Nebulae and Molecular Clouds

Nebulae are large, dense clouds of hydrogen, helium and cosmic dust. They are very dense (100 to 1000 molecules per cm3) compared to empty space, but are hardly dense at all compared to earth’s atmosphere (2.5 x 1019 molecules per cm3). In fact, they are less dense than the best of Earth's artificial vacuums. There are several types of nebulae.

Planetary Nebulae are clouds of hydrogen which are expelled from a low mass star upon its death. They are illuminated by the white dwarf which they surround. The nebula which results from a supernova is known as a supernova remnant.

Molecular Clouds are a particular type of nebula, typically ranging from 50 light years to 300 light years across and containing anywhere between 105 and 107 solar masses of material. They normally range in temperature from 10 Kelvin to 50 Kelvin and last for about 10 to 100 million years before dissipating.

Giant Molecular Clouds, such as the Orion Nebula, form stars by gravitationally coalescing, eventually accumulating enough mass in a single spot to form a protostar.

Worldbuilding in Practice
"The largest star systems may seem huge to us, but they are minuscule compared to the vast Nephele Nebula. With a diameter of about 86 pc and a total mass of around 106 solar masses, it is the cradle of millions of stars. In another 20 million years or so, the nebula will disappear completely, but until then, it remains one of the most beautiful sights in the Armstrong Sector, and one of the most popular desktop wallpapers across the galaxy."