John Kilgore, age 13, of Duluth, Minn., for his question:
Is it true that some stars are bigger than the sun?
The stars we see strewn across the night sky are at immense distances, and all objects appear to shrink with distance. A star of outstanding brilliance may be very near, as stars go, or very big or both near and big. When it comes to gauging either the distance or the size of the stars, we cannot depend upon the evidence of our eyes.
Comparing the sizes of the stars is a complex job, even for expert astronomers. Since objects seem smaller when they are farther away, the distance of a star must be known before its size can be estimated. This task requires a knowledge of math, delicate instruments and lots of patience. Star distances are measured in light years, and a light year is the distance a beam of light travels in a year. Since its speed is about 186,000 miles a second, the length of a light year totes up to roughly six million million miles which is 6 followed by 12 zeros.
The nearest stars are more than four light years away, and the sun is at an average distance of about 93 million miles. Hence, we cannot trust our eyes to estimate the sizes of the sun and stars. Astronomers have figured the diameter of the sun to be some 860,000 miles, which is almost 34 times greater than the diameter of the planet on which we live. The volume of the sun is more than a million times greater than the volume of the earth. Yet the experts tell us that this immense heavenly body is but a medium size star.
The largest stars are called red giants and supergiants. These glowing red whoppers are cooler and less dense than our sun, but in size some of them could swallow our sun a million times or more. Let's begin with a medium size giant and work up to the stunning supergiants. Arcturus is a bright red star in the constellation Bootes near the big dipper. The diameter of this smallish giant is some 30 time that of the sun.
One of the larger giants is Aldebaran, the glowing red eye in the constellation Taurus, the Bull. Aldebaran has a diameter some 72 times greater than that of the sun. One of the supergiants is Antares, the red star at the heart of Scorpius, the Scorpion. Its diameter is estimated to be greater than 300 times the diameter of the sun. And a certain supergiant in the constellation auriga is estimated to be some 2000 times wider than our dazzling sun.
Our sun would be lost inside one of the giant stars, like a baseball inside a monster balloon. To imagine their sizes, we can compare them with the solar system rather than with the sun itself. If Antares replaced the sun in our solar system, it would spread far beyond the orbit of mars. If the supergiant in Auriga replaced the sun, its edges would reach far beyond Mars, beyond Jupiter and Saturn and then half way to the orbit of Uranus.