Nebulae

A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. The word "nebula" is the Latin word for "cloud" and the plural of nebula is nebulae, nebulæ, or nebulas. Originally,"nebula" was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was referred to as the "Andromeda Nebula" before the true nature of galaxies was confirmed in the early 20th century.

Most nebulae are of vast size, even hundreds of light years in diameter. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created in an Earthen environment. For instance, a nebular cloud the size of the Earth would have a total mass of only a few kilograms.

Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the"Eagle Nebula". In these regions the formations of gas, dust, and other materials "clump" together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. The remaining materials are then believed to form planets and other planetary systemobjects.

Many nebulae were formed from the gravitational collapse of gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiationionizes the surrounding gas, making it visible at optical wavelengths. Examples of these types of nebulae are the "Rosette Nebula" and the "Pelican Nebula".

Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The materials thrown off from the supernovaexplosion are ionized by the energy and the compact object that it can produce. One of the best examples of this is the "Crab Nebula".

Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8 to10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off.

There are countless nebulae in the vastness of space. Each one is stunningly different and rely on the human imagination for a name. Here are some striking examples of nebulae.