LIFECYCLE OF A STAR
Astronomers believe that stars form in dense collections of gas and dust, molecular clouds that are often located in the spiral arms of a galaxy. As the force of gravity collects dust and gas particles, this material forms a ball of gas held together by the force of its own gravity. Deep within this ball, the tremendous pressure of that gravity creates heat and nuclear reactions in which hydrogen atoms are fused to form helium atoms. The energy from these reactions pushes outward, counteracting the inward pressure of gravity. The balance of these two forces prevents the newly forming star from collapsing. Electromagnetic radiation from the nuclear reactions spreads to the outer layer of the star and diffuses into space, causing the star to shine.
Death of a Star
A star with a stable balance of gravitational force and outward gas pressure is called a main sequence star, of which our sun is one example. The nuclear reactions that preserve a star's balance of pressure will last only as long as that star has a supply of hydrogen to burn into helium. Once the stock of hydrogen has been spent, gravity causes the center of the star to collapse. When it is dense and hot enough, the core will then begin to burn its helium into carbon, generating heat that pushes out the surface layers of the star. As a result, the star expands and turns red. This is referred to as the red giant phase of a star's life. When our sun enters this stage, it will swallow up the earth as it expands out toward Jupiter. Radiation emerging from the collapsing core will blow the outer layers of the star into space. Eventually, all that's left will be the hot carbon core of the former star and a surrounding layer of bright, radiating gas. This is called a planetary nebula. In time, the hot carbon core will cool and shrink until it is only a few thousand miles across -- a tiny white dwarf.
There are, of course, stars many times the size of our sun. When such a star exhausts its reserve of hydrogen, it collapses in the same manner as a sun-sized star. Yet these stars expand into much larger, cooler stars, called red supergiants. A red supergiant will burn its helium into carbon, but will also create enough heat to convert its carbon core into a number of different elements: oxygen, neon, silicon, sulfur, and finally iron. No more energy can be procured from this iron core, thus, there is no force to counteract the pressure of gravity, and this heavy iron core collapses. Electrons get pushed into the nucleus, where they combine with the protons and form a core of neutrons. This core is so dense that the outer layers of the star, as they collapse, simply bounce off in a sudden and explosive shock wave. For a month, this explosion, called a supernova, shines with the strength of an entire galaxy of stars. This material eventually disperses into interstellar space. In fact, the first stars were almost exclusively composed of hydrogen and helium. Oxygen and the rest of the heavy elements in the universe originated in supernova explosions. So, in a way it's true that we are all made of stars.
What happens to the remaining neutron core depends on the original mass of the star. In many cases, it will cool and become a neutron star, an object about the size of Manhattan with a mass 1.4 times that of the sun. Pulsars are spinning neutron stars that emit streams of particles from two magnetic poles. When the original star's mass is exceptionally large, not even the extremely dense neutron core will be able to resist the pull of gravity and it will collapse into a black hole. Black holes are so dense that nothing, not even light, can escape. So, how do we know they exist? While they do not emit light themselves, black holes may accumulate dust and gas from nearby stars or clouds that happen to drift within their vicinity. As this matter is drawn into a black hole, it is heated up and begins to emit x-rays into space, which we can see. According to Einstein's General Theory of Relativity, space and time becomes so warped in the vicinity of a black hole that time, as we know, it essentially stops. In the interior of a black hole, the laws of physics don't make sense.
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