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Tom Mazanec
2015-Sep-26, 12:31 PM
What fraction of all the stars ever to be formed in the universe's lifetime have already been formed?
What fraction of planets?

antoniseb
2015-Sep-26, 01:45 PM
Only wild guesses here but I'd guess about 50% for the stars answers based on the amount of gas still available, and a presumption that once it gets sparse enough it will never form stars.
As for planets, there were eight formed (plus some unknown number of dwarf planets ... at least five), and that's all there ever will be. If you want to include exoplanets, we're probably back to the 50% answer again.

DaveC426913
2015-Sep-26, 09:02 PM
Only wild guesses here but I'd guess about 50% for the stars answers based on the amount of gas still available, and a presumption that once it gets sparse enough it will never form stars.

You are excluding destruction and reformation. I'm not sure if the OP is excluding them as well.
Our universe is on its third population of stars.

Our Population I Sun formed from the dust and gas left by the novae and destruction of many earlier Population II stars, which themselves formed from the dust and gas blown off the destruction of very early Population III stars.

We don't see Population III stars anymore, they mostly went in the first few billion years, but there are lots of Pop II stars out there.

So, the universe has quite a lot more start formation to do in its future.

DaveC426913
2015-Sep-26, 09:03 PM
Only wild guesses here but I'd guess about 50% for the stars answers based on the amount of gas still available, and a presumption that once it gets sparse enough it will never form stars.

You are excluding destruction and reformation. I'm not sure if the OP is excluding them as well.
Our universe is on its third population of stars.

Our Population I Sun formed from the dust and gas left by the novae and destruction of many earlier Population II stars, which themselves formed from the dust and gas blown off the destruction of very early Population III stars.

We don't see Population III stars anymore, they mostly went in the first few billion years, but there are lots of Pop II stars out there.

So, the universe has quite a lot more star formation to do in its future.

https://en.wikipedia.org/wiki/Stellar_population

Cougar
2015-Sep-27, 01:21 PM
What fraction of all the stars ever to be formed in the universe's lifetime have already been formed?

Tough to estimate! What we do know is that the rate of star formation peaked about 5-6 billion years ago.