Download star

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.