In the beginning, the only elements in the universe were hydrogen, helium and lithium, if I’ve been told right.
Heavier elements were created in the intense pressures of dying, collapsing, pre-supernova stars of the first generation of stars post-Big Bang. Then the star’s explosion scattered those elements hither and yon, making them available to later solar systems for use as asteroids, amino acids and toaster ovens.
But (again, if I understand what I’ve read properly) it took a few stellar generations to create our current inventory of 89+ naturally-occuring elements.
But (and here’s my GQ) cosmologists say the universe is only 15 billion years old at most. And don’t most stars spend 5 billion years or more on the Main Sequence?
It seems to me that the universe isn’t old enough to have created so many heavy elements. How is this explained? Were the first few stellar lifetimes shorter than those of today? Is this something they’re still trying to figure out?
I’ve wondered this as well. My only guess is that not all stars take 5 billion years to burn out. IIRC the bigger the star the faster it eats its fuel and goes FOOM. I believe some stars have lifetimes on the order of only a few 100’s of millions of years.
Now, if I’m right about that to allow several generations of stars in the Universe’s lifetime, is there any validity to the notion that the early Universe was populated by giant stars? On that point I have no idea.
The really big stars, O,B,A which release most of the heavier elements into space can spend as little as a few million years on the main sequence before they start exploding.
Given the way these stars evolve, building up concentric layers of heavier and heavier, less and less energetic, fusion reactions, it should only take a single generation to produce most of the heavy elements. To get those elements distributed back into another batch of stars could take another generation of two.
Astronomers are still trying to figure out the details of all this. For example, the initial composition from the big bang still isn’t known exactly which can cause trouble when stars are found that have compositions that seem to indicate that the are older than the universe itself. (This result is obtained by back extrapolating stellar abundances of elements in back time to match what’s found in the spectrum of a star. The odd age implies that the extrapolation, and hence the numbers for element abundance versus time are wrong).
I recall reading somewhere (don’t ask for a cite… 'cause I can’t give you one…) that said earlier in the Universe there was a larger population of large stars that burned out quickly and went supernova…
Maybe this helps to explain why there are so many heavy metals around now…
Thank you, Squink. That makes sense. And I’m going to guess that maybe larger stars were more common early on, because the universe was smaller and the original matter was closer together, so you had larger original accretion disks?
So in the first stellar generation, larger, shorter-lived stars were more common than now, so the first several stellar generations only took a few billion years? And yielded all the heavier elements, yes?
There are still stars from the first generation around (called Population II stars, for historical reasons), but only the smaller, redder ones have survived that long. They’re mostly out in the globular clusters scattered in a sphere around the galaxy, but there’s a few closer in, too, including the next-closest star after Alpha Centauri, Barnard’s Star. Red dwarfs are expected to have lifespans in the trillions of years, and the Universe is only somewhere around 15 billion, so they’ll be with us for a while.
Now, I’m not dumb, but I can’t understand, why the stars from the first generation would be called “Population II” stars.
Also, how can we know certain existing stars have been around for that long? Math?
Wait, wait, wait. There are stars that old in the Milky Way galaxy? I had no idea our galaxy could be that old! Are all galaxies assumed to have been formed during the first few million years of the universe?
Well, our galaxy wasn’t necessarily one of the first to form (I don’t know the numbers offhand), but when it did, it was from fresh material: Whole galaxies don’t recycle, at least not yet. Therefore, the oldest stars in our galaxy can be considered first generation.
The reason the older generation is called Population II is just that they were discovered after Population I stars like our Sun. Of course, originally Pop I stars were just called “stars”. Then someone realized that those stars in the globular clusters were different somehow than most of the stars near the Sun, and so called them Populations I and II. Finally, after the name had been around a while and astronomers got used to it, someone else realized the reason why Pop II stars were different. Like I said, historical reasons.
Don’t even ask why the spectral type sequence is OBAFGKM.