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Lord Jubjub
2005-Feb-12, 01:13 AM
Just read an article on space.com that mentioned that Rigel is a red giant because it has started fusing helium and has expanded due to the increased energy output.

So if I really wanted to have an advanced fusion reactor I would want to use helium because I could get more energy, right? Of course, I would need large amounts of energy to start the fusion, but it would repay me faster than a hydrogen reactor would. What fusion reaction gives the highest energy? Is it helium?

I really don't think anti-matter would work because you have to use more energy to create than you would get out of it.

Secondly, when astronomers say that Rigel will likely die in the next few million years, are they saying it will become a white dwarf or will it supernova?

The Bad Astronomer
2005-Feb-12, 01:33 AM
Rigel isn't a red giant. It's a blue supergiant (http://www.astro.uiuc.edu/~kaler/sow/rigel.html). Do you mean Betelgeuse (http://www.astro.uiuc.edu/~kaler/sow/betelgeuse.html)? That's a red supergiant.

Anyway, it takes a huge amount of pressure to fuse helium, far more than can be done here on Earth, unless there is a catalyst that can be found.

Lord Jubjub
2005-Feb-12, 02:50 AM
oops, you're right. Somewhat Rigel and Red got conflated in my thinking.

Anyway, I was thinking about Sci-fi type technology--not current technology.

Brady Yoon
2005-Feb-12, 03:21 AM
Secondly, when astronomers say that Rigel will likely die in the next few million years, are they saying it will become a white dwarf or will it supernova?


Betelgeuse is really on the borderline. It's mass seems to be around 15 solar masses, so it's either way. It can either become a supernova or a neon oxygen white dwarf. On the other hand, Rigel will probably become a supernova because it is slightly more massive. The only reasonable close star that will definitely become a supernova is Deneb, which is substantially more massive than Rigel or Betelgeuse.. Hope that helps

Hamlet
2005-Feb-12, 03:34 AM
IIRC, hydrogen fusion in a star can begin when the core temperature reaches about 8-10 million K. Helium doesn't fuse until around 100 million K. So you can see that it's much harder to get helium to fuse than hydrogen.

A star like Betelgeuse is in a late evolutionary stage having converted the hydrogen at its core to helium and fusion has ceased. Once this occurs there is no longer outward pressure to balance the compression from gravity. The core begins to collapse along with the outer layers of the star. As the layers and core collapse, the gas is compressed and heated until a layer of hydrogen just above the helium core gets hot enough and dense enough to begin fusing.

The star continues to collapse and heat up until the core gets up to around 100 million K. At this temperature helium fusion can begin to fuse and eventually there is enough outward pressure to stop the collapse. The combined energy from the hot helium fusing core plus the energy from the hydrogen fusing shell causes the outer layers of the star to puff out and the star swells in size.

So Betelgeuse is outputting more energy now than when it was on the Main Sequence and it will continue to do so until all of the helium in the core has been converted to carbon and oxygen. The star will again go through a phase of collapse and heating. If Betelgeuse has enough mass it may be able to begin fusing the carbon/oxygen core, plus having a shell of helium fusing above the core and a shell of hydrogen fusing above the helium shell.

I'm not sure if Betelgeuse has enough mass to fuse elements all the way to iron and produce a supernova.

Maddad
2005-Feb-12, 04:49 AM
So if I really wanted to have an advanced fusion reactor I would want to use helium because I could get more energy, right?No sir. Per gram of fuel, hydrogen fusion gives you more energy than any other process. The energy output steadily declines as the mass of the reactant increases towards iron, at which point you get zero energy out of fusion. In the opposite direction, you get less energy from the fission of lighter reactants as you approach iron.

The reason that a helium fusing core produces more energy than a hydrogen fusing core is that far more helium mass fuses each second than does hydrogen. A 25 solar mass star fuses hydrogen for 7 million years (40 million kelvin). However, it fuses nearly that same mass in helium fusion in only 500,000 years (40 million kelvin), 14 times faster. It then fuses carbon in 600 years (600 million kelvin), neon in one year (1.2 billion kelvin), oxygen in six months (1.5 billion kelvin), and silicone in one day (2.7 billion kelvin). Since each successive stage releases more energy per second, the only way to get those numbers is if you get less energy per gram of fuel as you move away from hydrogen.

Brady Yoon
2005-Feb-12, 05:18 AM
So if I really wanted to have an advanced fusion reactor I would want to use helium because I could get more energy, right?No sir. Per gram of fuel, hydrogen fusion gives you more energy than any other process. The energy output steadily declines as the mass of the reactant increases towards iron, at which point you get zero energy out of fusion. In the opposite direction, you get less energy from the fission of lighter reactants as you approach iron.

The reason that a helium fusing core produces more energy than a hydrogen fusing core is that far more helium mass fuses each second than does hydrogen. A 25 solar mass star fuses hydrogen for 7 million years (40 million kelvin). However, it fuses nearly that same mass in helium fusion in only 500,000 years (40 million kelvin), 14 times faster. It then fuses carbon in 600 years (600 million kelvin), neon in one year (1.2 billion kelvin), oxygen in six months (1.5 billion kelvin), and silicone in one day (2.7 billion kelvin). Since each successive stage releases more energy per second, the only way to get those numbers is if you get less energy per gram of fuel as you move away from hydrogen.

But if hydrogen burns longer, and the star is less luminous at the time, doesn't that mean hydrogen fusion releases less energy?

Brady Yoon
2005-Feb-12, 05:19 AM
The reasoning behind what you say is definitely clear. I'm probably missing something. :)

Tobin Dax
2005-Feb-12, 05:36 AM
But if hydrogen burns longer, and the star is less luminous at the time, doesn't that mean hydrogen fusion releases less energy?

Remember, though, that luminosity depends on surface area as well as temperature. Red giants are brighter than the corresponding main sequence starts because of that-their convective envelopes of unburned hydrogen expand by a factor of about a million times. So the luminosity itself isn't a good measure of which method is better. The temperature could be, but the luminosity depends on more than just that.

Brady Yoon
2005-Feb-12, 05:45 AM
Thanks for correcting me. How could I forget the radius luminosity temperature relationship? #-o

Romanus
2005-Feb-12, 08:57 AM
Three items:

1.) For stars as massive as Rigel and Betelgeuse, there often isn't a dramatic difference in their luminosity from the main sequence to blue or red supergiantdom; these stars tend to "move" horizontally across the HR diagram. The luminosity increase is much more marked for stars like the Sun.

2.) When a star does increase in luminosity as a red giant, it really isn't because helium is a more efficient fuel--it definitely isnt. It has to do with the higher temperature in the core being able to jump-start fusion in a shell of previously non-burning hydrogen around the core. This increases the energy output of the star, and thus its luminosity.

3.) Helium fusion is much less efficient than hydrogen fusion; the energy per unit reaction is less, which means that more helium is required to generate a given amount of energy compared to a hydrogen fusion reaction. It also requires much higher temperatures, because the repulsion between the nuclei is greater; helium fusion even in the laboratory will probably be out of our league for many, many years.