Question for Astronomers regarding star formation

[Connoisseur]

I was watching yet another science channel show on star formation during my down time (yes I literally just watch science channel during off peak hours), it got me thinking about the theory on star formation. Here's my confusion:
1) Nebula are supposedly star "nurseries"
2) Nebula are the remnants of a previous supernova
3) A supernova occurs because the previous star did not have any remaining hydrogen for fusion and it eventually fuses heavier and heavier elements till it hits iron and then loses its ability to carry out fusion

So my question is this: If a nebula is the remnants (or at least outer layers) of a star that didn't have any remaining hydrogen, what is the source of hydrogen for new stars in a nebula? Shouldn't all of the remaining elements be heavier elements and thus be unable to start fusion? Does a star going nova still have a source of hydrogen somewhere or is more hydrogen created during the supernova process?

I'm sure this is a basic high school question, but I think it's worth asking. Google doesn't quite give me the answer i'm looking for.

 

[Fritzo]

Stars are coagulated balls of hydrogen. Matter tends to stick together when it's floating around like that, and when you get a ball of a certain immensity, the pressure in the center is so intense it "fuses" the hydrogen atoms together, turning two fused atoms into helium. This process is called fusion, and it produces a LOT of energy...heating up the gas around the star. This heat causes an outward pressure, keeping the star at a certain size and stability.

As the star ages and runs out of hydrogen, it starts to burn helium. Since the energy output of helium fusion is higher, the heat pressure causes the star to expand. When the helium runs out it will shed its outer gas shell, creating a small nebula.

Very large stars will have enough pressure to start burning lithium and expand even more. As they burn elements higher and higher on the periodic table, the sustained fusion reaction becomes less likely. When a large star runs down the periodic table and gets to iron, you're in trouble. Iron fusion does not create enough energy pressure to keep the giant gas ball around the core at bay. You get a big "slurp" and all of the gases crush into a neutron star. Matter in a neutron star is compacted so tightly, a single teaspoon of neutron star matter would weigh several tons.

Gigantic stars are even more interesting. There's an upper limit to the amount of mass you're allowed to have in one area of space at one time. These stars contain so much mass that when they collapse, the gravity they create effectively "rips" the fabric of space/time, creating a black hole. Gravity is a "dimple" in space, and the dimple these stars create is so deep it never ends.

Nebula that you are describing are formed by one or more supernova explosions. It's hard to picture how large a star can get, but one supernova could have enough hydrogen to create 100's of sun-like stars. Here's some pics to put that in perspective:

 

[Connoisseur]

I uhh appreciate the lesson in the star life cycle, but I know all of this pretty thoroughly and it doesn't answer my question. How can stars form in a nebula if the nebula is the blown out matter from stars who no longer have hydrogen to initiate fusion? Where is the hydrogen for new stars coming from?

 

[Fritzo]

I uhh appreciate the lesson in the star life cycle, but I know all of this pretty thoroughly and it doesn't answer my question. How can stars form in a nebula if the nebula is the blown out matter from stars who no longer have hydrogen to initiate fusion? Where is the hydrogen for new stars coming from?

There has to be enough hydrogen at the core of the star to sustain it. There's still a lot of free-roaming hydrogen in the star's outer layers. You may be referring to nebula that are just "glops" of gas that coagulated through the eons. These are your most likely star forming nurseries as they'll have as high as 97% hydrogen in them. Nebulas don't necessarily have to come from supernovas.

 

[SMOGZINN]

If a nebula is the remnants (or at least outer layers) of a star that didn't have any remaining hydrogen, what is the source of hydrogen for new stars in a nebula?

It is not that the star has burnt through all it's hydrogen, it is that it has burnt through all it's available hydrogen at it's core. There is still a huge mass of hydrogen in the shell that can't get down to the core due to the outward pressure of the fusion reaction. When the star runs out of sustainable fusion at it's core the star collapses and a whole lot of hydrogen rushes in, and there is a massive explosion flinging it all out and leaving only a small core behind.

You also have atomic decay creating hydrogen atoms but that is a much smaller source then the original source of hydrogen in the universe.

 

[phucheneh]

He's watching Wonders of the Universe, which I've seen a few times and wondered the same thing. Its explained as, star burns like mofo, star collapses and fuses new elements under intense heat and pressure, star burns like mofo some more, rinse repeat.

I had always assumed that the various expansions and contractions of the star keep blowing out lighter elements. It seems like, yes, eventually you would run out of lighter elements...

...until you look at the info on the elemental composition of the universe. Hydrogen and helium are just too abundant to 'run out.'

 

[Paul98]

I uhh appreciate the lesson in the star life cycle, but I know all of this pretty thoroughly and it doesn't answer my question. How can stars form in a nebula if the nebula is the blown out matter from stars who no longer have hydrogen to initiate fusion? Where is the hydrogen for new stars coming from?

Because that isn't correct.... type 2

 

[The Boston Dangler]

a nebula is just a gigantic space cloud. supernova nebulae are much smaller than the primordial clouds that created first-generation stars.

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

These youngest stars, including the Sun, therefore have the highest metal content, and are known as Population I stars.

that means the sun is a 3rd generation star.