Helen Brown, age il, of Helena, Mont., for her question:
WHAT EXACTLY IS A NEBULA?
A nebula is a cloud of dust particles and gases in space. The word "nebula" comes from the Latin word meaning "cloud." In the olden days, astronomers used the term for distant galaxies outside of the earth's galaxy, the Milky Way.
Those distant galaxies, called extragalactic nebulae, looked like hazy patches of light among the stars.
When modern telescopes came along, astronomers discovered that extragalactic nebulae were definitely not clouds of dust particles, as had been believed. Instead, the astronomers found that they were systems of stars similar to the Milky Way.
Today, most astronomers use the term nebulae only for the actual clouds of dust and gases in the Milky Way and other galaxies. They classify these masses into two general types: "diffused" nebulae and "planetary" nebulae. Both types also are called "gaseous" nebulae.
Diffused nebulae are the larger of the two types. Some contain enough dust and gases to form as many as 100,000 stars the size of the sun.
A diffused 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 diffused nebula of this kind is called an "emission" nebula.
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.
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 small telescope, a planetary nebulae appears to have a flat, rounded surface like that of a planet. Because of this similarity, early astronomers called these nebulae "planetary" nebulae.
In an emission nebula, pressure and temperature build up within the mass as contraction continues through millions of years. In time, the mass becomes hot enough to shine as a new star is formed.
A diffused 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 diffused nebula is called a "reflection" nebula.
If a diffused nebula occurs in an area that has no nearby stars, it neither emits nor reflects enough light to be visible. In fact, its dust particles blot out the light from the stars behind them. Astronomers call such a diffused nebula a "dark" nebula.