سؤال قراءة ستيب محلول — الإشارة والمرجعية (تدريب 4800)

(1) If we look at the night sky carefully, we will see that the stars are of many different colors. Some are red, others are yellow and some are blue. This is also shown when we take color photographs of the night sky. You can take such a photograph with an ordinary camera as long as it is kept steady. A thirty second exposure is sufficient. (2) Astronomers have been able to classify stars according to color. They have found that blue stars are the largest and red stars the smallest. However, there are a few stars which cannot be classified in this way. These are the superstars. For example, Ryiejol is a blue superstar as big as 40,000 suns and Beetle juice is a superstar with a size equal to 17,000 suns. (3) Suppose an astronomer observes two stars, one brighter than the other. If neither of them is a superstar, he will know immediately that the brighter star is closer. Astronomers have instruments like light meters which can measure the brightness of a star quite accurately. It is possible to measure a star’s distance from the earth if astronomers know the color, brightness and whether or not it is a superstar.