Monday, April 16, 2012

How to make iron (6) Red giants

Red giant Mira with white dwarf companion
Credit: X-ray: NASA/CXC/SAO/M. Karovska et al.; 
Illustration: CXC/M.Weiss

Well, this really is not a simple subject but truly interesting to try to understand what scientists are telling us.

We have so far seen Sun and placed it in the main sequence with hydrogen to helium fusion in its core.
We have learned that bigger mass stars have a more complex and effective nuclear process called carbon cycle which produces carbon and oxygen.

So far so good.

Now, in a nice simple line of evolution we would get to the next step in God's creation, where the lego pieces, atoms and their sub-particles fuse into even heavier elements, right?


Professor Kaler tells us that in the evolution of the star there is a complication at this point. Hmm... really not so simple, this stellar astronomy... These giants pulsate by switching the hydrogen and carbon nuclear reactors on and off in sequence. The fusion processes create huge Red giants out of ordinary stars. So big that if they were in place of the Sun they would reach the orbit of Earth. (This growing and blowing is one possible scenario for the future of Sun, as well).

When the helium in the core has turned to carbon and oxygen,
  • the core shrinks again 
  • the helium begins to fuse to carbon and oxygen in a shell around the old core 
  • this shell surrounded by another one fusing hydrogen into helium 
the two turning on and off in sequence.

The star now brightens again, expands even more, and becomes cooler and even redder than before. As the star brightens it becomes unstable and begins to pulsate, the pulsations making it vary, or change in brightness.

Massive factories of elements
Red giants are massive factories producing some heavier elements in some of the more awesome processes in nature. There are a number of parallel nuclear reactions producing heavier elements and breaking them up again. Quite a mess but I guess there is order in the powerful reactions of millions of megatons each, as everywhere else where we see God's handwork in action.

The gases of red giants can circulate upward to the tops of the stars, carrying the by-products of nuclear fusion with them.

Oxygen is normally more abundant than carbon. If conditions are right, the surfaces of some stars can change their chemical compositions, some becoming very rich in the carbon that was made below by helium fusion, resulting in the reversal of the normal ratio. Mira variables and other old red giants thus divide into oxygen-rich stars and "carbon stars."

Raised up along with the carbon are elements such as zirconium and many others that have been made in a huge variety of nuclear reactions that go on at the same time as helium fusion.

Other stars' surfaces are enriched in helium and nitrogen.

Such huge giant stars have low gravities and lose mass through powerful winds that blow from their surfaces.

Some of the gas condenses into molecules and dust. There may be so much that the star can be buried in it and become invisible to the eye, the glow of the heated dust seen only by its infrared (heat) radiation.

Oxygen-rich giant stars make silicate dust, while
carbon stars make carbon-dust similar to graphite and soot.

Most of the dust that inhabits interstellar space began this way, though since inception it has been highly modified in the freezer of interstellar space.

As a giant star loses almost all of its remaining outer hydrogen envelope, it comes close to revealing its intensely hot core. A fast wind from the core first compresses the inner edge of the old expanding wind. High-energy radiation from the hot core then lights up this inner compressed portion, which is now many times the size of the whole Solar System.

From dust to dust (molecular cloud to planetary nebulae).

Simply awesome!

Ring nebula M57 in Lyra

Location of M57 in Lyra
Jodrell Bank Observatory

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