Though the most familiar star, our own sun, stands alone, about three of every four stars exist as part of a binary system containing two mutually orbiting stars.
No one knows how many stars exist, but the number would be staggering. Our universe likely contains more than 100 billion galaxies, and each of those galaxies may have more than 100 billion stars.
Yet on a clear, dark night Earth's sky reveals only about 3,000 stars to the naked eye. Humans of many cultures have charted the heavens by these stars.
Appearance
Some stars have always stood out from the rest. Their brightness is a factor of how much energy they put out, which is called their luminosity, and also how far away from Earth they are.
Stars in the heavens may also appear to be different colors because their temperatures are not all the same. Hot stars are white or blue, whereas cooler stars appear to have orange or red hues.
Stars may occur in many sizes, which are classified in a range from dwarfs to supergiants. Supergiants may have radii a thousand times larger than that of our own sun.
Hydrogen is the primary building block of stars. The gas circles through space in cosmic dust clouds called nebulae. In time, gravity causes these clouds to condense and collapse in on themselves. As they get smaller, the clouds spin faster because of the conservation of angular momentum—the same principle that causes a spinning skater to speed up when she pulls in her arms.
Building pressures cause rising temperatures inside such a nascent star, and nuclear fusion begins when a developing young star's core temperature climbs to about 27 million degrees Fahrenheit (15 million degrees Celsius).
Life Cycle
Young stars at this stage are called protostars. As they develop they accumulate mass from the clouds around them and grow into what are known as main sequence stars. Main sequence stars like our own sun exist in a state of nuclear fusion during which they will emit energy for billions of years by converting hydrogen to helium.
Stars evolve over billions of years. When their main sequence phase ends they pass through other states of existence according to their size and other characteristics. The larger a star's mass, the shorter its lifespan will be.
As stars move toward the end of their lives much of their hydrogen has been converted to helium. Helium sinks to the star's core and raises the star's temperature—causing its outer shell to expand. These large, swelling stars are known as red giants.
The red giant phase is actually a prelude to a star shedding its outer layers and becoming a small, dense body called a white dwarf. White dwarfs cool for billions of years, until they eventually go dark and produce no energy. At this point, which scientists have yet to observe, such stars become known as black dwarfs.
A few stars eschew this evolutionary path and instead go out with a bang—detonating as supernovae. These violent explosions leave behind a small core that may become a neutron star or even, if the remnant is large enough, a black hole.
