My friend was asking me yesterday what stars were made of. I had to tell her that I had no idea, since I was hardly an expert on astronomy. But does anyone here know what they’re made of?
I think they’re made of gas and energy myself, but would like a more detailed explanation, or the correct one if thei is wrong. Thanks in advance! 
F_X
A star is pretty much a big ball of hydrogen and crap. It’s so big that the immense gravitational force it creates makes the gas undergo nuclear fusion as it gets compressed under its own weight, and things start to get somewhat hot.
Well, that’s my layman’s take on it.
Stars are made out of atoms and atomic nuclei… just like you or I. Old stars (sometimes called Population I) do not contain much else other than Hydrogen and Helium as they were theoretically created before many of the other elements were around (incidentally, they are all made by stars). Population II stars contain small but not trivial amounts of other atoms, ions, and neuclei. Generally in the upper atmosphere there is more chance for the atoms to be less ionized (surrounded by more electrons). In especially cool stars you even see some chemical compounds! As you approach the core plasma (that’s basically ionized gas) is the dominant state of matter.
There’s a basic onion skin model for stars as they get older. The core of the star is where fusion happens (it’s at the hottest temp and highest pressure) to keep everything warm and cozy here on earth. Initially that means that Hydrogen fuses to Helium. It can’t go farther because there’s not quite enough pressure to convert into usuable activation energy for fusion. Eventually the core runs out of hydrogen and will start burning Helium to Carbon while outside of that there will be Hydrogen burning. In a similar fashion you get layers of Oxygen and Nitrogen neuclei, Silicon, Neon, and other layers. That’s the onion skin. Stars the size of our sun and smaller usually cannot get beyond Oxygen neuclei and eventually puff out their outer atmospher in what’s called a planetary nebulae before becoming white dwarfs and slowly dying out.
For larger stars, when you get to the onion skin layer of 26 (Iron) on the periodic table, fusion is no longer exothermic. Suddenly, everything begins to change and dramatic processes take place… the long and the short of it is something called a supernovae. It’s a rather spectacular way to die, if you’re a star. The explosion is often considerably brighter than all the stars in the galaxy. That’s a lot of energy. During that explosion all the elements beyond Iron are made that we find in nature.
As Carl Sagan said, “We are starstuff!”
Backwards, actually; Pop II stars are the older, low-metallicity ones. Another historical quirk of astronomical nomenclature.
Minor nitpick, but planetary nebulae develop from stars up to 5-8 solar masses, including a lot more stars than just those the size of the Sun and smaller.
Just to throw some numbers into the mix, most stars are roughly 3/4 hydrogen and 1/4 helium, reflecting the abundances in the universe as a whole. Heavier elements make up much less than a percent by number of nuclei, and up to 2-3% by mass, of a typical star. A star like the Sun will fuse roughly 10% of its hydrogen over its lifetime.
Stage presense, personality, and a winning smile.
What?
All together now!
“The sun is a mass of incandescant gas,
a gigantic nuclear furnace…”
[sub]Deja vu[/sub]
Thanks, guys. I think I’ll just email my friend the link to this thread, as I can imagine trying to explain all the details in JS Princeton’s post to her on the phone.
“Atoms, ions, nuclei, and what else again?”
“What do you mean by all that?”
etc., etc.
F_X
Sorry about the Pop I, Pop II mixup. I always do that!
It’s kind of like Early Type galaxies being younger than Late Type. So backwards!
Another nitpick then: stars more massive than that can make PNe too.
You’re hardly alone - this is just a varient of the Baconian Paradox which has perplexed many over the centuries. From The Advancement of Learning:
[Carl Sagan]
They are made of “star stuff”… and we are all made of “star stuff”…
[/Carl Sagan]
*queue Cosmos theme.