To establish the distance of stars that are even farther away, astronomers use the stars temperature and brightness. They can measure a star’s temperature surprisingly easily: a bluish star is hot, around 20,000°C; or white or yellow star has a medium temperature and orange or red stars are cool about 3000°C.
The hotter a star, the brighter it is. A star with a temperature of 10,000°C, for example, is 40 times brighter than the sun (which has a temperature of 5500°C). So if a star of 10,000°C is found which appears very dim, then it must be a long way off in space, its brightness diminished by its great distance. Before astronomers can use this relatively simple method they need to know the relationship between brightness and temperature, and the distance from earth. This is why the first use methods such as Parallax, on nearby stars. After measuring the brightness of those stars they can then use what they know as a guide to ascertain the relative brightness of more distant stars.
The measurement of star brightness allows astronomers to measure distances to any star in the Milky Way, some lying as such as 100,000 light years away.
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