluster NGC 2099 and has a mass of $1.28^{+0.05}_{-0.08}\ M_{\odot}$, is likely made of an O/Ne mixture, and had an estimated progenitor mass on the main sequence of $>3.5\ M_{\odot}$ (Cummings et al. 2016).

In order to get a more massive white dwarf, up to the Chandrasekhar mass (about 1.38 solar masses for a C/O or O/Ne white dwarf in general relativity), it almost certainly needs to accrete mass from a close binary companion or be the result of some sort of merger.

This is the leading candidate to explain type Ia supernova.

Initial final mass relation

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Although the maximum mass for a white dwarf is 1.4 solar masses, stars don't need to have 1.4 or more to become white dwarfs. A star with .5 solar masses will likely end up as helium white dwarf composed chiefly of helium-4 nuclei. (From source: Laughlin, G.; Bodenheimer, P.; Adams, Fred C. (1997). "The End of the Main Sequence". The Astrophysical Journal 482: 420–432. Bibcode:1997ApJ...482..420L. doi:10.1086/304125.)

A white dwarf less than 1.4 solar masses can reach that number (the Chandrasekhar limit) by accretion from a star or collision with another white dwarf. At which point, it blows up as a supernova and becomes a neutron star.

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