Nebulae are found inside galaxies, filling the space between stars or enveloping stars like a cloak. They’re made of dust and gas and can appear as either bright or dark clouds. The gas is mostly hydrogen mixed with some helium.

Dark nebulae are usually seen together with emission and reflection nebulae. The Horsehead Nebula in Orion is probably the most famous example of a dark nebula. It is a dark region of dust in the shape of a horse’s head that blocks the light from a much larger emission nebula behind it.

There are five types of cloudy or nebulous objects in the sky: planetary nebulae, emission nebulae, reflection nebulae, dark nebulae and supernova remnants.

The star becomes a white dwarf, and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution. For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the resulting plasma.

The nebula is illuminated by a central star, which sometimes is too faint to be seen. Although initially grouped with galaxies and star clusters under the class of “nebulae”, we now know that galaxies and star clusters are made up of stars, whereas planetary nebulae are gaseous.

: a usually compact luminous ring-shaped nebula that is composed of matter which has been ejected from a hot star at its center.

The Helix Nebula (aka NGC 7293) is one of brightest and closest examples of a planetary nebula, a gas cloud created at the end of the life of a Sun-like star. A classic planetary nebula, the Cat’s Eye (NGC 6543) represents a final, brief yet glorious phase in the life of a sun-like star.

These colors are the result of different elements within the nebula. Most nebulae are composed of about 90% hydrogen, 10% helium, and 0.1% heavy elements such as carbon, nitrogen, magnesium, potassium, calcium, iron.