In article ,
Yousuf Khan writes:
....
extremely tiny stars, the smallest red dwarfs, which I can't imagine go
through the same process as even our Sun. ... red
dwarf stars are mainly convective, even inside their cores. As such,
these red dwarfs would be very efficient at converting most of their
store of hydrogen into helium, unlike the more massive stars which only
convert the hydrogen inside their physically distinct cores.

This looks correct to me. I _think_ it was discussed in a stellar
interiors course I took, but that was a long time ago.

So these red dwarfs will last hundreds of billions and trillions of
years at the main sequence, doing exactly what they are doing right now.

Far longer than the Sun for sure. Let's see... maybe 1/1000 of a
solar luminosity and 1/10 of the mass so 100 times the lifetime. Put
in another factor of two for burning a greater fraction of the
hydrogen, so yes, a few trillion years looks right.

So what happens when their hydrogen stars running out finally? Do they
go through the same phases as stars like the Sun do?

I'm sure it has been studied, but I don't know the answer. My naive
guess would be they just slowly contract and cool, becoming white
dwarfs, once the hydrogen runs out. I'd not be surprised at all to
be wrong, though. A good source might be the book _Extreme Stars_ by
James Kaler. At least it has promising chapter titles, and he's a
good writer and knows the subject.

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