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Rosette Nebula    
The Rosette Nebula has recently given birth to a star cluster. The star cluster in the middle is releasing supersonic winds that are making the nebula spread out and eventually the nebula will disappear. Click the image for a closeup of the central star cluster.    
Image copyright R. Gendler

Stars are born inside massive clouds of gas and dust called nebulae. There are many types of nebulae such as emission, reflection and dark. Nebulae contain hydrogen and helium and this condenses inwards to clump together to form protostars. These protostars become larger and become main sequence stars. The Sun is currently in its main sequence phase of its life and will die in 5 billion years time.

When material inside an intergalactic nebula reaches a specific density, then the strong forces of gravity collapse inwards until an accretion disk is formed. Then the temperature increases until nuclear fusion starts at the core. Eventually the star that is formed releases blowback winds that causes the nebula to dissipate and eventually disappear.

Protostars that blow surrounding gas away are called T Tauri stars. Nearby dust that the stars are shrouded in is turned into protoplanetary disks, which are the early precursors of solar systems. When the nebula clears away, the disks evolve into fully fledged solar systems with planets. Many extrasolar planets orbiting far away stars have been discovered.

   
   
There are three types of nebulae involved with the birth and life of stars. These are emission, reflection and dark. There is also a fourth type of nebula called planetary but this is involved with star death.
   
 
The Lagoon Nebula is an emission nebula that is 6000 light years away. Mouseover for a more detailed false colour image.  
Image copyright R. Croman
Emission nebulae glow from the ultraviolet light produced by stars inside them. The radiation ionizes the gas and this in turn makes it glow brightly. Many starforming nebulae are of this type and they are occasionally referred to as HII regions as they contain a lot of hydrogen. Emission nebulae are also sites of very active star formation. The above image for example shows a nebula that has many stars being formed inside it. The colour of emission nebulae is dependant on the internal elements that can be ionised. Many are red because they contain hydrogen although there are pink, purple and turquoise nebulae.
   
Iris Nebula    
The Iris Nebula is a very close reflection nebula. It is quite small and has only produced one single star instead of a cluster.    
Image copyright R. Jay GaBany
 
Reflection nebulae are as the name suggests, clouds of gas and dust that reflect the light of either embedded or adjacent stars. Some of them are starforming regions but others are not, which are runaway stars that are travelling at high speeds that crash into dark clouds that are therefore lit up by the star. Examples are the Pleiades star cluster and the Flaming Star Nebula. The clouds contain heavy elements and the starlight reflects from these particles and the light is scattered in the blue wavelength or the same wavelength as the star. Most reflection nebulae are blue because they are more efficient at the scattering of blue light than any other colour.
   
Horsehead Nebula  

Dark nebulae don't glow as the gas inside them isn't ionized by the embedded stars and they don't reflect light from nearby stars. The only way that they are visible is by blocking the light of whatever is behind them. The most noticeable ones are the one found near the centre of the Milky Way in Sagittarius, Scorpius and Ophiuchus. They are surprisingly active sites of star formation and are often found near emission nebulae.

There are also other types of nebulae that don't really fit into any of these categories such as Wolf Rayet nebulae and Herbig Haro nebulae.

There are also many areas in the Milky Way that contain many different types of objects. Examples are the Trifid Nebula, which has emission and reflection parts and the Christmas Tree Cluster region, which contains open clusters, emission nebulae, reflection and dark nebulae.

The Horsehead Nebula is a famous dark nebula in the constellation of Orion. It blocks an emission nebula behind it and as you already shoud know, the blue nebula is a reflection nebula.  
Image copyright R. Jay GaBany
           
NGC 2170      
This region found in the constellation of Monoceros is an exotic and colourful mix of emission, reflection and dark nebulae. It is referred to by the unceremonious name of
NGC 2170 as it doesn't look like anything.
     
Image copyright R. Croman