Outer space contains large cool clouds of Hydrogen gas molecules. These clouds are comosed of mostly hydrogen and some helium. Most of these intersellar gas clouds may not collapse and form stars unless they are triggered by the compression of a shock wave. The shock wave can originate from the following sources. Examples of lage gas clouds include the Orion nebula, Lagoon nebula, and the North American Nebula.
1) The spiral arms of a galaxy are the dominate trigger of star formation. The arms act like a shock wave traveling through space.
2) Nearby supernova explosions can provide the shock wave necessary for star formation.
3) The nuclear ignitions of young stars forming can create a shock wave.
As the Hydrogen gas cloud collapses, the core begins to heat up due to the gas pressure. When the core reaches billions of degrees, nuclear fusion occurs and the star begins to shine.
Average stars like our sun produce energy through nuclear fusion by the proton proton chain reaction. In the core of the sun temperatures are so high that the positive hydrogen nuclei are moving fast enough that they can overcome their positive charges and combine together. In Hydrogen fusion, four hydrogen nuclei join to produce one helium nucleus. One Helium nucleushas a mass that is 0.7 percent less than the four hydrogen nuclei so it appears that there is some missing mass. It turns out that the missing mass is converted into energy by the equation e=mc squared. In this equation, e represents energy, m represents the missing mass, and c squared is the speed of light squared.
As a star ages it accumulates more and more helium and less hydrogen. This helium is more dense so it accumulates in the star's core. Eventually, as temperatures rise above 100,000,000, the star will begin to fuse helium into beryllium and carbon. Carbon fusion does not occur until temperatures exceed 600,000,000 K.
Stable stars like our sun are in a balance. Gravity wants to compress the star while heat from nuclear fusion in the core wants to expand the star. As the star fuses heavier elements the temperature in the core rises. This causes the star to expand into a red giant. The star appears redder because its surface heat is spread out over a much larger area that its surface is cooler even though the core is hotter.
The Death of a Star
As stated above, when a star nears the end of its life, it begins to fuse heavier elements which raises the core's temperature and causes the star to swell into a red giant. What happens after this stage depends of the initial mass of the star. The three final fates of a star are as follows.
1) Medium mass stars (between .4 - 3 solar masses) In the final stages of the red giant phase the star's strong stellar wind blows off its outer gas atmosphere like an expanding shell of gas. These objects are called planetary nebulas because they look like the greenish disk of Uranus and Neptune through the telescope although they have nothing to do with planets. There are numerous planetary nebulas we can see through the telescope. These include M57 the ring nebula or the Helix nebula. The core left behind eventually cools and forms a White Dwarf. A White Dwarf is esentually a dead star which is about the size of a planet. This will be the final fate of our sun. The bright star Sirius has a companion that is a white dwarf called Sirius B.
2) The Deaths of Massive Stars (between 3 - 9 solar masses) We saw that low and medium mass stars die relatively quietly as they use up their nuclear fuel. The most violent event in these medium mass stars is when they blow off their outer shell. Larger stars use their fuel exponentially faster and so they live a much shorter life. They die in one of the most spectacular astronomical events called a supernova. When a massive star is in the red giant stage it's core is hot enough to fuse heavier elements beyond carbon. As the star develops iron in the core, energy production drops because the fusion of iron requires more energy than it produces. This causes the core to collapse rapidly triggering a supernova explosion. In 1054 A.D. Chineese astronomers recorded the supernova explsion in the constellation Taurus. Today we see the crab nebula through the telescope where this supernova occured.
The core left behind from a supernova will be one of two things.
1) Neutron Star If the core left behind from a supernova explosion has a mass between 1.4 to 3 solar masses then gravity will be strongh enough to create a neutron star. Due to the collapse of atoms the neutron star has a size of a small city. Neutron stars have a density beyond any material on earth and they spin very fast. Beams of radiation emerging from the magnetic poles sweep through space so neutron stars are also called pulsars.
2) Black Holes If the core left behind from a supernova explosion is greater than 3 solar masses, then no known force can stop the gravitational collapse of the star. When this occurs a black hole is formed. A black hole is where a star once was and now there is only gravity so strong that not even light can escape. Since we cannot see black holes we must look for their gravity such as binary stars revolving around an invisible partner. As matter is pulled into a black hole, it speeds up to incredible velocities in a disk around the black hole. This matter streaming into the black hole is so energetic that it emits x rays which are like the "paw print" of an invisible black hole.