- A combination of three images taken by the Hubble Space Telescope shows the center of a star …
The thousands of pinpricks of light that shine in the night sky are stars. Stars are huge, glowing balls of gas that have fascinated humans throughout history. From earliest times people have looked at the night sky and imagined patterns in the stars. They gave these patterns, or constellations, names and told stories about them. Eventually the branch of science called astronomy developed as scientists began to study the stars and the rest of the universe. The sun is a starThe most well-known star is the sun. The sun is an average star. It is a giant ball of mostly hydrogen and helium gas that is 865,000 miles (1,392,000 kilometers) in diameter. The hydrogen atoms in the sun's core are continually combining to form helium atoms in a nuclear reaction called fusion. Fusion reactions release large amounts of energy, which is why stars shine so brightly. The sun's surface temperature is about 9,980° F (5,527° C), while the temperature in its core, where the fusion reaction takes place, is roughly 28,080,000° F (15,600,000° C). A count of starsNo one knows how many stars there are in the universe. Stars are clustered into huge groups called galaxies. The sun and its solar system are part of the Milky Way galaxy, which contains billions of stars. Through telescopes it is possible to see that the universe is in turn filled with billions of galaxies. The distances between stars are equally enormous. The closest star to the sun is Alpha Centauri. It is 4.3 light-years away. A light-year is the distance light travels in a year —about 5.8 trillion miles (9.5 trillion kilometers). Galaxies themselves are separated by as much as a million light years. Because of the huge distances to even the nearest of them, the stars appear from Earth as points of light that change position very little in the sky. The night sky today looks almost exactly as it has for hundreds, and even thousands of years. Classifying starsStars vary in size, temperature, density, composition, brightness, and color. They are classified in a system first developed by astronomer Henry Draper in the early 20th century. In this system, each star is assigned a letter code—O, B, A, F, G, K, or M—based on its surface temperature. The surface temperature can be determined by measuring properties of the stars' spectra—the particular colors of light they give off. The spectrum of colors ranges from blue for hot temperatures to red for cool temperatures. O stars, also known as blue supergiants, are hotter, and hence bluer stars, and M stars are cooler, and hence redder. The sun is a type G star. Stars are also often classified by magnitude, which is a measurement of how bright they appear in the Earth's night sky. The brighter the star, the lower is its magnitude. A magnitude 6 star is barely visible, while the brightest star in the sky, Sirius, has a magnitude of about -1.5. The life of a starStars most likely begin as clouds of hydrogen and dust that slowly come together, due to the force of gravity, into thicker and thicker clumps. These clumps form clusters of young stars, each of which begins to heat up as its pressure rises and it compresses. Once a star reaches a certain critical temperature and density, the fusion reaction begins. The star then burns its hydrogen in this so-called main sequence stage, which lasts for billions of years. When the star finally runs out of hydrogen for the fusion reaction, it cools and contracts. What happens next depends on the star's mass. Low-mass stars become cooler red giant stars, and eventually burn away, leaving a compact white dwarf at their centers. High mass stars, on the other hand, expand into huge, cool red supergiants when they run out of hydrogen. If the conditions are right, these red supergiants sometimes then explode in a huge, violent blast called a supernova that lasts for only days. During a supernova a star may briefly burn billions of times brighter than normal. Supernovas are very rare, but a number have been observed, including one especially bright one in 1987. Other types of starsThe material left after a supernova may collapse into a neutron star—an incredibly dense ball of material. Some neutron stars rotate very quickly and give off strong periodic pulses of radio waves. They are called pulsars. If the material left after the supernova has enough mass, it may collapse inward and become a black hole, which is an object so dense that nothing that gets near it can escape its gravity, not even light. Since black holes emit no light, they are difficult to detect, but many scientists believe they exist at the centers of galaxies. Other starlike objects include quasars, which are extremely bright, distant objects that emit huge amounts of radio waves. |