Types of nebulae

The word nebula comes from the Latin 'nebula' i.e. cloud. Clouds of gas and dust among the stars, nebulae are both active nurseries and cemeteries of stars. These wonders of the sky are illuminated by the stars they contain or by nearby stars located behind them. These nebulae far are the best pictures of astronomy telescopes that we offer.
Nebula contain wonderful agglomerations of stars, dust and gas often forming figures that allow us to recognize them. Of course, more the telescope could be powerful, more images are beautiful and colorful. Only long exposures reveal the entire palette of colors, the stunning pink hydrogen, blue helium, nitrogen, red, blue-green oxygen.

But the nebula can be dark. It's William Hershel, who discovered the dark nebulae. These nebulae are clouds of gas and dust without visible star, too dense to let the light of stars behind.

NB: NGC (New General Catalogue) is one of the most popular catalogs in the field of amateur astronomy with the Messier catalog.

Image: In one image all the splendors of the constellation of Orion (left to right) of the Flame Nebula and the dark channels (NGC 2024), the Horsehead and Barnard 33, NGC 1977 (blue and purple) and the Orion Nebula and M 42 (orange-red at right).

A dark nebula is a cloud of dust and cold gas, which does not emit visible light, it hides the stars it contains. However, the dust that make up these clouds have an mean diameter of 1 micron (0.001 mm). Their density is that of cigarette smoke. These small grains of material are assembling a small number of molecules (carbon grains, silicate grains, grains with ice mantles).

The diffuse reflection nebulae are composed of hydrogen, the most abundant element in the universe, but also dust.
The reflection nebulae reflect light visible only send them nearby stars where the stars they contain.
This dust has the distinction of returning the color blue.
The nebulae surrounding the Pleiades, are excellent examples of this type of blue nebula.

The emission line nebula and emission nebula emit their own light.
The hydrogen atoms that are excited by the powerful ultraviolet light of nearby stars. The hydrogen is then ionized, that is to say, it loses its single electron emitting a photon. This generates the brightness of the nebula.
The stars of spectral type O can ionize the gas within a radius of 350 light-years. The Swan Nebula or M17 is an emission nebula discovered by De Chéseaux in 1746 and rediscovered by Messier in 1764.
It is located in Sagittarius. Also known by the names of Omega Nebula, the Swan, the Horseshoe, or Lobster, this nebula very bright, color pink is visible to the naked eye in low latitudes (apparent magnitude 6).
This is because it is home to young stars born from the nebula that radiate around the gas, creating an HII Called HII regions, regions formed cloud composed mainly of hydrogen and most of atoms are ionized and extend over several light years. The ionization is produced by the proximity of one or more very hot stars of spectral type O or B, which radiate strongly in the extreme ultraviolet, and ionizing the gas around, from which these stars formed.  region, the red color of the nebula is also that of ionized hydrogen.

Infrared was able to observe a significant amount of dust in favor of star formation.
Within the nebula would be an open cluster consists of thirty stars obscured by the nebula. The diameter of the nebula around 40 light years.
The total mass of gas which forms the Omega Nebula is about 800 times that of the Sun.
M17 is located 5500 light years from our solar system. M16 and M17 are in the same spiral arm of the Milky Way (the arm of the Sagittarius or Sagittarius-Carina) and are perhaps part of the same complex of giant clouds of interstellar matter.

Image: Omega Nebula and Horseshoe, covers about 3 light years away. It was taken in 2003 to celebrate the thirteenth year of the cosmic journey of the Hubble telescope.

The emission nebulae are often mixed with reflection nebulae.
This is true of the Orion Nebula, also known as the M42. It has a telescope one of the finest performances of the sky.
The glowing gas of M42 surrounds hot young stars located at the edge of a huge molecular cloud located about 1 500 light-years from Earth.
At the heart of the nebula, four blue stars, forming a trapezoid, illuminate long-range matter dispersed in space.
The atoms absorb the starlight and re-emit according to their own colors, those of oxygen that is in the green, hydrogen and nitrogen in the red.
Radio astronomical observations reveal that the Orion Nebula is a small part of the large opaque cloud of Orion.
Contraction of the cloud were born the Trapezium stars, and a group of proto-stellar nebulae located behind the Orion Nebula.
The Orion Nebula offers one of the best opportunities to study the birth of stars. It is the largest and the closest region of star formation.

The energy star of the nebula have blown the dust and the surrounding gas, which allows us to see.

Image: The Orion Nebula, also known as the M42 and NGC 1976, is an emission nebula and reflection. It is green in the heart of the constellation Orion.
The nebula is a large part of the center of the image, the small nebula located on the left above the nebula is M43.

If the diffuse nebulae are associated with the birth of stars, planetary nebulae are the remnants of stars. The name "planetary nebula" comes from the first observations of these objects sometimes have a circular aspect. End of life a star shines especially in the ultraviolet. This radiation illuminates the previously expelled gas by ionizing radiation, and thus forms a planetary nebula. The colors observed from the various elements more or less ionized emitting each in a very specific wavelength. The hydrogen atoms emit a red light, while the oxygen glows green.
The Helix Nebula is a cosmic star often photographed by amateur astronomers for its vivid colors and its resemblance to a giant eye. Discovered in the 18th century, is located about 650 light years away in the constellation Aquarius.
It belongs to the class of objects of planetary nebulae. Planetary nebulae are remnants of stars similar in their past to our Sun.

When these stars die, they expel into space outside their gaseous layers. These layers are heated by the hot core of the dead star, a white dwarf, and shine in the infrared wavelengths and visible.

Image: The Helix Nebula, or NGC 7293 as seen by the Spitzer Space Telescope.
The outer gaseous layers are shown in blue and green light. The tiny white dot in the center of the photograph is the white dwarf star immersed in a surprisingly bright infrared glow.
The red color in the middle of the eye is the final layers of gas blown at the death of the star.
The red circle in the center light is the light of a dust disk surrounding the white dwarf star. All the inner planets of the system were charred or sprayed while the volume of the dying star rose.