Victor Densmore, age 13, of Tucson, Arizona, for his question:
How do galaxies differ from nebulas?
A nebula is a hazy cloud of light and some are vaguely visible among the stars of our own Galaxy. A fairly powerful telescope reveals many others in the vast reaches of space beyond our star system. For a long time all of them were called nebulas. Then more powerful telescopes revealed that some really are just hazy cosmic clouds, but others are remote star systems similar to our own Galaxy.
The word "nebula" has a nice sound and it calls forth a serene celestial picture of hazy light. Such a word is hard to forget, even when a modern telescope reveals that a foggy halo is a precise, cart wheel system of perhaps 100 billion separate stars. Obviously it is a galaxy, but it still may be called a nebula. If a galaxy's stars are arranged in twirling arms, it is called a spiral nebula.
We now know that the cosmic universe is strewn with clusters of galaxies at various stages of development. It is thought that those shaped like enormous spheres of glowing gases are the youngsters. Older galaxies, similar to our own, are shaped like wheels with spiraling arms and most of their original gases have become stars.
All of these nebulas are true galaxies, destined to become systems of separate stars, rotating around a brilliant, crowded hub. In the skies above the Southern Hemisphere, there are two hazy nebulas called the Clouds of Magellan. We now know that these are galaxy type star systems, perhaps satellites of our own Galaxy.
From north of the equator, we can see a dim nebula in Andromeda. Actually it is almost a twin of our Galaxy. The beauteous celestial object is catalogued as M 31 better known as the Great Nebula in Andromeda. Many hazy nebulas viewed through old style telescopes turned out to be galaxies. But not all of them. An enormous amount of cosmic material is in the form of gaseous cosmic clouds. These are the true nebulas.
Our dominant winter constellation is Orion. On a clear night we may see a pale blue, just below one of the three stars in Orion's belt. This true nebula is a gaseous cloud that spans a width of 16 light years. Its hazy light is borrowed from a group of nearby stars. Several other bright nebulas are strewn along the Milky Way.
Other vast cosmic clouds are in sparsely populated parts of the Galaxy. Unlit by neighboring stars, they look like black storm clouds. Several of these dark nebulas are between us and the center of the Galaxy. They blot out our view of the crowded hub. Unlike the galaxies, the light and dark nebulas have no definite shapes.
The word "nebula" is used to describe galaxies and cosmic clouds though this is not all. One of these oddities looks like a dim star in the constellation Taurus, near the Milky Way. It was long known as the Crab Nebula because its gaseous filaments sprawl outward like skinny legs. We now know that these gases are the debris of a supernova. This explosion of a star was recorded in 1054 A.D. and its nebulous gases still are spreading through space.