Death of a Star? How about the Death of a Galaxy!

Objective 14 wants me to explain how stars evolve and eventually die. With this blog post I will explain the process, and then relate the process to the article linked above in an attempt to make it understandable.

After stars are formed and reach main sequence is when they begin to die or degenerate. But depending on the mass of the star, their death is slightly different. Stars like our sun can expect to have it’s helium core contract and the hydrogen shell will begin to burn, during this the energy balance will be lost. When energy balance is lost, the star will start to gain in size and go from it’s main sequence to a sub-giant, and then a super giant until the stars core reaches a temperature at which helium can fuse, there are a few moments of tremendous pressure and then thermal pressure takes over, the star then returns to the main sequence becoming dimmer than it was as a red giant.

The process of a low mass star death photographed from The Essential Cosmic Perspective by Bennett, Donahue, Schneider, and Voit.

Low mass stars have a different death than our sun. These stars will never be able to fuse anything but helium to carbon for the pressure in the core is not enough to force carbon to fuse. the core will continue to shrink while the hydrogen and helium fusing causes the star to grow bigger and more luminous than it’s been yet. Because the star is so much larger than it can handle, it will begin to lose it’s grip on it’s outer layers and the solar wind pulls that particles away bit by bit. These particles cool and form nebulae or flow out into space and drift along with stellar wind and into interstellar space to mix with other dusts and gases in the galaxy.

Stars with a high mass react much more dramatically in their death than the deaths of stars with lesser mass. In addition to the protein-protein cycle that happens in the the fusion of hydrogen and helium, the high mass star has what is called the Carbon, Hydrogen, Oxygen (CNO) cycle image below. This cycle produces energy at a much faster rate than the normal protein-protein cycle.

The protein-protein cycle visualized

The CNO cycle visualized in its 6 steps.

The high mass star begins to fuse a number of elements and the core begins to host a select number of fusions in order from outer layer of the core to the absolute center of the core: non-burning hydrogen, hydrogen fusion, helium fusion, carbon fusion, oxygen fusion, neon fusion, magnesium fusion, silicon fusion, and the inert iron core. When the star runs out of fuel, the gravitational pull from the core becomes overwhelming and suddenly pulls all the material of the star into the core. The material is pulled down with such force that it slams into the core and then bursts out creating a Type II supernova. In this explosion enough energy is put into the molecules to create all the elements heavier than iron (which before now the star was incapable of creating). A nebula of heavy molecules and dust are left in the wake of this stars death, and in the middle of all this is a neutron star. Scientists do not know much about the neutron star except that it has an unfathomable mass and thus possesses a tremendous gravitational pull. But neutron stars aren’t the only possible result from a super nova. Stars with an even higher mass can create a black hole upon their final display. The star goes through the same changes as it would in creating a neutron star except when all the materials are pulled in for the final explosion, they keep getting pulled in and the star collapses in on itself creating a black hole. Black holes and neutron stars are subjects of a great amount of study because as of now, scientists know very little about either of them.

The article linked at the top of this blog post speaks about a large, dim-red, galaxy that astronomers have calculated to have been active in the early period of the universe. This galaxy is much larger than most, but has burnt out and lived a unexplainably short life of ~1.65 billion years. “If the galaxy runs out of gas star formation will stop” said Dominik Reichers, and astronomer who was not involved in the study of this galaxy. Of course, without star birth, you cannot have star death. This being said, the galaxy had to of gone through either a great death of stars, or a long period of stars dying and not producing enough nebulae to create new stars; meaning that they were either very low mass stars, or very high mass stars. The low mass stars would simply not create enough nebulae to start the growth of new stars; whereas the high mass stars would of only created materials suitable for creating asteroids, space debris, and planets.

I find this to be an interesting article despite the fact that there was not a lot of information other than speculations made by scientists that weren’t involve in the study. One of the speculations caught my attention more than the rest and that was the one stating that the reason for this galaxy’s death may have been a collision with another galaxy. The mental image of such a monumental fender-bender really puts the size of life in perspective.


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