Deep Sky Objects

Beyond our solar system lie million of stars, nebulae, galaxies, and clusters that are visible from Earth. Often these objects are difficult to see except with a large telescope, but there are also many fascinating deep sky objects that can be seen with small telescopes. Charles Messier, a French astronomer who lived in the late 1700s, created a catalog of 110 objects. His reason for doing this was so that amateur astronomers searching for comets would not mistake bright nebulae, clusters, and galaxies for comets. However, today Messier’s catalog serves as a good guide to the most brilliant objects that can be viewed with a small telescope.

A STAR is, like our sun, an enormous ball of gas that releases energy created through nuclear fusion in the star’s core. All stars are not the same, however. Some are hotter than others, some are more massive, and some are dying while others are being born. The life cycle of a star is complicated, and depends on its mass and other factors. For instance, a very heavy star might explode in a supernova and then become a black hole. A lighter star will eventually run out of fuel, cool down, and become a black dwarf. Stars usually occur in groups called clusters, which are gravitationally bound.

DOUBLE and MULTIPLE STARS are linked by gravity, so that they orbit one another in some way. In this respect, our own sun is somewhat unusual in being single, because about three quarters of all stars are in groups.

OPEN CLUSTERS are groups of dozens to hundreds of stars which are loosely bound by gravitation. Open clusters generally lie in the spiral arms of the Milky Way.

GLOBULAR CLUSTERS are usually spherically shaped, and contain hundreds of thousands of old stars. These clusters lie around our galaxy in a halo.

A NEBULA is a cloud of gas and dust where new stars are being formed. For this reason, nebulae are sometimes called ‘stellar nurseries.’ A nebula is created from the remnants of a star in the last stage of its life. For instance, a supernova explosion will eventually result in the formation of a nebula, which will in turn become a birthplace for new stars. Nebulae are some of the most beautiful objects in the sky, and are often named after their shape, like the North American Nebula, the Rosette Nebula, and the Horsehead Nebula.

A GALAXY is a huge grouping of stars, nebulae, and dust that is held together by gravitational forces. Our galaxy, which has about 100 billion stars in it, is called the Milky Way, because of the bright yet hazy band that we see sweeping across our sky. The nebulae, stars, and clusters that were just discussed are all in our galaxy. From Earth, we can also see through our own galaxy to identify other galaxies in our corner of the universe. There are several different kinds of galaxies: spiral ones have hazy arms that twist around the center (the Milky Way is a spiral galaxy), barred spiral ones have a thick band of stars near the center, and elliptical ones are nearly circular in shape. Galaxies are also grouped together in clusters. Our galactic cluster has about 30 galaxies in it, of which the Andromeda Galaxy is the largest.


Ian Ridpath, Eyewitness Handbooks: Stars and Planets, D.K. Publishing, 1998.
The photographs used on this page are reproduced for educational purposes from the NASA, STScI, and Hubble Space Telescope Picture Galleries.