Right now about 95% of the universe is hydrogen or helium. How much longer will it be until stars have used up all the hydrogen and helium and cannot perform fusion anymore? Can stars exist without hydrogen or helium? I have read that iron is the largest possible atom formed by fusion, so could there be stars hundreds of billions of years from now when there is alot less H & He that are formed completely out of atoms like nitrogen and argon or do you need hydrogen and helium?
Well, iron (specifically iron-56) is the most stable atom. Larger elements are created when twin-star systems go supernova and so much fusion energy is introduced (by matter falling onto the surface of a white dwarf, etc.) that it gets kicked upstairs, all the way to uranium.
It’s an interesting question. I’d say without fear of contradiction… a long, long, long time, exceeded only by proton decay.
Anything below Iron-56 on the curve of binding energy will fuse, given the right circumstances (actually anything above it will, too, but it’s an endothermic reaction, requiring net input of energy).
Fusion will occur at the right temperature and pressure, with minima different for every isotope. Of the lot, the lowest temperature appears to be Tritium (H-3) and Helium-3. Deuterium is slightly higher, and “normal” Hydrogen-1 (Protium) higher yet. But that’s still at the bottom of the sequence.
To cause 3 helium atoms to fuse into carbon takes much hotter temperatures, and hotter yet to get to oxygen, silicon, etc. It’s been estimated that massive stars get hot enough at the core to fuse silicon into iron for only about six hours before hitting the catastrophe that induces a supernova, and less massive stars will never get that hot.
I don’t have actual temperatures to report here, only the fact that successive core collapses with consequent higher temperature and pressure are necessary to fuse the higher elements in the H-1 to Fe-56 sequence.
As for the basic question, stars are perpetually forming from hydrogen clouds, and presumably there will be a substantial residual of uncoalesced hydrogen far into the future.
Red Dwarfs are the longest lived stars.
The usual estimate for the hydrogen burning lifetime of red dwarfs is about 100 billion years, so find the date at which stars stop forming, and add 100 billion.
Star formation in the Milky way:
-Of course that doesn’t tell us how long it’ll take the remaining 10% to be swept up into stars.
Huh. I thought white dwarfs held that record. Oh, well…
/hijack/
If, near the end of the Universe, all the dead (Iron) stars were to coalesce into one big mass, what would happen? More fusion?
I don’t think it would be fusion. What you’r describing, as far as I know (which isn’t all that far) is what would happen if the expansion of the universe is halted by gravity. Once stopped it would fall into itself and produce another Big Bang.
I don’t think you can call this either fusion or fission. My guess is that in forming a new universe the rules would be different enough that we can say nothing about what it might be doing and so cannot name it other is some general manner such as, “Nonspecific Particle Energy Production Mode.” Or something.
Only in the same sense that vampires can live for a long time. White dwarfs are already dead. They only glow because they’re still hot from the star-death which formed them, but they’re cooling off. As they get colder, they get dimmer and dimmer, until they effectively wouldn’t glow at all. Unless they collide with another star, or something, they’ll continue to exist indefinitely, but they won’t be all that impressive, and not exactly “live”.
On another note since Iron is the largest atom formed by fusion where do all the atoms with atomic numbers 27-92 come from?
Nevermind.
Note that while massive stars can create elements up through iron, they can just as easily destroy them. During a core-collapse supernova, the temperature near the center of the star can reach around 10[SUP]9[/SUP] K. When it’s this hot, the core is replete with gamma rays with enough energy to actually break iron nuclei into helium nuclei in a process called photodisintegration. Much of the heavy element content of the core, millions of years in the making, is lost during the very end of a star’s life.
So I would say that the length of time that the universe can continue to perform fusion is not limited by the abundance of fusable atoms; iron can be recycled into helium. The universe is much more likely to first run out of the energy necessary to fuse those atoms.
That’s okay, at best mine was a half-explanation. A google on “supernova uranium” will discuss it in greater detail. Essentially, a white dwarf absorbs additional matter from its binary-partner, passes the Chandrasekhar Limit (about 1.44 times the mass of our own sun), undergoes catastrophic collapse (a type 1a supernova) during which elements heavier than iron are created and then dissipates these elements to the universe, where billions of years later they might collect on newly-formed planets and eventually formed into pipes (“plumber” is from a Latin word for lead), worn as jewelry or burned in reactors.
Subrahmanyan Chandrasekhar won a well-deserved Nobel Prize for all this, by the way.