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While low mass stars (initial mass approximately $0.1 M_\odot$ to $0.8 M_\odot$) are quite numerous, their evolution seems to draw relatively little attention. The lower central density and temperature, compared to massive stars, result in a very long lifetime. The star being fully convective, a larger fraction of the hydrogen is available to fusion in the center. This further extends the star's lifetime to more than 10 Gy.

Apart from this, what are the principle differences in evolution of a solitary low mass star compared to the Sun's? Will a red dwarf ever burn helium, will it have a central helium flash, will it become a red giant and then a white dwarf?

Red dwarfs with a present age of 10 Gy were formed when the Galactic metallicity still was low. How does the original metallicity of a red dwarf influence it's evolution?

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At solar metallicity, only stars with masses below 0.35$M_{\odot}$ are fully convective. Stars with $0.35<M/M_{\odot}<0.8$ will have radiative cores on the main sequence.

There is a standard reference that deals with the low-mass end of this range - Laughlin et al. (1997). To summarise:

Stars below $0.16 M_{\odot}$ don't ascend the giant branch. Stars with $0.16 < M/M_{\odot} <0.25$ begin the ascent of the giant branch but don;t get as far as being very red or giant. Stars from $0.25<M/M_{\odot}<0.5 M_{\odot}$ become red giants, but never make it to helium burning before complete dgeneracy sets in. Above this, stars behave "normally" and become red giants, go through He burning and end up as C/O white dwarfs. The lower mass objects ultimately end up as He white dwarfs, with almost all hydrogen consumed, first by core burning and convection and then when the core is exhausted, by a shell-burning phase.

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  • $\begingroup$ #1 Thank you for yet another quick and informative answer. Posting the question I thought red dwarfs were dull and boring. On the contrary, the article by Laughlin et al. is fascinating, shedding some light on the physics that let an MS star evolve into a red giant (or not). Once again, opacity seems important. $\endgroup$ – gamma1954 Dec 27 '19 at 22:48
  • $\begingroup$ #2 Could you please explain that stars between a quarter and half a solar mass "become red giants, but never make it to helium burning before complete dgeneracy sets in"? Does degeneracy in this situation prevent helium burning? $\endgroup$ – gamma1954 Dec 27 '19 at 22:50
  • $\begingroup$ #3 On page 423, left column, Laughlin et al. write "Between 1.5 and 4 Gyr" and a few more times "Gyr", whereas the preceding text and the inset diagram of Figure 1 mention "trillion years". If Gyr is 10^9 years and a trillion is 10^12, there seems to be a factor of 1000 between the Gyr and trillion years? $\endgroup$ – gamma1954 Dec 27 '19 at 22:56
  • $\begingroup$ @gamma1954 I believe the Gyr mean a trillion years. The graphs are clear. I don't understand this error. $\endgroup$ – Rob Jeffries Dec 27 '19 at 22:58
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    $\begingroup$ Complete degeneracy allows the core to be supported without an increase in temperature. Like a He white dwarf. $\endgroup$ – Rob Jeffries Dec 27 '19 at 23:00

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