"shrinking" stars

[ihwip]

I have a question about small stars that are on the verge of losing their mass to the point that they become brown dwarves. How would this happen? If a red dwarf was rapidly losing its mass, how would this effect the light it emits and the nuclear reactions in its core? Would it suddenly wink out etc?

 

[dragon04]

Red Dwarves don't "lose their mass" in the way, say a more massive star does when it comes to the end of its fusion days. Red Dwarves are much less massive, and therefore, less mass is packed into its core. They fuse H2 at a much slower pace than Main Sequence stars.

Comparatively, a red dwarf might fuse Hydrogen for 100 billion or maybe a trillion years while our Sun gobbles up its fuel in 10 billion or so years. When a red dwarf can't fuse hydrogen anymore, pretty much just "turns off" without significant loss of mass.

 

[ihwip]

I should have specified that said red star would be losing mass by other means like a sibling star syphoning off its envelope etc.

This makes me wonder if there are other ways a star could lose mass beside the stellar cycle. Would it be possible for a red giant to blast off the surface of a companion star with the sheer force of its stellar wind? Could a nearby supernova blast off mass? Has there ever been evidence found of stars losing mass this way?

 

[MeteorWayne]

No, not really. Red dwarves will outlast the lifetime of Sol by hundreds or millions of times without losing any significant amount of mass.

They don't eject large amounts of mass from their surface, and basically reach the point where they stop fusing. They will never be brown dwarves, just slowly cooling red dwarves. The elemental composition sets them apart.

 

[3488]

No, not really. Red dwarves will outlast the lifetime of Sol by hundreds or millions of times without losing any significant amount of mass.

They don't eject large amounts of mass from their surface, and basically reach the point where they stop fusing. They will never be brown dwarves, just slowly cooling red dwarves. The elemental composition sets them apart.

Hi Wayne,

That's my take on it. IIRC the universe is not yet old enough for dead Red Dwarves, due to their immensely long life cycles.

IMO, a dead Red Dwarf will be as you say, they will be cooling, perhaps as they cool, become for a time 'Infrared Dwarves' & then be super mega massive 'Jovian' type rogue planets. But this is in the far far future.

Andrew Brown.

 

[Saiph]

you guys missed his laboration:

I should have specified that said red star would be losing mass by other means like a sibling star syphoning off its envelope etc.

The process is pretty simple, if enough mass is stolen/stripped away. The fusion reaction in the core decreases as the pressure drops, less hits per minute basically as the core environment thins out. This slowing reaction causes the temperature to drop, which also decreases the rate (but increases the pressure....)

Eventually it'll sputter and die, and the star will cool like a giant hot rock.

 

[eburacum45]

Bear in mind that most red dwarfs will get a lot brighter before they eventually go out.

from here
http://arxiv.org/PS_cache/astro-ph/pdf/ ... 1131v1.pdf

The steady luminosity increases expected by aging M dwarfs will have a considerable effect on the mass to light ration of the galaxy. For example, as a 0.2 M xSun sar evolves, there is a relatively fleeting epoch when the star has approximately the same radius and luminosity as the present day Sun.

This is because the density of the star increases as the helium builds up (just as it does in our own star).
..

 

[neilsox]

"This is because the density of the star increases as the helium builds up (just as it does in our own star)."
eburacum45
I know that is mainstream opinion, but counter intuitive that diluting the hydrogen fuel with helium in the core increases total fusion. (sort of like expecting lots of carbon dioxide to make fire burn better) Also increased fusion should increase the star volume thus lowering the density at the outer boundary of the core and out to the photosphere. This reduced density may speed the transfer of energy from the fusion site to the photosphere which takes 50,000 years in our Sun according to one source. Can someone clarify? Also why should fusion occur at the outer boundary of the core instead of near the center, unless the center is 99% helium and heavier elements? It seems to me that the helium ash would be scattered though out the volume of the Sun by slight non-symmetry, just as dense gases and vapors find their way from Earth's surface to the top of our atmosphere. Does plasma mix differently than molecular gas and vapor? Neil