Nebulas, also referred to in the plural as nebulae, are among the most beautiful celestial objects that can be seen by telescope. These giant clouds of gas originate in the outer layers of old, dead stars, which are blown away in the last moments of a star's life to form giant clouds of dust and debris.
When it was first used, the term "nebula" actually referred more vaguely to any large astronomical object. We now realize that some of the objects once classified as nebulae were actually distant galaxies, while others were true nebulae, as we now use the term. A nebula is a giant interstellar cloud, made up mostly of hydrogen gas with smaller amounts of helium gas, trace other gases, and dust. Very large nebulae form stellar nurseries, areas of unusually dense dust and hydrogen where clumps gradually come together to form new stars. The closest stellar nursery is located in the Orion Nebula, 42 light-years away.
The first nebulae would have formed in the early life of the universe, in areas where the first gas clouds began to form. Today, however, nebulae actually originate through entirely different processes: the deaths of stars. Stars, as they approach the end of their life, begin to run out of hydrogen in their cores, the result of millions or billions of years of fusing it into helium. (The smallest red dwarf stars will do this for trillions of years, so that none has ever reached the end of its life - yet.) When this happens, they begin to lose the battle against their own immense gravity.
What happens next depends on the size of the star: small stars shed their surfaces and leave behind a glowing ember called a white dwarf, while large stars explode much more explosively and leave behind more exotic remnants like neutron stars and black holes. In any case, what happens to the surface of the former star is key to understanding the causes of nebulas. The outer layers of the star are blown outwards, forming massive clouds of dust, gas and debris. These clouds still consist almost entirely of hydrogen because the hydrogen fusion in stars is largely confined to the inner core.
Nebulae are very long-lived, but they are not permanent. Once they are formed, if they become stellar nurseries, then the process of star formation will eventually use up the free-floating gas and dust that makes the nebulae appear so beautiful today. Some will coalesce into new stars, while the rest will be dispersed into the interstellar medium as it is tugged first one way and then another by the gravity wells of the infant stars.
- Sources -
Case Western Reserve University. Nebulae.
NinePlanets.org. "Types of Nebulae."
University of Tennessee-Knoxville. The Origin of the Solar System.