As anãs vermelhas e castanhas são estrelas de baixa massa. São muito ténues e apenas as primeiras conseguem fazer fusão nuclear. Como é que estas estrelas morrem? É algo que não aparece muito na literatura. Decidi por isso falar com algumas pessoas para saber a sua opinião. Aqui estão as primeiras impressões. Já tinha a ideia de que as anãs vermelhas podiam morrer na forma de anã branca e que as anãs castanhas nunca podiam lá chegar. Pelas opiniões colhidas parece-me óbvio que as anãs vermelhas chegam mesmo à fase de anã branca (ao contrário do que diz o Geraint Lewis) e que as anãs castanhas nunca se transformam em anãs brancas. O tema continua nos próximos dias.
Low mass stars like red dwarfs and the even lower mass brown dwarfs just fizzle out at the end of their lives, and don't leave a white dwarf, just a rather dark corpse. It takes a more massive stars to leave a white dwarf, neutron stars and eventually black holes.
Geraint Lewis; Dept. de Física da Universidade de Sydney
It is thought that all stars except for the large mass ones will end their lives as white dwarfs. The lives of red dwarfs are so long that almost none of them have yet turned into white dwarfs. A 0.5 solar mass red dwarf is calculated to have a life of 56 billion years before it turned into a white dwarf, much longer than the currently accepted age of the universe!
A brown dwarf would not turn into a white dwarf because it would not have the mass or density to produce a planetary nebula and become degenerate. Brown dwarfs would just slowly cool into something like a massive Jupiter type planet. The exact mass limit between a brown dwarf and red dwarf is uncertain, but below about 0.08 solar mass, the fusion of H to He cannot occur.
James G. Hill, Rainwater Observatory & Planetarium
Yes, red dwarf's will end their life as a white dwarf. Any star which is big enough to have hydrogen fusion in the core (i.e. bigger than a brown dwarf), but not big enough to undergo a supernovae (i.e. less than about 5-8 times the mass of the Sun) will end as a white dwarf. A star on the main sequence (burning hydrogen in the core) is in balance between the force of gravity trying to compress it and the radiation pressure due to the fusion in the core. When the star runs out of fuel (the details of which are quite complex) there is no longer radiation pressure to fight of gravity and the star contracts. What eventually stops it is something called electron degeneracy pressure. Basically the Pauli exclusion principle limits how close two electrons in the same quantum state can be to each other. So this is the fate of all small stars, although for the very small stars the evolutionary process takes a very long time and the Universe is not old enough for them to have become white dwarfs.
And in the case of brown dwarfs? Do they end in something?
I don't know much about brown dwarfs (so this is my best guess, not hard facts). I think that the answer is no, a brown dwarf is too small to have nuclear fusion in the core and so it can't evolve the way a normal star does. I guess that over time they must get colder, but I think that they remain small clumps of dense gas which gradually cool over time.
Simon Ellingsen University of Tasmania
Red dwarfs should end as degenerate white dwarfs as i suspect would the more massive brown dwarfs. I am not sure about the lower mass brown dwarfs though, and would have to dig into the literatrure on that. It has of course not yet happened, as the main sequence as the time scales are too long. But i do want to look into this more.
Jim Kaler University of Illinois
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