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Stars |
A star is a large ball of hot gas that is held together by gravity and gives off its own light. Constellations are a number of stars that appear to form a pattern. Some stars are brighter than others. The word magnitude is used to describe the brightness of a star. Scientists can tell how far away a star is by the way the star appears to shift in its location. This apparent shift when an object is viewed from two different locations is called parallax. Astronomers describe distances in space in units called light-years that are the distance light travels in a year.
Stars have colors that are related to their surface temperatures. The hottest stars are blue-white and yellow. Cooler stars are red and orange. At the center of a star, nuclear reactions create the light we see. Surface temperature, size and the distance from Earth determine how bright a star is.
Stars begin life as enormous clouds of gas and dust in space called nebulas. Gravity causes the nebula to contract and heat up to become a protostar. When the protostar becomes hot enough, nuclear reactions create energy that moves outward against gravity and becomes a main-sequence star. As stars use up the hydrogen at their cores they become giant or supergiant stars. In their final stages, lower mass stars become red giants and are eventually torn apart. More massive stars may explode as supernovas or collapse into black holes.
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