FLPhotoCatcher wrote:I know that most of the mass of the dust in the pictured nebula is hydrogen, but what if less than 25% was hydrogen? Has anyone figured what would happen if dust that low in hydrogen combined to make an object approximately the mass of our sun? Would you have something that looked and acted like a star?
I am not
the best person to answer that question, but what I do know is that hydrogen is fantastically good at producing energy through fusion. All other elements (except perhaps lithium) require higher temperatures before they will produce energy through fusion.
So I guess that a universe that contained only 25% hydrogen, but contained the same mass as our own universe does, would be generally worse at making stars. By "stars", I mean objects that shine by their own light through fusion. Or perhaps the universe would have been just as good at making stars as it is today, but its stars would have had shorter lifetimes. Since stars spend most of their lifetimes fusing hydrogen into helium, the 25% hydrogen stars would use up their hydrogen sooner and have shorter lifetimes than the 75% hydrogen stars, or so I think.
The reason why massive stars in our 75% hydrogen universe have short lifetimes is because their cores get so hot due to their great mass, and they have to fuse their hydrogen at a furious rate to combat their own gravity. In a 25% hydrogen universe, a 25% hydrogen massive star would still have to produce a lot of energy to fight its own gravity and prevent itself from collapsing, but now it would have only a third of the hydrogen available to fight its own great mass.
So if our own Sun had started out life with the same mass as today, but with only 25% hydrogen, its core temperature would have been the same as today. The Sun would also have had to fuse hydrogen at the same rate as today in order to fight its own gravity, but since it would contain only a third of the hydrogen that our own Sun does, its core hydrogen might have been used up three times faster. Humanity might not have evolved fast enough in a 25% hydrogen universe to see a stable main sequence Sun. The 25% hydrogen Sun would have turned into a red giant with no difficulties, and if the Sun's core helium supply had been considerably greater than it will be in our present-day 75% hydrogen universe, it is just possible that its red giant lifetime (where stars primarily fuse helium) might have been longer.
But I think that on the whole, the Sun would have been a less suitable host for habitable planets in a 25% hydrogen universe than it is today.