Wednesday, February 13, 2008

Super-Asymptotic Giant Branch Stars

As I promised in my last post, here's an attempt to explain why in the world I would travel to London for a one-day visit. The challenge is to see if I can both explain the science and keep this post reasonably short.

First, we need to talk briefly about how stars work. Stars shine by nuclear fusion. They spend most of their lives turning hydrogen into helium. But eventually the hydrogen fuel runs low, and the star starts to burn helium into carbon. Almost all stars bigger than half the sun's mass go this far.

For stars up to about 7 times the mass of the sun, this is as far as the star gets. Once it has burned all of its helium, it ends its life, throwing off its outer layers as a planetary nebula and leaving behind a glowing lump of carbon ash called a white dwarf.

For stars more massive than about 10 times the sun's mass, the star is big enough to start fusing the carbon into oxygen, neon and magnesium, which then ignites and fuses into silicon and related elements, which then fuse to make iron and nickel and related atoms. And then the star explodes as a supernova star, spewing most of these elements out into the universe.

My story above, though, has a bit of a hole in it. I said that stars smaller than 7 times the mass of the sun make carbon white dwarfs, and stars bigger than 10 times the mass of the sun explode after fusing elements up to iron. But what happens to stars that are 8 or 9 times the mass of the sun? These stars are big enough to burn carbon into oxygen, neon and magnesium, but they are not big enough to fuse these elements into silicon and iron.

From the standpoint of theory, it is possible to make a white dwarf out of oxygen, neon and magnesium. It is also possible to have oxygen, neon and magnesium explode in a special kind of supernova explosion. But we don't know which of these scenarios happen.

Our meeting in London was therefore to discuss these stars. We talked about what we know about these stars between 7 and 10 times the mass of the sun, and what we don't know about these stars. Frankly, there are a lot of mysteries, and not a lot of answers. What happens to these stars depends a lot on how fast the stars are rotating, how quickly they are shedding their outer layers, and how much the inside of the star is mixed up by the slow boiling that happens inside many stars. But these mysteries are what makes the science interesting, and will keep me working for years to come!

I talked about the white dwarfs I have been studying, and a British group talked about their studies of the types of stars that explode as supernovae. If we combine our two areas of study, it seems that these stars must explode and not make white dwarfs. But there is a lot of careful study we need to do before blindly combining our work! I may make different assumptions than the other group which can affect the outcome of our data, or maybe one or both of us have made mistakes in our analysis. Only time will tell!


  1. You wrote: "Stars shine by nuclear fusion.", "sans attribution"!
    Is this a fact you know?! At least
    a citation be required therefor, I
    believe. And, I have a letter from
    the late Dr. Hans A. Bethe, confirming that I had found and corrected a quantitative error in his Nobel Prize Lecture in Physics for 1967 entitled "Energy Production in Stars", so I am not joking when I challenge your reputedly authoritative assertion! If you'd like, I could bring in the original for you to the P-M-A Library. Just ask Mr. Dave G. or Ms. Molly W. if they ever see
    me in there - really studying - please?!
    In Summary, professional Astronomers have to more carefully distinguish fact from theory, especially when dealing at my lower level of intellectual comprehension (sic[k]!), I believe!

  2. Anonymous11:33 AM

    Re: Comment by joseph1110.

    I would have thought that this comment was at best unnecessary as the phrase "Stars shine by Nuclear Fusion" is as contentious as "Stars twinkle in the night sky".

    Best regards, Brian