NEBULA
CLASS A: ABSORPTION
Also known as a dark nebula, these interstellar clouds are so dense that they obscure the light behind them. Often located within larger molecular clouds, dark nebulae are rich in hydrogen, helium, and ammonia, and often serve as a birthplace for stars and planets.
Their forms are often irregular, with no clearly defined boundaries and many contort into unique serpentine shapes.
A nebula is an interstellar cloud of dust, hydrogen, helium, and other ionized gases. Most nebulae are vast, and the largest can span hundreds of light years. They are visible due to fluorescence caused by stars within, and while they are denser than the space around them, a nebular cloud the size of the planet Earth would have a total mass of only a few kilograms.
Nebulae are often rich in heavy metals, and as a result are ideal star-forming regions. Pillars of gas and dust clump together into denser regions, which attract further matter and over the course of many eons, grow dense enough to form stars.
CLASS E: EMISSION
An emission nebula is a large, self-illuminating molecular cloud composed of ionized gasses including hydrogen, helium, oxygen, and nitrogen. Their densities are highly varied, though the largest emission nebulae may span hundreds of light years and grow dense enough to support star formation.
CLASS P: PLANETARY
Despite the name, planetary nebulae have nothing to do with planets. They are, in fact, the final stage of life for many lower-mass stars, and take shape when the star is no longer able to sustain nuclear fusion. As the star’s core contracts, it ejects ionized gases into space, creating a planetary nebula. This plays a crucial role in the evolution of the galaxy, for the process returns material to the interstellar medium.
CLASS R: REFLECTION
Reflection nebulae are large clouds of dust that reflect light from nearby stars. The nearby stars are not usually hot enough to cause ionization, but are bright enough to make the dust visible. Reflection nebulae are usually less dense than most other kinds of nebula.
CLASS S: SUPERNOVA REMNANT
When a massive star reaches the end of its life, it explodes in an immensely powerful supernova. This event blows the entire star apart, leaving in its wake a remnant nebula that expands into the interstellar medium. The dispersal of heavy elements made during the explosion make these remnants crucial to the evolution of galaxies.
