Kevin Curtsinger, age 14, of Concord, N.Y., for his question:

HOW DOES A STAR FORM?

A star forms from a cloud of interstellar gas and dust. Such clouds can be seen by astronomers as dark patches in front of the bright, distant stars of the Milky Way. The cloud that is to become a star consists mostly of hydrogen mixed with dust.

Astronomers say that some of the hydrogen and dust in a forming star comes from old stars that may have exploded. Some of the hydrogen and dust may be a collection of gases thrown from the surface of rotating stars.

The first step in the formation of a new star is the concentration of part of an interstellar cloud into a ball. Although astronomers have never watched a new star flash into life, they have discovered several dark, ball like interstellar clouds that may be new stars starting to take shape.

It actually takes millions of years for the cloud of gas and dust to be pulled together by gravity. As the material forms into a ball, the pressure of the gas increases and the gas at the center of the ball becomes extremely hot.

When the temperature at the center of a forming star reaches about 2 billion degrees Fahrenheit, nuclear fusion reaction starts. The hydrogen in the center starts to change into helium and produces great amounts of nuclear energy. This energy heats the gas that surrounds the center and the gas starts to shine.

And so a new star has come to life.

The kind of star that takes shape depends on the mass or size of the contracting cloud. A cloud with a mass about one twentieth that of the sun becomes a red, low luminosity main sequence star. A cloud with a mass about 50 times that of the sun becomes a blue, high luminosity, main sequence star.

Most of the stars we see shining today were formed between 1 million and 10 billion years ago. The life of a star is many billions of years.

Astronomers receive much of their information about the life of stars by studying star clusters. The stars in a cluster probably formed about the same time and thus are probably all about the same age.

Some star clusters include many blue giants and such stars are believed to be young. Other clusters include many red giants that are thought to be old stars. From observations and theory, astronomers have been able to understand how a star forms and how one may end its existence.

After a star starts to shine, it starts to change slowly. The speed with which it changes depends on how rapidly the nuclear energy producing process takes place inside it.

How fast a star changes depends on its mass. The greater the mass, the higher its luminosity and temperature, and the faster it changes.

Stars with great mass may take only a few million years to change. Very small stars may take hundreds of billions of years to change.