Deep Sky Objects - Nebulae

The word nebula means "mist" in Latin.  The plural form of nebula is nebulae. Hubble Space Telescope photos of some popular nebulae are below. Nebulae are large clouds of gas (usually hydrogen), dust and/or plasma.  

Many nebulae form when gas begins to coalesce due its gravitational effect.  When the gas collapses, stars begin to form which emit extreme levels of ultraviolet radiation.  These are generally known as diffuse nebulae because they have no well defined boundaries.  This radiation causes the gas surrounding the stars to glow.  There are two main types of diffuse nebulae that emit visible light, emission and reflective.  Emission nebulae emit visible light due to the glowing ionized hydrogen regions.  Reflective nebulae do not emit a great deal of visible light themselves, but reflect light from nearby stars.  Dark nebulae are nebulae that are diffuse, but do not emit or reflect much, if any, visible light.  We only see them because they are in front (from our perspective) of stars or other bright nebulae and we see them as dark clouds of gas.  The Horsehead Nebula is a fantastic example of a dark nebula.

Other nebulae form when a star explodes to make a supernova explosion.   Supernovas occur when a star is at the end of it's life.    When a star has burned out the hydrogen and helium at its core, nuclear fusion ceases, causing the star to collapse.  When it collapses, the heat becomes so intense, the star explodes.  The gaseous material that is thrown off in the explosion forms a nebulae, sometimes called a supernova remnant.  Technically, a supernova remnant, is a type of diffuse nebulae.

The other type of nebulae form as a result of a red giant star that slowly sheds matter from its outer layers.  As the star begins to shrink, it greatly increases the heat of the star, thus releasing more ultraviolet radiation causing the matter that was released to glow.  This is a planetary nebula.  They don't have anything to do with planets in our solar system.  They usually only last a few ten thousand years or so.  Approximately 1500 have been identified in the Milky Way Galaxy

No two nebulae look alike, their size, brightness, density, and composition can vary  greatly.  Some, such as the Orion Nebula, are extremely bright and large.  Others, such as the Horsehead Nebula are very faint and are difficult to view.  They are amongst the best objects to view providing both challenge and reward.

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Veil Nebula Supernova Remnant	Carina Nebula Emission Nebula	Orion Nebula Emission and Reflection Nebula
Crab Nebula Supernova Remnant	Cat's Eye Nebula Planetary Nebula	Bug Nebula Planetary Nebula
Horsehead Nebula Dark Nebula	Spirograph Nebula Planetary Nebula	Eagle Nebula Emission Nebula

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