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Classifying Stars (pages 753–754) Key Concept: Characteristics used to classify stars include color, temperature, size, composition, and brightness. A star’s color gives clues about the star’s temperature. The coolest stars appear red. The hottest stars appear blue. Very large stars are called giant stars or supergiant stars. Our sun is a medium-sized star. Most stars are smaller than the sun. Brightness of Stars (pages 754–755) Key Concept: The brightness of a star depends upon both its size and temperature. Stars differ in how bright they are. A hot star shines brighter than a cool star. A large star shines brighter than a small star. A star’s apparent brightness is the brightness you see from Earth. A hot, large star that is very far from Earth does not look very bright. But the sun looks very bright because it is so close to Earth. A star’s absolute brightness is the brightness the star would have if all stars were the same distance from Earth. Measuring Distances to Stars (pages 756–757) Key Concept: Astronomers use a unit called the light- year to measure distances between the stars. A light-year is the distance that light travels in one year. That distance is about 9.5 million-million kilometers. A light-year is a unit of distance, not time. You could also measure distance on Earth in terms of time. For example, if it takes you 1 hour to ride your bike to the mall, you could say the mall is “1 bicycle-hour” away. Key Concept: Astronomers often use parallax to measure distances to nearby stars. Parallax is the change in an object’s position you seem to see when you change your own position. The object does not really change position. It only seems to change because you change your position. Astronomers use parallax. They measure how far a star seems to move when Earth moves from one side of the sun to the other. The distance the star seems to move tells an astronomer how far the star is from Earth. The Hertzsprung-Russell Diagram (pages 758–759) Key Concept: Astronomers use H-R diagrams to classify stars and to understand how stars change over time. The Hertzsprung-Russell diagram, or the H-R diagram, shows how the surface temperature of stars is related to their absolute brightness. The points on the H-R diagram form a pattern. Most stars on the H-R diagram fall into a band that spreads from the top left corner of the diagram to the bottom right corner. This band is called the main sequence. Stars in the main sequence are called main-sequence stars. About 90 percent of all stars are main-sequence stars. Life Cycle of Stars Video