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Why Do Stars Change?
In order for stars to go through the process of nuclear fusion, they need fuel. Stars are made out of this fuel, and have only a limited amount. These fuels are mostly hydrogen and helium, as well as smaller amounts of carbon, nitrogen, and oxygen. As a star uses up one type of fuel, it must change its size and pressure in order to use another. Sometimes these changes are very subtle, while other times the are literally quite explosive. These changes take place over a course of millions to billions of years.
Birth
A star is born in a huge cloud of gas and dust known as anebula (plural: nebulae). This nebula is about 21 light-years(125 trillion miles) across. Part of the nebula begins to shrink under the pull of its own gravity. This forms a protostar which is about 60 million miles across. The star begins to take shape. The temperature continues to rise and nuclear fusion begins to take place. The pressure from inside the star finally equalizes the gravity pushing in, and the star stops contracting.
Nebula M16 (click image for larger one)
Life
In order for nuclear fusion to take place, there must be tremendous amounts of pressure and heat. This pressure crushes together elements to create more massive elements and energy. Stars begin fusing hydrogen first because it is the least dense and the easiest to fuse. Four hydrogen nuclei fuse together to form one nucleus of helium. By-products of this is the production of two positrons, two neutrinos, and the release of energy. Stars that are going through this hydrogen burning process are known to be on the main sequence. Stars spend most of their life (aprox. 90%) on the main sequence.
Death
A star will eventually use up most of it's hydrogen and be left with helium. At this time there is not enough pressure crushing down on the star to create a nuclear reaction with helium. Nuclear reactions cease inside the star, and because there is no longer any outward push from fusion, the star begins to collapse upon its self. Here is where the star leaves the main sequence. This collapse begins to create more and more pressure inside the star until it is sufficient to have the fusing process of helium begin in the core, while some of the remaining hydrogen burns just outside of it. The products of this helium burning is carbon and oxygen. The star swells, and depending on its size, either becomes a red giant or a red supergiant.
Small Stars:
After the hydrogen burning process is complete in stars with an initial mass of less than 8 solar masses, they become red giants. These red giants have a diameter of roughly 60 million miles. Helium is burning in the core producing oxygen and carbon, while a thin layer of hydrogen is burning around it where there is not sufficient pressure for helium burning. The red giant begins to brighten between 1,000 and 10,000 times. The hydrogen-rich covering on the surface of the star begins to swell and becomes as large as as the orbit of Earth or Mars. Because there is so little pressure now in the outside areas of the star, the surface temperature drops to about 5,000 - 6,500 degrees Fahrenheit. This temperature is actually very cool for a star. A strong solar wind begins to blow, and jettisons away most of this hydrogen covering. All that is left is a long-period variable star. This shed material is known as a planetary nebula. It can get as big as 1 light-year across.
The center of the star has now met its demise. During the formation of the planetary nebula the star ceases all nuclear reactions. The star is still very hot, up to several hundred thousand degrees Fahrenheit. Over a few hundred million years, the star cools and becomes a white dwarf. As the star cools more, it becomes dark and barely detectable. It is now known as a black dwarf.
a white dwarf is circled
The white/black dwarf is composed of carbon and oxygen. Surrounding this is a thin layer of helium, sometimes surrounded by hydrogen. The star is very compact. Although only about the size of earth, it's mass can be from a little less than one half a solar mass to a little more than one solar mass.
Large Stars:
If a star starts off with a mass of less that 8 solar masses, then it will stop at the red giant stage. More massive stars continue to burn. The carbon and oxygen produced in the previous stage begin to fuse. Carbon begins to be crushed into neon and magnesium, while oxygen is being crushed into silicon and sulfur. Silicon and sulfur get crushed into an iron core.
This iron core now just sits in the center of the star. The reason for this is that iron doesn't burn. Nuclear burning is only possible if an object is releasing energy. In order for iron to go the the fusing process, energy must be added. This leads to the collapse of the star. The addition of energy that the iron needs will only occur during the supernova explosion caused by the collapse of the star.
Because the iron is not fusing, it does not create any outward pressure do balance the effects of gravity. As the iron gets a mass of about 1.4 solar masses, gravity gets the upper hand and the core collapses from a size of about 5,000 miles to about 12 miles in less than a second. This sudden crush makes protons and electrons combine to form neutrons. This expels high-energy subatomic particles (known as neutrinos.) This huge energy release is equivalent to 100 of our stars burning for more than 10 billion years. A small amount of energy is deposited in the lower layers of the shell surrounding the core, triggering the supernova explosion. The energy deposited around the core creates a shock wave that runs outward toward the stars surface. As it is passing through, it heats up the shell sounding the core, starts nuclear burning, and throws off the shell faster than 10 million mph. This is when the iron fuses to create heavier particles. When the shock wave reaches the surface, it heats them very quickly and causes them to glow. In a day or 2, the star is brighter than a billion suns. In a couple of weeks, the explosion diminishes, although it may remain visible for months or years.
[Background on the words Nova and Supernova]
the rings of supernova 1987A
What's left is two distinct parts. There is a rapidly expanding gaseous shell that barges through the surrounding interstellar medium and interacts with it, and there is the compact stellar remnant which is either a neutron star or a black hole.
Neutron stars are super-dense remnants of supernovae. They have about 1.4 solar masses, but only have a diameter of 12 miles. Because they are so small and faint, they can't be seen with visible light. Neutron stars spin very fast. Usually they spin about once every second, but some can spin much faster. For example, a neutron star in the Crab nebula has been found to spin 30 times per second. This rapid rotation creates a large magnetic field. The neutron star begins to emit radiation out of it's poles. This radiation ranges from radio waves to visible light to x-rays to gamma rays.
Neutron stars can lead to pulsars. Pulsars are short for "Pulsating radio sources". Because neutron stars emit beams of radiation out of their poles, if they're positioned right they will sweep across earth and a pulsating signal will be detectable.
[Pulsars and Little Green Men]
If an object with four or more solar masses remains after a supernova explosion, it will become a black hole. Because there is so much mass and no nuclear reactions inside of the star to compensate for it, the gravity continues to crush the star. Eventually, the gravity gets so strong that it even holds back light. When material first begins to get pulled into a black hole, it swirls around it for awhile. It will heat up and eventually give off visible x-rays that are detectable before finally crossing theevent horizon and enter the black hole.
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