|
|
Space Facts |
|
|
Star AgesHow old are the stars?Star ages are typically measured in billions (109) of years, also called "giga-years" (Gyr). The heavier a star is, the faster it burns its fuel, and there are a few stars that burn bright enough and fast enough to last only a few million years.
Stellar EvolutionAfter stars are born (when they begin the nuclear process of converting hydrogen into helium), they enter what is called the "main sequence." Hot, blue-white stars spend only a few million years here. Our sun (already 4.5 billion years old) will spend a total of about 10 billion years in the main sequence. Dim, red dwarf stars might spend as much as several hundred billion years there.After this stellar "adulthood," the star runs out of hydrogen at the core and begins to collapse. Collapsing causes compression which generates more heat, so the end of the hydrogen phase is an uneasy period, giving rise to fluctuations in brightness. When the core temperature rises high enough, a star starts to burn the helium it spent the better part of its lifetime creating. When helium is ignited, the core temperature shoots up, and this pushes the upper regions of the star's body outward. With expansion comes cooling, so helium ignition may happen more than once. Once the helium is burning steadily, the outer portions of the star have expanded enough to make them significantly cooler than they were during the main sequence. This is the star's red giant phase. Stars roughly the mass of our sun, or smaller, end up sloughing off the outer layers leaving a burnt out husk called a "white dwarf." This is an ultra-dense form of matter where one teaspoon full of it would weigh several tons. The matter has collapsed to the point where only the electrical pressure of the electrons is holding the matter back from complete collapse. Stars much larger than our sun may end their lives with a spectacular bang! This is the supernova — for many days brighter than the entire galaxy. At the core, when the bang is over, there may be a neutron star or possibly a black hole.
Star Life Cycle—Main Sequence and BeyondFor a chart on the life cycle of stars, click the preceding link. References: |