Q:

How many suns are there?

A:

Earth's solar system has only one sun, but if the word "sun" is taken to mean "any star with planets in orbit," the Milky Way might have as many as 400 billion suns. If the observed ratio of approximately one planet per star is consistent everywhere, the observable universe might contain 10^24, or 1 septillion, suns.

The sun of Earth's solar system is large when compared with other planet-supporting stars. Of the 50 stars within 17 light-years of the solar system, the sun is the fourth most massive. The majority of stars that last long enough to support the formation of planets are classified as red dwarfs. These stars are smaller and less massive than the sun, they burn through their smaller complements of hydrogen fuel more slowly, and they shine much longer than stars like the sun.

Very large stars, such as the blue giants, tend not to support planetary systems. These stars' high mass drives accelerated fusion reactions in their cores and exhausts their entire complement of hydrogen on a scale of tens of millions of years as compared to billions of years for stars like the sun, which might not allow time for planetary systems to form. However red dwarfs could potentially endure for trillions of years and form full planetary systems.

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