Birth and Death of a Star
Astronomers think that a star begins to form as a dense cloud of gas in the arms of spiral galaxies. Individual hydrogen atoms fall with increasing speed and energy toward the center of the cloud under the force of the star's gravity. The increase in energy heats the gas. When this process has continued for some millions of years, the temperature reaches about 20 million degrees Fahrenheit. At this temperature, the hydrogen within the star ignites and burns in a continuing series of nuclear reactions. The onset of these reactions marks the birth of a star.
When a star begins to exhaust its hydrogen supply, its life nears an end. The first sign of a star's old age is a swelling and reddening of its outer regions. Such an aging, swollen star is called a red giant. The Sun, a middle-aged star, will probably swell to a red giant in 5 billion years, vaporizing Earth and any creatures that may be on its surface. When all its fuel has been exhausted, a star cannot generate sufficient pressure at its center to balance the crushing force of gravity. The star collapses under the force of its own weight; if it is a small star, it collapses gently and remains collapsed. Such a collapsed star, at its life's end, is called a white dwarf. The Sun will probably end its life in this way. A different fate awaits a large star. Its final collapse generates a violent explosion, blowing the innards of the star out into space. There, the materials of the exploded star mix with the primeval hydrogen of the universe. Later in the history of the galaxy, other stars are formed out of this mixture. The Sun is one of these stars. It contains the debris of countless other stars that exploded before the Sun was born.
In 2006, astronomers were excited about star formation in Arp 220, a super galaxy created by the collision of two other galaxies 250 million light-years from Earth. The Hubble Space Telescope has observed more than 200 huge star clusters, giving scientists a glimpse of what occurred when the universe was young. A surprising find was that the mix of gas and dust in this new galaxy is very similar to our own older Milky Way.