There are a great number of stellar wallpaper images available for the Macintosh. It isn't possible for us to know which one you are referring to.
However, it is quite likely that the image you are referring to is one of these:
http://hubblesite.org/gallery/wallpaper/
In astronomy - there does not appear to be a nebula with that name.Other options could be:The Green Leopard Plague which won the nebula awards.A fictional wallpaper made with photoshop and used in Mac OSX
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"The Crab" is a nebula.
The crab nebula.
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protostar or nebula
The nebula you are referring to is likely the Swan Nebula, also known as the Omega Nebula. It is a famous emission nebula located in the constellation Sagittarius. The name "Swan Nebula" comes from its resemblance to a swan when viewed in certain images.
Galaxy
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The Nebula Theory is also known as the Solar Nebula Theory. It is a widely accepted model for the formation of our solar system.
The Horsehead Nebula can be found in the constellation Orion. Its name comes from its resemblance to a horse's head. The Horsehead Nebula was first discovered by Williamina Fleming in 1888.