Low mass post-main-sequence on HR diagram

[Zorba]

Can anyone describe to me the path a low mass body takes as it leaves the main-sequence? Or is it the case that a low mass body just moves along the main sequence, down to the right?

 

[mathman]

Lower mass stars end up as white dwarfs when they run out of fuel.

 

[Nik_2213]

Can we still count 'Brown Dwarfs' as Main Sequence ??

 

[Vanadium 50]

Brown dwarfs are usually not considered stars, so they are not on the main sequence.

There is no observational evidence as to what red dwarfs do when they leave the main sequence, since the universe is not old enough to have any examples. For the same reason, it's not been of intense theoretical interest.

We know they won't form white dwarfs as we know them, because they are too small to fuse helium. I imagine that they travel vertically up the H-R diagram until they run out of fuel, and then form a cold ball of helium, possibly with an electron degenerate core.

 

[qraal]

Can anyone describe to me the path a low mass body takes as it leaves the main-sequence? Or is it the case that a low mass body just moves along the main sequence, down to the right?

This paper... http://adsabs.harvard.edu/abs/1997ApJ...482..420L" ...describes the late-time evolution of very low mass stars.

But there are many other papers available on the ADS, the NASA Astrophysical Database Server. Try this search result for just a few...
http://bit.ly/jfiGfM".

 

[Nik_2213]

Thank you for the link to that fascinating article on the end of the main sequence.

 

[harcel]

Brown dwarfs are usually not considered stars, so they are not on the main sequence.[/QOUTE]

That is not a very good reason. In principle we can put any object in the Universe we like on the HR diagram, even though they have nothing to do with stellar evolution. Besides: brown dwarfs get their energy from gravitational contraction, just as some stars do in some parts of their life, so I would think they fit perfectly well on there in the same context.

There is no observational evidence as to what red dwarfs do when they leave the main sequence, since the universe is not old enough to have any examples. For the same reason, it's not been of intense theoretical interest.

There is _plenty_ of theoretical interest in low mass stellar evolution. The Universe is not old enough, so indeed we cannot find these objects, but that does not mean that we cannot calculate these objects. And we do. If stars leave the main sequence (i.e. did _did_ burn hydrogen in their core), they would indeed move upwards, along the Hayashi line: their core contracts, their envelope grows, so it cools down. They will become small variations of red giants. If they do not have enough mass to fuse helium, I guess they would just fade from there (and do some deuterium burning first).

Brown dwarfs will never move upwards, as they won't run out of the hydrigen they cannot even start burning. They will just cool down and fade.