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Star Life Cycles We Classify Stars by Their Characteristics All stars are huge balls of glowing gas that produce or have produced energy by fusion Stars differ in size, brightness, and temperature We Classify Stars by Their Characteristics Stars look like small points of light because they are very far away Light-year – the distance light travels in one year Brightness and Distance The amount of light a star gives off and its distance from Earth determines how bright it appears to an observer Parallax – the apparent shift in the position of an object when viewed from different locations Size Giant and supergiant stars range from ten to hundreds of times larger Because giant and supergiant stars have such huge surface areas to give off light, they are very bright Size Stars called white dwarfs are about 100 times smaller in diameter than the Sun, or roughly the size of Earth White dwarfs cannot be seen without a telescope Color and Temperature If you observe stars closely, you may notice that they vary slightly in color The differences in color are due to differences in temperature Color and Temperature The coolest stars are red - the hottest stars are blue-white The colors and lines in a spectrum reveal which gases are present in the star’s outer layers Stars Have Life Cycles Although stars last for very long periods, they are not permanent The life cycle of a star varies, depending on the mass of the star Stars form inside a cloud of gas and dust called a nebula Stars Have Life Cycles Gravity pulls gas and dust closer together in some regions of a nebula – as the matter contracts, it forms a hot, dense sphere – the sphere becomes a star if its center grows hot and dense enough for fusion to occur When a star dies, its matter may eventually become part of new stars Stages in the Life Cycles of Stars Lower-Mass Stars – The stage in which stars produce energy through the fusion of hydrogen into helium is called the main sequence – When a lower-mass star runs out of hydrogen, it expands into a giant star, in which helium fuses into carbon Stages in the Life Cycles of Stars – Over time a giant star sheds it outer layers and becomes a whitedwarf – the dead core of a giant star – Although no fusion occurs in white dwarfs, they remain hot for billions of years Stages in the Life Cycles of Stars High-Mass Stars – Stars more than eight times as massive as our Sun spend much less time in the mainsequence stage because they use their fuel rapidly – After millions of years, a higher-mass star expands to become a supergiant star – fusion produces heavier and heavier elements Stages in the Life Cycles of Stars - When an iron core forms, fusion stops and gravity causes the core to collapse and the star erupts in an explosion called a supernova Neutron Stars and Black Holes The collapsed core of a supergiant may form an extremely dense body called a neutron star Neutron stars emit little visible light Some neutron stars emit beams of radio waves as they spin – these stars are called pulsars because the seem to pulse as the beams rotate Neutron Stars and Black Holes Sometimes a supernova leaves behind a core with a mass more than three times that of the Sun – it collapses forming an invisible object called a black hole The gravity of a black hole is so strong that no form of radiation can escape from it Stages in the Life Cycle of Stars Star Systems A binary star system consists of two stars that orbit each other A multiple star system consists of more than two stars