Nebulae

= zN ﻿ eNe = 

**Nebulae**
= b is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases. = 

Nebulae
The Great Orion Nebula in the sword of Orion is pictured at left. Stars are being formed in the cloud of gas and dust. Click on the image to see a larger version. Gas, dust and debris from star explosions and around star forming regions are called nebulae. A nebula can reflect light (Reflection Nebulae) or absorb light (Dark or Absorption Nebulae) while Emission Nebulae emit light on their own. The remnants of a star explosion are called Planetary Nebulae because they looked like planets to early astronomers with small telescopes. Some are named because of their recognizable shapes like the Ring Nebula in the constellation Lyra. The image at right is the Eagle Nebula in the constellation Aquila. You can see the darker clouds in the center look like a diving eagle. This emission nebula glows red because of hot hydrogen atoms.

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Galaxies== Early in this century, there was a great debate as to the nature of the nebulae like this one which at that time could not be resolved into individual stars. Thanks in large part to the work of [|Edwin Hubble] whose famous paper "The Realm of the Nebulae" finally put the issue to rest, we now know that these are really vast conglomerates billions of stars which are very much more distant from the [|Earth] than other nebulae. Our own Milky Way galaxy is just one of the billions of galaxies now known to exist. A typical galaxy is 100,000 [|light-years] in diameter. (M 51 shown; see the [|SEDS Messier catalog] for more examples.) ==

Globular Clusters== Globular clusters are gravitationally bound groups of many thousands (sometimes as many as a million) of stars. They consist primarily of very [|old] stars. Globular clusters are not concentrated in the plane of the galaxy but rather are randomly distributed throughout the halo. There are several hundred globular clusters associated with our galaxy. A typical globular cluster is a few hundred light-years across. (M 13 shown; see the [|SEDS Messier catalog] for more examples.) ==

Open Clusters== Open clusters are loose aggregations of dozens or hundreds of [|young] stars. They are generally not gravitationally bound and will disperse in a relatively short period of time, astronomically speaking. They are often associated with more diffuse nebulosity, as well. Also called "galactic clusters" because they are usually found in the plane of the galaxy. A typical open cluster is less than 50 light-years across. (M 44 shown; see the [|SEDS Messier catalog] for more examples.) ==

Emission Nebulae== Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation. ([|M 42] shown) ==

Reflection Nebulae== Reflection nebulae are clouds of [|dust] which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae. ([|NGC 7023] shown)

[[image:http://astro.nineplanets.org/twn/thumb/n2264.JPG align="jpg" link="ftp://ftp.aao.gov.au/pub/dfm_images/aat013.jpg"]] Dark Nebulae
Dark nebulae are clouds of [|dust] which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. (NGC 2264 shown; see also the [|Horsehead Nebula]) ==

Planetary Nebulae== Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our [|Sun] will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with [|planets]; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across. ([|M 57] shown) ==

Supernova Remnants== Supernovae occur when a massive star ends its life in an amazing blaze of glory. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant. A typical supernova remnant is at most few light-years across. ([|M 1] shown)