Stellar Life Cycle
The stellar life cycle describes the formation and death of stars. Stars begin as large clouds of dust and gas. These immense clouds (up to 25 light years in diameter) have a low density although each cloud contains as many particles as a single star. Hydrogen gas occupies as much as 99% of the cloud; less than one percent is made up of dust particles. The dust particles are tiny, each having a diameter of less than four hundred-thousandths of an inch (one ten-thousandth of a centimeter). These particles contain such compounds as silicon carbide and elements such as carbon and nitrogen. These clouds of dust and gas probably begin as a dying star exploding into a nova or a supernova.
The dust particles are gradually pulled close together by the force of gravity. An outside force, such as a shock wave from a supernova, usually triggers this pulling together of the particles. As the cloud is pulled together, it reaches extremely high temperatures. Eventually, the cloud becomes dense enough to be called a protostellar nebula, Latin for clouds. Nebulae can be luminous or dark. Luminous nebulae result when a nebula is located near a bright star, for example, the Great Nebula in the constellation Orion. The light from the star creates a glow in the nebula. Dark nebulae are those that are not located near stars, for example, the Horsehead Nebula, also seen in the constellation Orion. These nebulae block some of the light from distant stars, and appear as dark spots in space. Many nebulae are not visible to the unaided eye because they are not near a bright star or they do not block the light of distant stars.
The nebula glows as its temperature and density increase. This hot, glowing cloud of gas and dust particles is called a protostar. If the protostar reaches high enough temperatures, fusion reactions begin inside the core. During these reactions, two hydrogen nuclei combine to form one helium nucleus. Some of the mass from the hydrogen atoms is converted into energy. Once the fusion reactions begin, the protostar has become a star. The energy released from the fusion reactions work against the force of gravity and the cloud of dust and gas stops contracting. When this occurs, the star is said to be in its stable state. Reaching the stable state may take hundreds of thousands to millions of years, depending on the size of the star. Less massive stars reach the stable state faster than stars with great mass.
A star in the stable state remains the same size and emits a steady amount of radiation for millions of years. Eventually, the quantity of hydrogen gas driving the fusion reactions diminishes and the star emits less energy. At this point, the energy no longer balances the force of gravity and the star begins to contract. As the star contracts, the temperature of the core increases further, which causes the outer layers of the cloud to expand. This makes the star appear brighter because of its increased size. Simultaneously, fusion reactions begin to take place in the outer layers as the core supply of hydrogen gas is expended. This causes the star to expand, and it is now called a red giant or a supergiant. Sometimes, the star's temperature continues to rise as it expands to a temperature high enough to facilitate another type of fusion reaction in its core. When this occurs, two helium nuclei fuse to form heavier elements such as oxygen, carbon, or iron.
Eventually, the hydrogen and helium inside the star are exhausted until no fusion reactions occur. At this point, the inner core cannot support the weight of the outside layers, and the giant or supergiant collapses, forming a white dwarf. White dwarfs are usually smaller than Earth and are formed from stars of equal or less size as the Sun. Without the fusion reactions taking place, the dwarf's temperature decreases. Enough heat remains from the fusion reactions for the dwarf to glow for a few million years. Occasionally, a white dwarf will become extremely bright when it has collided with another star. The white dwarf then becomes a nova, or new star. The increased luminosity usually lasts only a few years, at which point the nova returns to its original brightness as a white dwarf.
A larger star may form a supernova when they collapses from a red giant. Fusion reactions in the red giant form an iron core. When the red giant cools, the core begins to collapse, causing the temperature and pressure inside of the core to increase. The temperature becomes high enough that the iron atoms undergo fusion reactions to produce even heavier elements. When the giant collapses further, a violent explosion occurs and the star's luminosity increases dramatically. A supernova lasts for a few weeks or months at most. After the explosion occurs, only about half of the original star's mass remains. This remaining mass is called a neutron star. A neutron star is a highly dense cloud of neutrons. The violent explosion essentially crushed the atoms together, causing electrons to combine with protons to form neutrons. A neutron star may only be several kilometers in diameter, although it is millions of times more dense than the Sun.
The largest red giants, or supergiants, may form a black hole when they collapses. In this scenario, the collapse is so great that all of the atoms are condensed into a very small volume. The force of gravity between the atoms is strong enough that no light can escape. Black holes are invisible structures that can only be identified when matter from a nearby star is drawn into them.
The stellar life cycle varies from star to star, depending on its original size. The more massive stars have greater temperatures and, therefore, have shorter lives. Smaller stars have longer life cycles.
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