Expert: James Gort - 9/27/2011

Question
Hi,
 I have a hypothetical question. Ok lets start with a large 1st generation star. Now lets say that soon after it was created it was sent away on a path of its own that will never take it anywhere near anything, it is and always will be completely isolated.
Now lets say that the star lives out its life until it finally goes supernova.  Now eventually the remnants for a 2nd generation star consisting of heaver elements.
Is this correct so far?
OK now again lets say that it lives its life and then also goes supernova.
Will there be enough elements left to form a 2rd generation star from only this one star? Granted, I would assume that the star would become a less massive star each life cycle  but since most of the time stars form in clusters or at least star forming regions where they probably feed off each other and the "community" gas do they have enough on their own to keep continuing the cycle?
If so how many generations could a large star go before it did not have the mass to become a full blown star on its own?
Now I understand that at a certain point it may get to a point where a black hole or neutron star would form which would form but lets just ignore that if we can and hypothetically assume all the expelled gas from the exploding star staying in that same small region.
Am I making any sense?
lol
Thank You

Answer
Hi Amy,

Interesting hypothetical situation. Although your assumption that a second generation star needs a first generation star if it is to have heavy elements is correct, a second generation star could certainly form even it was far removed from first generation stars and didn't have any supply of heavy elements. It would form (just as massively) from the primordial hydrogen gas clouds that permeate space (as a relic of the big bang). So as long as the cloud was massive and dense enough to collect and condense by its own gravity (meets or exceeds the "Jeans Criterion"), then a star (or star cluster) will form. If that second generation star didn't have its supply of heavy elements, it wouldn't be blessed by cold (terrestrial-like) planets, however, which need heavy elements to be gravitationally stable. As long as the contracting star has a mass greater than about 0.072 solar masses, it will ignite thermonuclear reactions. Below that mass, it will be a brown dwarf.

So I hope that answers your question. A second generation (or third generation) star doesn't need "close" supernovae in order to form - it can still form and be "heavy element poor". But most second (and above) generation stars do have at least small amounts of heavy elements from those first generation donors.

As as final note, suppose there was no primordial gas cloud, and your single lonely massive star went supernova. Could the released gas be sufficient to form a second generation star just from the remnants of the supernova? My guess would be probably not. The gas expelled from a supernova has extremely high velocity. According to the "Jeans Criterion" for gravitational collapse, the thermal energy of the gas must be less than the gravitational energy of the gas molecules. That is not likely to occur - the gas is moving too fast to contract to form another star (even a small one). So what would normally happen is the fast moving molecules would eventually hit a "stationary" hydrogen cloud, slow down through collisions, and "seed" the cloud with heavy elements. That "seeded" cloud might then collapse to form a second generation star (based on the mass of the cloud and the "Jeans Criterion").

Cheers,

Prof. James Gort