Martha White, age 16, of Washington Boro, Pa., for her question:
WHAT IS A NEBULA?
The nebular hypothesis is a theory that was advanced by the French astronomer Pierre Simon Laplace during the early 1800s which attempted to explain how our solar system was formed. He said the planets and sun were formed from a cloud of intensely heated gas. Then. gravitation caused the cloud, or nebula, to condense and form globes. Some scientists still believe in a modified and updated version of this theory.
A nebula is a cloud of dust particles and gases in space. Early astronomers used the word to describe distant galaxies outside of the earth’s galaxy. These galaxies, called extragalactic nebulae, resembled hazy patches of light among the stars. Now, however, modern telescopes have shown that extragalactic nebulae are really systems of stars similar to the Milky Way.
The term nebulae today is used by most astronomers for the clouds of dust and gases in the Milky,Way and other galaxies. These masses are classified into two general types: diffused nebulae and planetary nebulae. Both are called gaseous nebulae.
Planetary nebulae are ball like clouds of gases and dust that can be found surrounding some stars. They form when a star begins to collapse and throw off part of its outer gaseous envelope. A nebula appears to have a flat, rounded surface like that of a planet.
Diffused nebulae are the larger of the two types. Some contain enough gas and dust to form as many as 100,000 stars the size.of our sun.
A diffused nebula may be found near a bright, extremely hot star. Gas atoms are energized by intense ultraviolet light, and this enables the mass to emit light. Astronomers believe some emission nebulae are places where new stars are forming. Gravity causes part of a nebula’s dust and gas 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 scientists believe, the mass becomes hot enough to shine—and forms a new star.
If a diffused nebula occurs in an area that has no nearby stars, it neither emits nor reflects enough light to be visible. Its dust particles then blot out the light from the stars behind them. Astronomers call such a diffused nebula a dark nebula.
A diffused nebula may also occur near a cool star. When this is the case, the ultraviolet light from the star is too weak to energize the nebula’s gas atoms and make them give off light. The dust particles in the nebula, however, reflect the starlight. This kind of diffused nebula is called a reflection nebula.