In my article from last week I promised to explain the origin of the most massive stars. All stars, massive or not, form in clusters and not individually. When the Sun was born, for example, it is thought to have been born with many siblings.
Most siblings would have lower masses and a few would weigh in at masses higher than the Sun. At most only 1 star forms with mass equal to 100 times the mass of the Sun, or 100 _solar_ masses. A natural question to ask is: is this the limit, or are there stars that are still more massive? And, which is the highest mass star that one can point to on the sky?
Even though the more massive a star is, the brighter it is, it turns out to be exceedingly difficult to see massive stars shining in the sky. This is because really massive stars have exceedingly short lifetimes. While the Sun will live for about 10 billion years, a 100 solar mass 'sibling' lives for only about 10 million years. After that, a massive star will blow up in a supernovae and likely leave behind a black hole.
So the most massive stars can really only be found by looking for their black hole remnants. Now then, where does a 30 solar mass black hole (not a 30 solar mass star) come from?
A 30 solar mass black hole must have come from a star with a mass well in excess of 30 solar masses, and probably closer to 100 solar masses or more. Such a star is close to the limit that physics will allow given that massive stars shine so strongly that the photons of light emitted from them will actually serve to push the outer parts of the star apart. This sets a kind of natural maximum limit on the mass of a star.
The only conceivable way around this limit is if the star is formed by a whole new route, which would be possible for stars made only from pure hydrogen and helium and not from any other element. Such environments are realized in the early universe, so it may very well be that the 30 solar mass black holes we detect today could very well be the remnants of the very first stars to turn on in the universe!