Andrew Martin, age 11, of Gary, Ind
What is a double star?
On a clear night, the dark velvety sky is spattered with sparkling stars. Some are bright and some are dim ‑ but each seems to shine alone in solitary splendor. The telescope shows that this is not so. Many of the stare we see as singles are actually twins or triplets, sometimes quadruplets or even quintuplets, Twin stars are called binaries and the larger groups are called multiple stars,
Each pair of twins and each multiple group is held together by their combined gravitational force, Many of them have been observed to rotate around each other in orbits like the links of a chain, The stars, however, are so far away and the family groups so close together that most of this is invisible to the naked eye, The average star could be oompared to a golf ball at a distance of 100 miles, If there were another golf ball just a few inches from the first, we would see them as one ‑ if we saw them at all.
Only a few of the binary stars can be spotted without the aid of a good telescope. They are large, widely separated. pairs and they have been known for centuries. They were called double stars before it was discovered that they were bound together by the force of gravity. One such star is Mizar, the sparkler in the middle of the handle of the Big Dipper. If you have good eyes, on a clear night you may be able to see that this star is actually a pair of twins.
Astronomers use the telescope and the spectroscope to find binary and multiple stars and so far some 23,000 of them have been discovered. Even after a group has been discovered it often takes years to calculate the details about it. A pair of twins may be closer than Mercury is to the sun. The Mizar twins are separated by sore 30,000 million miles of space.
Some twins swing around their orbits in hours and some in days, some take centuries and soave change too slowly to be detected.
A pair of binary stars both may be big, as are the Mizar twins. One may be large and one small, as in the case of Sirius the Dog Star and its tiny companion, the Pup. Sirius has a diameter almost twice that of the sun and its mass i s about two and a half times that of the sun, The Pup is about four times the size of the earth, However, the mass of the small companion is almost equal to the mass of our sun. The mass is the amount of material packed into a star.
This points up a general rule concerning the many heavenly twins. They may vary in size and certainly in brightness. But they do .not vary greatly in mass. The two together usually contain about as much star material as a pair of suns. Sirius is light and bulky with a density only half that of our sun. The gases of the Pup are tightly packed together. A pint of its starry gases weighs about 24 tons.