Stars

Introduction: Each individual star is unique, but all stars follow a common cycle of life. They are born from interstellar gas clouds, shine by nuclear fusion and then die. The Sun, which is the source of virtually all light, heat and energy reaching the Earth, is our nearest star. A Star's Birth: Stars are born in cold interstellar clouds like the Orion Nebula and the Eagle Nebula. In these stellar nurseries, dense regions undergo gravitational collapse to form a rotating gas globule. As the globule collapses, the temperature and pressure increase and it spins faster. Material in the central core of the cloud is squeezed so tightly that it becomes hot enough to ignite nuclear fusion. The surrounding flattened disk of dust may coalesce into planets and asteroids. The process of collapse takes between 10,000 and 1,000,000 years.

A Star's Life: A star's life is an extended battle between two opposing forces: gravity and pressure. A star can maintain its internal pressure only if it continually generates energy to replace the energy that it radiates into space. This energy comes primarily from nuclear fusion of light elements into heavier elements, through which a star shines for millions or billions of years. As a star has a limited amount of material in its core, it cannot rely on thermal energy to resist gravity forever and its ultimate fate depends on whether something other than thermal pressure manages to halt the unceasing crush of gravity.

A Star's Death: The outcome of a star's struggle between gravity and pressure depends entirely on its birth mass. As a star - such as the Sun, Vega, Sirius or Spica with a mass below about 5 solar masses - depletes its supply of hydrogen, the core will shrink and temperatures will climb high enough for it to burn helium instead. The surface will puff up like a balloon, growing cooler, brighter, and redder, forming a red giant. Examples of red giants are Betelguese, Arcturus, Aldebaran and Antares. Subsequently, the envelope is ejected as a planetary nebula, while the core becomes a white dwarf. Examples of planetary nebulae are the Ring Nebula, Eskimo Nebula, Helix Nebula and the Cat's Eye Nebula. Under certain conditions, a white dwarf may become a nova. High mass stars with masses above 5 solar masses end their lives much more violently. The stronger gravity of high mass stars compress their cores to higher temperatures, and consequently, these stars are much brighter than low mass stars. In the final stages of their lives, they proceed to fuse increasingly heavier elements until they have exhausted all possible fusion sources. When fusion ceases, gravity drives the core to implode, resulting in a titanic supernova explosion, leaving behind a neutron star or a black hole. Examples of supernova remnants are Vela, Crab Nebula, Veil Nebula and Supernova 1987A.

Adapted from: Stars