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Chapter 26 Section 2 Stars Distance to the Stars A star is a large, glowing ball of gas in space which generates energy through nuclear fusion in its core. Astronomers use units called light-‐years to measure the distance between stars. A light-‐year is the distance that light travels in a vacuum in a year. That’s about 9.5 trillion kilometers!! Proper0es of Stars Astronomers classify stars by their color, size, and brightness. Other important properties of stars include their chemical composition and mass. Color and Temperature A star’s color indicates the temperature at its surface. The hottest stars, with temperature hotter than 30,000 K, appear blue. The surfaces of relatively cool red stars are about 3000 K. Stars with surface temperatures between 5000 and 6000 K appear yellow, like the sun. Brightness The sun appears very bright to us because it is much closer to Earth than other stars. In fact, the sun is really a star of only average brightness. Apparent brightness-‐ the brightness of a star as it appears from Earth. What would happen to a stars apparent brightness the farther away the star got from you? Absolute brightness-‐ how bright a star really is. Absolute brightness does not depend on how far away it is from Earth. Size and Mass There is no direct way of finding the mass of a star. Astronomers can calculate the masses of groups of starts based on gravity and how the stars interact with one another. Astronomers have also discovered there is a relationship between mass and absolute brightness. Many stars are similar to the sun in size and mass. Composi0on Astronomers use an instrument called a spectrograph to identify elements in a star’s atmosphere. Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium making up 96-‐99% of the star’s mass. The H-‐R Diagram An H-‐R diagram is a graph of the surface temperature, color, and absolute brightness of a sample of stars. They are used to estimate the sizes of stars and their distances, and infer how stars change over time. If two stars are the same temperature, the brighter one must be larger. Hotter stars are brighter than cooler stars of the same size. Main-‐Sequence Stars Most stars are found along a diagonal band running from the bright hot stars on the upper left to the dim cool stars on the lower right. This is called the main sequence. 90% of stars are found on the main sequence. Giants and Dwarfs The very bright stars at the upper right of the H-‐R diagram are called supergiants. Giants are large bright stars that are smaller and fainter than supergiants. A white dwarf is the small dense remains of a low or medium-‐mass star.