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Fraser
2004-Sep-16, 05:37 PM
SUMMARY: Mira stars are a special class of variable red giants which pulsate. Over the course of 80-1,000 days, a Mira star can vary in brightness by a factor of 10 times or more during the cycle. An international team of astronomers has observed the environments of five Mira stars, and found that they're surrounded by a shell of water vapour and carbon monoxide; this makes them seem larger than they actually are. These new observations bring the size of Mira stars in line with mathematical models that predict their size and composition. By observing Mira stars, astronomers will get a preview of the fate that could befall our own Sun when it bloats up to become a red giant in a few billion years.

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lswinford
2004-Sep-16, 07:49 PM
Another boundary layer problem. I remember hearing about the tropopause, the troposphere boundary layer that causes cumulus clouds to flatten out, or at least until some upward plume breaks through. There was talk here about the heliopause, a boundary layer for our solar system. The earth's crust is a comparatively cool boundary layer floating above the liquid or at least plastic mantle and core. The sun is supposed to be a ball of gas, but has features that appear more liquid or even solid in many respects as the intense balances of gravity, magnetism, radiant energy/heat, with physical flows and clumping of what appears to be mostly hydrogen, some helium, and a salting of heavier elements. It has an energetic and gaseous outer zone, the corona. But what if, as with the mira variables, the outer zone chills? As exemplified with the sunspot interactions we see today, the sun's surface is a comparatively cool zone, relative to the corona. Our sun today has a denser surface beneath a high temperature gaseous zone. But if our sun cools and the corona cools and fades, we would seen a denser outer shell. Then, when pockets or zones of hydrogen bubble up or collapse into the remaining sustained fusion zones and suddenly more energy begins to temporarily flow out again, before that bubble is exhausted and the lesser efficient fuel processes begin again, there may be that gaseous bubble pushing out the cooler, denser surface. The outward flow of heat then dissolves that comparatively cooler surface zone and the star's surface retreats like the foam on a freshly poured beer or soda pop.

Okay, that's my picture of what's happening. I'll watch to see how it gets sliced and diced.

Dave F
2004-Sep-17, 02:48 AM
Originally posted by lswinford@Sep 16 2004, 07:49 PM
Another boundary layer problem. I remember hearing about the tropopause, the troposphere boundary layer that causes cumulus clouds to flatten out, or at least until some upward plume breaks through. There was talk here about the heliopause, a boundary layer for our solar system. The earth's crust is a comparatively cool boundary layer floating above the liquid or at least plastic mantle and core. The sun is supposed to be a ball of gas, but has features that appear more liquid or even solid in many respects as the intense balances of gravity, magnetism, radiant energy/heat, with physical flows and clumping of what appears to be mostly hydrogen, some helium, and a salting of heavier elements. It has an energetic and gaseous outer zone, the corona. But what if, as with the mira variables, the outer zone chills? As exemplified with the sunspot interactions we see today, the sun's surface is a comparatively cool zone, relative to the corona. Our sun today has a denser surface beneath a high temperature gaseous zone. But if our sun cools and the corona cools and fades, we would seen a denser outer shell. Then, when pockets or zones of hydrogen bubble up or collapse into the remaining sustained fusion zones and suddenly more energy begins to temporarily flow out again, before that bubble is exhausted and the lesser efficient fuel processes begin again, there may be that gaseous bubble pushing out the cooler, denser surface. The outward flow of heat then dissolves that comparatively cooler surface zone and the star's surface retreats like the foam on a freshly poured beer or soda pop.

Okay, that's my picture of what's happening. I'll watch to see how it gets sliced and diced.
From what I understand the Sun is supposed to cool off slowly over the next few billion years as its available hydrogen supply starts to run out. Only when it stops burning and collapses somewhat does it start fusing helium atoms, and that's where the rapid changes start occurring.

That's not to say the earth won't be affected by this gradual change. Over time the Earth will cool off, barring other circumstances such as a fundamental change in the Earth's atmosphere. I don't know if that would cause every continent to tranform into Antarctica, but I'm sure the ice ages would be much less pleasant than they are now (geologically speaking).

dancd
2004-Sep-20, 11:20 AM
Mira (or Omicron Ceti) is probably a double star which pusate because a litle star is orbiting around. This second star will be included in the central star next period.
Just remember that a thermonuclear bomb does not blow up twice.

lswinford
2004-Sep-20, 04:34 PM
Dancd, but for stars to 'burn' their 'thermonuclear' bomb must not only blow up twice, but continuously. The fusion of stars must be a sustained fusion in the first place. However, if the fusion fuel changes, the heavier elements need more energy to do their fusion, and release less energy in the process.

I was picturing that a star doesn't just flip a switch from hydrogen one day and burn only helium the next, and then still heavier elements a little later. Perhaps the variable-ness of variable stars are the same things that make wood fires (or house fires, or auto fires) variable. Different fuels, with their varying burning requirements (in this case, temperatures, pressures, and the relative abundance of the particular fuel elements) cause one to flare up. A preponderance of helium in a hydrogen fusion zone may cause some of the helium to burn. A preponderance of burned helium products will cause some of them to make the next stages. But as those nuclei build and bond, they have used up several times their number of their higher, lighter precursor elements, which changes the porportions in that zone of the star. With so much of the residue of previous fusings having transpired, there is a larger quantity, say, of hydrogen to fuse again, now that the junk has coalesced. The hydrogen fusion that re-erupts does so with a greater energy release than the heavier elements had been doing. Now a fresh wave of heat has to bubble up through the star that had relatively cooled as it had previously burned batches of oxygen or carbon or iron. A variable star, then, is as a car whose engine drags down from carbon build-up or impure fuel, then backfires to partially clean some lines for a while until the next build-up and instability causes another booming backfire.