Hydrogen and the formation of stars

[fly2cast]

This might be a dumb question and the answer may be obvious so I appologize if it is.

If our sun or any other star was made as a result of a previous supernova in which a star exploded after all of its hydrogen, helium, etc had been used, how would there be enough hydrogen left for any other stars to form? Wouldn't the hydrogen all get used up from previous stars?

[antoniseb]

Good question. In general, the Sun and stars like it are largely made of gasses that were never previously in stars, but were "snow-shoveled" (to coin a term) by the shock-fronts from supernovae. The shock-fronts also polluted the gasses that collapsed to form the Sun, but those pollutants are much in the minority.

[fly2cast]

Thanks. It makes sense. I'm guessing then in the distant future when enough stars have formed, all of the hydrogen will have been used up and no new stars will form.

[korjik]

Also: In most really big stars, the total amount of hydrogen that gets fused is a fairly small amount of the total mass. Most of the hydrogen gets blown back into space.

[baric]

Also: In most really big stars, the total amount of hydrogen that gets fused is a fairly small amount of the total mass. Most of the hydrogen gets blown back into space.

Dang. You beat me to that.

[cjameshuff]

Also: In most really big stars, the total amount of hydrogen that gets fused is a fairly small amount of the total mass. Most of the hydrogen gets blown back into space.

In fact, only the smallest stars are "fully convective", mixing the surface layers back into the core and eventually burning all their hydrogen. Stars big enough to explode blast away outer layers that are still rich in hydrogen.

[neilzero]

Likely the number of new (several solar mass) stars formed per century in our galaxy (and most other galaxies) is decreasing. In a billion times a billion years 10E18 the hydrogen shortage will mean only a few new stars all under 0.2 solar mass, which don't scatter much hydrogen. By then there may be no naked eye visible stars as seen from most locations throughout the Universe. Exceptions will be a few locations where star forming just began in the last billion years or so. There will be a few free flying protons even in a trillion times a trillion 10E24 years. When one is absorbed by an electron, you will have one lonely hydrogen atom, unless protons (and/or electrons) decay. Neil

[WayneFrancis]

This might be a dumb question and the answer may be obvious so I appologize if it is.

If our sun or any other star was made as a result of a previous supernova in which a star exploded after all of its hydrogen, helium, etc had been used, how would there be enough hydrogen left for any other stars to form? Wouldn't the hydrogen all get used up from previous stars?

Not a stupid question. There are 2 factors.
1) 2nd and 3rd generation stars pick up most of their hydrogen from the nearby space. IE there are regions where there is a lot of hydrogen out there that hasn't collapsed. These are called star forming regions. So its not that the hydrogen is all from the supernova
2) Not all the hydrogen gets burnt up. Depending on the type of star there will be different amounts of hydrogen. The nova/supernovas happen when a critical point is reached when there isn't a sufficient ratio of fuel to burn and the collapse of the core happens.

Thanks. It makes sense. I'm guessing then in the distant future when enough stars have formed, all of the hydrogen will have been used up and no new stars will form.

There is a LOT of hydrogen out there. The universe is still about 75% hydrogen and most of the remaining ~25% is Helium which is also from the big bang. Very little of the original matter of the universe has been converted.

[fly2cast]

Thanks for your replies. The stuff I have read about stars typically states that when the hydrogen "gets used up" it will slowly cool off or go supernova depending on its size. I was assuming that they meant all of the hydrogen gets used up. I did not realize that a lot of the hydrogen didn't fuse to become heavier elements.

[antoniseb]

... The stuff I have read about stars typically states ...

As you can imagine, the details vary depending on lots of factors, but in a simple article or textbook for school children, the focus is on where the heavy elements come from, not so much what else is involved, so there are a lot of side processes that get ignored.

[cjl]

Thanks for your replies. The stuff I have read about stars typically states that when the hydrogen "gets used up" it will slowly cool off or go supernova depending on its size. I was assuming that they meant all of the hydrogen gets used up. I did not realize that a lot of the hydrogen didn't fuse to become heavier elements.

That tends to be oversimplified, but in general, stars (aside from very small ones) are not well mixed. Stuff in the core tends to stay in the core, and stuff in the envelope tends to stay in the envelope. As a result, the core can be completely out of hydrogen and unable to fuse, while the outside of the star is still nearly 100% hydrogen. Usually, when stars run out of fuel, it's the core itself that runs out, not the whole star.

[Middenrat]

That kinda slants the oft-trundled out calculation of the Sun's life expectancy based on matter conversion rates correlated to its weight, I trust we're not all going to go Red Giant swimming next year

[cjl]

Nope. The standard calculations do account for the fact that only the core is fusing, and we have another 5-6 Gyr to go.