A nebula comprises a cloud of dust, hydrogen, helium and other ionized gases resulting in some of the most beautiful objects in the Universe.  Most are very large; some in excess of several hundred light-years in diameter.  While large, they are extremely tenuous; for example, a volume of a nebula about the size of the Earth would have a mass of only a few kilograms. 

Nebulæ

Astronomy & Cosmology -

Galaxies

WILLIAM & DEBORAH HILLYARDWILLIAM & DEBORAH HILLYARD
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Eagle Nebula - M16
The Eagle Nebula is a young open cluster of stars famous for the "Pillars of Creation" and the "Spire", two out of a number of active star-forming regions in the nebula.