The weight of a cloud changes during its lifetime. Before it is born, it is water vapor and this gas is a bit lighter than the other gases of the air. Then the vapor becomes fine droplets of water and water is too heavy to float in the air. But so long as the droplets stay sin all with lots of air space between them, the misty cloud is light enough to float.

But gradually, the water droplets become bigger and the mf.sty cloud gets heavier. Soon itbtoo heavy to float in the air   and down spills the rain.

Are the stars really different colors?

It is summer time, and the constellation Scorpius the Scorpion sprawls over the evening sky. Antares, the big star at the he rt of the scorpion, glows like a hot coal   but a powerful telescope shows it to be two stars, a giant ruby and a miniature emerald. One of the stars marking the great square of Pegasus is actually three stars   one red, one green and one blue.

If you are a careful star watcher, you will notice even without the help of a telescope that the stars seem to sparkle with different colors. This is not imagination. A star actually burns like a glittering jewel and it may be red or orange, yellow, white or blue. This can be seen clearly when a powerful telescope is turned on a double star, a pair of twins so close together that their contrasting colors can be compared.

An astronomer can tell the temperature of a star from its color. It also tells the chief gases from which the star is made. The stars may be divided according to their colors into seven major classes and several minor classes.

To the naked eye, Arares, the big star in the constellation Scorpius, looks ruby red. And so it is. This red giant and its class¬mates have temperatures ranging from 2,000 to 3,000 degrees centigrade. This may seem hot, but it is only half as hot as our sun.

Aldebaran, the star that looks like a pumpkin lantern in Taurus the Bull, is an orange star. Stars in this class are from 3,000 to 5,000 degrees centigrade. Our sun is a yellow star, with a surface temperature of 6,000 degrees. Its classmates range from 5,000 to 8,000 degrees.

Bright Sirius, the Dog Star, sparkles like a clear diamond. It is a white star and the stars of this class seethe at temperatures from 8,000 to 10,000 centigrade degrees. Bluish stars, like dazzling Spica in the constellation Virgo, have temperatures from 12,000 to 20,000 degrees and some in this class may be as hot as 30,000 centigrade degrees.

All stars contain an abundance of hydrogen gas, plus traces of other elements in gaseous form. But the hottest stars have a greater proportion of hydrogen and the cooler stars contain more metallic elements, Our sun, which is fairly cool as stars go, contains traces of iron and perhaps all of the other metals we have on the earth.

Each element burns with it own special color. The astronomer uses the rainbow colors of the spectrum to identify the elements in a seething star. The spectrum can give him detailed information about a star which is perhaps millions of light years from the earth.