Is it possible for a star to gain mass throughout it's life?

[liubare]

Could a star gain more mass from elements that "collide" with it? If a ×-sized meteor crashed into the sun, does it ultimately accrete to the sun?

 

[Chronos]

A star steadily looses mass via fusion, but, also accretes mass. There is a limited amount of mass for any star to accrete, so the odds are very good it will eventually exhaust its fuel source.

 

[stargazer3]

While it is possible to gain weight for a star, this change is usually insignificant. One common exception happens in binary systems. Two stars evolve, one of them starts to enter a giant phase, and eventioally, as it's radius overflows the Roche lobe of a pair, it starts accreting mass on it's companion. And there you have it, a huge increase in mass of a second star! There's only one other realistic example of huge mass increase I can think of: in globular star clusters there is a possibility of stellar collision, which can lead either to a black hole or to a larger star.

 

[liubare]

While it is possible to gain weight for a star, this change is usually insignificant. One common exception happens in binary systems. Two stars evolve, one of them starts to enter a giant phase, and eventioally, as it's radius overflows the Roche lobe of a pair, it starts accreting mass on it's companion. And there you have it, a huge increase in mass of a second star! There's only one other realistic example of huge mass increase I can think of: in globular star clusters there is a possibility of stellar collision, which can lead either to a black hole or to a larger star.

Do we know of such a gravitational limit that would force mass to become a black hole? How big could a star actually get before it has no choice but to collapse on itself becoming a gravity well? This is slightly confusing since I once believed the early universe to be filled with "mega stars" that were very energetic and unstable, and mostly just hydrogen - the stars and planets today are just derivatives of the earlier stars.

 

[Chronos]

We do not know the critical mass limit before a star has no option but to collapse into a black hole. There is, in fact, an interesting mass gap between the biggest neutron stars [~2 solar mass] and smallest known black hole [~5 solar mass] that remains unexplained. The earliest stars that formed in the universe are called population III stars, and may have been extraordinarily massive compared to stars that formed later [pop I and pop II stars]. The reason for this is the universe was virtually metal-free [essentially no elements heavier than helium] until these oversized pop III stars burned through their fuel and exploded. For these extremely massive stars this would have occurred in just a few millions of years. We believe pop III stars were more massive than latter generations of stars because it would have taken longer for them to initiate fusion during gravitational collapse.

 

[acesuv]

A star steadily looses mass via fusion, but, also accretes mass. There is a limited amount of mass for any star to accrete, so the odds are very good it will eventually exhaust its fuel source.

So if a star were to consume enough mass, it could sustain fusion longer than it normally could?

 

[stargazer3]

So if a star were to consume enough mass, it could sustain fusion longer than it normally could?

Consider something continiously "resupplying" H2 onto a star, causing a delay in shifting it off the main sequence (the star wandering off the main sequence is like flying on a jetpack into volcano — while you'll still live for some time, it's sort of short compared to your age). This effect of "delaying He burning", however, should overcome the increase of the H2 burning rate (depends on pressure and temperature of the core), and the core, where the fusion takes place, is *very* sensitive to temperature. So quite the opposite should happen: if a star consumes too much gas, making it more massive, it will burn hydrogen faster through more energetic fusion cycles, so it's lifespan will be shorter. Although I'm not absolutely sure if it still holds for a small mass increases.