Jeff Waldrep, age 14, of Gastonia, N.C., for his question:
WHAT IS A NEBULA?
Nebular hypothesis is a theory advanced by a French astronomer named Pierre Simon Laplace to explain how our solar system was formed. Laplace lived from 1749 until 1827. He believed that the sun and planets were formed from a cloud of intensely heated gas. Gravitation caused the cloud to condense and form globes. New discoveries have changed and modified his theory.
A nebula is a cloud of dust particles and gases in space. The name comes from the Latin word nebula which means cloud.
Early astronomers used the name to describe distant galaxies outside of the earth's galaxy, the Milky Way. They called these galaxies extragalactic nebulae because they looked like hazy particles of light among the stars.
As time passed, however, modern telescopes showed that extragalactic nebulae were actually systems of stars very similar to our own Milky Way.
Today the term nebulae is used by astronomers only for clouds of dust and gases in the Milky Way and other galaxies. There are two general types: diffuse nebulae and planetary nebulae. Each type is also called gaseous nebulae.
Some diffuse nebulae contain enough dust and gases to form as many as 100,000 stars the size of the sun. An intense ultraviolet light energizes the gas atoms of the nebula and enables the mass to emit light. Astronomers say that some diffuse nebulae, called emission nebula, are places where new stars are forming.
Planetary nebulae are ball like clouds of dust and gases that surround certain stars. They form when a star starts to collapse and throw off a part of its outer gaseous envelope.
When viewed through a telescope, a planetary nebula seems to have a flat, rounded surface like that of a planet. Because of this similarity, early astronomers called these nebulae planetary nebulae.
One of the most famous nebulae is the Great Nebula in the constellation Orion. It is a huge cloud of dust and gas that can be seen even without a telescope.
Sometimes a diffuse nebula will be found near a cool star. When this happens, the ultraviolet light from the star is too weak to energize the nebula's dust and gas atoms and make them give off light. This kind of diffuse nebula is called a reflection nebula.
In the Milky Way, many of the clouds of dust block out stars that are located behind them.