How Big Is a Star?
Stars vary in size from less than 8 percent of the size of the sun to those with diameters of more than 1,800 times that of the sun. In terms of mass, stars must have enough mass to support the nuclear chain reaction that takes place in the core of a star, which is about 10 percent of the sun’s mass. There is also a limit to how massive starts can grow, and scientists suspect that the limit is about 150 times the mass of the sun.
Scientists have only measured the size of two stars directly: the sun and a relatively close and large star called Betelgeuse. All other stars are measured through the combination of observing their perceived brightness, distance and temperature. The sun’s diameter is about 864,000 miles, and it weighs more than 300,000 times the mass of the Earth. The sun is a fairly average-sized star. By comparison, Betelgeuse is about 700 times the size of the sun and much brighter.
Stars can change size as they age. For example, the sun will increase to almost 300 times its current size when it becomes a red giant. Alternatively, when large stars collapse, they sometimes form neutron stars. Some of the most dense items in the universe, neutron stars are only about 12 miles in diameter, but a single teaspoon of neutron star material would weigh billions of tons.