, Research Paper
The night sky, unimaginably deep, is a breathtaking sight. Some three thousand stars can be seen with the naked eye, twinkling points of light that have inspired the human spirit since the dawn of time. Study of the stars, based on data collected from visible-light telescopes, radio telescopes, and detectors wavelengths can now reveal extraordinary amounts of information: size, temperature, chemical composition, internal structure, distance and rotation rate, among other factors. One of the most important discoveries that scientists and astronomers have made is mapping out the life cycle of a star. Little by little, they have discovered all the different stages of a star; from its birth to its eventual death.
As giant molecular clouds orbit the center of a galaxy, they are tugged by gravitational and magnetic fields. How fast their constituent particles move depends on their temperatures: the colder the cloud, the slower the particles. Fast moving particles resist collapsing together, and so stars can form only in the dense cores of cold clouds. Typically, these clouds are only about 15 degrees above absolute zero. Periodically, the clouds begin to collapse. The trigger mechanisms for such collapses are thought to be collision between giant molecular clouds or entry onto galactic spiral arms.
Both of these occurrences set up compression waves within the cloud, which cause isolated regions to become so dense that gravity overwhelms all other processes and the could collapses. These isolated regions can often contain enough mass to create several hundred stars of similar mass to the sun. They are known as Barnard objects, and often appear as black regions in front of stars. Sometimes regions with emission nebulas reach the appropriate density and collapse. These appear as round, black ?bubble? within the glowing gas. They are referred to as Bok globules. As Barnard objects and Bok globules collapse, isolated regions within them collapse as well. In this way, the cloud fragments on many different scales. It is the smaller-scale collapses from which stars form.
At the center of the collapsing regions, concatenations of matter build up. Three-quarters of this matter is in the form of hydrogen gas. The rest is nearly all helium with 2 percent being made up of the heavier elements. This region is known as the protostar and, as material pours down upon it, the gas becomes so compressed that the temperature begins to rise dramatically. The rise in temperature makes the gas move faster and thus creates more pressure. This pressure gradually balances the inward pull of gravity and halts the collapse of the protostar. As more material accumulates on the protostar, instead of collapsing, it is squeezed gently. This raises the temperature even more.
Although there are no nuclear processes going on within the protostar, it is still giving off energy from the material that is striking its surface. This is given off as radiation but is very quickly absorbed by the dusty envelope raining down on the surface of the protostar. This action heats the dust, which then re-radiates the energy at infrared wavelengths. The envelope that surrounds the envelope is vast; typically, it is 20 times larger than out entire solar system.
The first, young infrared star to be found was discovered in the Orion star-forming region. It was discovered in 1967 by Eric Becklin and Gerry Neugebauer of the California Institute of Technology, and is now known as the Becklin- Neugubauer object. The youngest protostar, however is in the constellation of Ophichus and is known as VLA 1633, named for the Very Large Array telescope from which it was discovered. It is thought to be less than 10,000 years old.
In stars of more than seven solar masses, the inert carbon core is so massive that it collapses sufficiently to ignite carbon fusion. The temperature needed to ignite the carbon is in the region of several hundred million degrees. The carbon fusion produces magnesium. The star begins to take on a layered structure, with each shell within the center of the star undergoing nuclear fusion of a different element. Hydrogen fusion takes place in the outermost shell of the core region and below that, helium is converted to carbon and oxygen. The star develops concentric rings of material, each of which is fusing a specific chemical element into another one and feeding the shell below it. The shells contain such elements as neon, sodium, magnesium, silicon, sulphur, nickel, cobalt and iron. These High Mass stars race through their final evolutionary phases at extraordinary speed, compared with their initial phases. Carbon fusion is usually complete within a thousand years neon and oxygen fusion takes places within a single year. The silicon burning, which produces the iron core, usually takes place within a mere day or two.
Iron builds up in the core of the star and does not fuse into anything else. This is because all of the nuclear fusion processes so far have released energy but, after iron, the energy needed to fuse elements together is greater than the energy released in the fusion process. There is nowhere for that energy to come from, and so the iron accumulates in an electron-ddgenerate mass at the center of the star. The electron pressure is not infinite, however and as the mass gradually builds up, the core begins to become unstable. When the mass contained in the core reaches just under one-and-a-half times the mass of the Sun, known as the Chandrasekhar limit, the electron pressure can no longer resist the pull of gravity and the core collapses even further. This collapse has the same effect as knocking the foundations out from beneath a building. The overlying structure, in this case the rest of the star, begins to collapse downward.
As the star crashes down upon itself, it releases so much energy that it explodes and virtually blows itself to bits. This is known as a supernova. The energy released in supernova explosions initiates the production if the elements heavier than iron. Stars that explode in this way are called supernovas, type II. Type I supernova involve white dwarf stars. If a white dwarf star is close enough to another star, that star can transfer some of its outer atmosphere onto the white dwarf. This builds up on the white dwarf until a catastrophic nuclear detonation tales place. This can destroy the white dwarf and produce a supernova type I.
Supernovas seed the interstellar medium with elements that are heavier than helium. The Universe is composed of 75% hydrogen and 23% helium; heavier elements make up the remaining 2%. Heavier elements, called metals by astronomers, make planets and life possible. Every atom on Earth an in out bodies was once at the center of a massive star that exploded as a supernova before the Sun and the planets formed. The shock waves from the supernova explosion are one of the mechanisms by which the interstellar medium is compressed and thus new stars formed.