The stars are the most obvious feature of the night sky, yet for millennia they were one of the most mysterious. To the untrained eye, the visible universe still appears to be just a disorganized scattering of stars, but closer inspection reveals that it is host to an enormous variety of objects. Quite apart from the nearby worlds of the solar system (covered in the next chapter), there are stars of many kinds, nebulae formed in a number of different ways, star clusters, and countless galaxies.

The stars are so far away that even the most powerful telescopes show them as nothing more than points of light. Until recently, there was no way to tell what they were—though some astronomers guessed correctly that they were stars like the Sun. It took a series of breakthroughs in the 18th and 19th centuries to create the science of astrophysics, the study of the physical properties of stars (see pp.22—23). Today, astronomers are able to explain nearly all the phenomena they observe in stars as aspects of a single model of stellar evolution.

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