Ken Schumacher, age 13, of Birmingham, Ala., for his question:

WHAT EXACTLY IS A NEBULA?

A nebula is a cloud of dust particles and gases in space. The term comes from the Latin word for cloud.

Early astronomers used the term for distant galaxies outside our galaxy, the Milky Way. Such galaxies, called extragalactic nebulae, look like hazy patches of light among the stars. However, modern telescopes revealed that extragalactic nebulae are actually systems of stars similar to the Milky Way.

Most astronomers today use the term nebulae only for the clouds of dust and gases in the Milky Way and other galaxies. They classify these gases into two general types: diffuse nebulae and planetary nebulae. Both types are called gaseous nebulae.

Diffuse nebulae are the larger of the two types. Some diffuse nebula contain enough dust and gases to form as many as 100,000 stars the size of our sun.

A diffuse nebula may occur near an extremely hot, bright star. The intense ultraviolet light from the star energizes the gas atoms of the nebula and enables the mass to emit light. A diffuse nebula of this kind is called an emission nebula.

Planetary nebulae are ball like clouds of dust and gases that surround certain stars. They form when a star begins to collapse and throws off a part of its outer gaseous envelope.

When viewed through a small telescope, a planetary nebula appears to have a flat, rounded surface like that of a planet. Because of this similarity, early astronomers called these planetary nebulae.

Astronomers believe that some emission nebulae are places where new stars are forming. Gravity causes a portion of a nebula's dust and gases to contract into a much smaller and denser mass.

Pressure and temperature build up within the mass as contraction continues through millions of years. In time, the mass becomes hot enough to shine, and forms a new star.

A diffuse nebula also may occur near a cool star. In this case, the ultraviolet light from the star is too weak to energize the nebula's gas atoms and make them give off light. However, the dust particles in the nebula reflect the starlight. This kind of diffuse nebula is called a reflection nebula.

A French astronomer named Pierre Simon Laplace, who lived from 1749 to 1827, came up with the theory to explain how our solar system was formed. Laplace called the idea his nebular hypothesis.

Laplace said the sun and planets were formed from a nebula, or cloud of heated gases. Gravitation caused the nebula to condense and form globes.

Laplace's theory has been changed and modified by new discoveries and different analyses of known facts.