Expert: Jayendra Upadhye - 2/18/2009

Question
What is the complete life cycle of a normal star, starting
with a protostar and ending with a black dwarf?

thanks.

Answer
Hi,
The protostar.
In this phase of the star's life, the star has an accretion disk.
Depending on the prevailing conditions of the stellar nursery, it will have either an accretion disk of gas alone, or a differentiated disk of inner solid matter, and outer gaseous matter.
If a large star in the gas cloud seeds it with Solid matter such as silicates and Oxides of Alkalis etc towards the end of its supernova phase, and ends up triggering the star formation by its shock wave, there will be dense silicates and other salts in the inner rings of the accretion disk, leading to rocky inner planets. The silicates and salts (and water) will form as the supernova debri cools as it expands, and reaches the recombination temperatures of each compounds. The solids loose their kinetic energy to the gas in the accretion disk, and descend inwards as in a cyclone separator. My guess - mind you - not proven theory, is that as a rule, all stellar systems should have inner rocky planets wherever solids are part of the accretion process.

mid life.
The rest of the life of the star will follow the main sequence path of all sol like stars over 10 to 15 billion years. The smaller the mass, the longer the life.

end phase.
There will be the helium burning phase, the planetary nebula, and the remnant white dwarf.
The dwarf will take a time longer than the known age of the universe to cool!!
For to date, no cool white dwarf has been observed anywhere.

So black dwarf's are out of question.

Yes, if the protostar ends up as a brown dwarf, the brown dwarf itself will run through its fusion life in a few million years, deplete most of the lithium in its entire volume by convetion and fusion to helium. There will be no helium flash marking the start of the helium burning phase, and it will slowly radiate away its energy by continued gravitational slow collapse.
As predicted for supernova debri, Methane and alkali salts have already been found including water, on cool brown dwarfs! (The dwarf GD 165B of spectral class T).
Interestingly, all brown dwarfs are close to jupiter in their diameter if not density! And jupiter itself radiates more energy than it recieves from the sun!!

refer:- http://en.wikipedia.org/wiki/Brown_dwarf

regards
Jayen