A nebula is a cloud of gas and dust in space. Some are nurseries where new stars form, others are the remains of dying or exploded stars. They are classified mainly by how they shine, or fail to.
Emission nebulae
Hot, young stars flood nearby hydrogen gas with ultraviolet light. The gas is ionized and glows, often red from hydrogen-alpha light. Most are active star-forming regions, sometimes called H II regions.
- Orion Nebula (M42): visible to the naked eye as a fuzzy patch in Orion’s sword, about 1,300 to 1,400 light-years away.
- Lagoon Nebula (M8) and Eagle Nebula (M16), home of the “Pillars of Creation”.
Reflection nebulae
Dust clouds that do not glow themselves but reflect the light of nearby stars. They usually look blue, for the same reason the sky is blue: shorter wavelengths scatter more. The wisps around the Pleiades are a classic example.
Dark nebulae
Dense clouds of dust that block light from behind. They appear as dark shapes against brighter regions. Examples: the Horsehead Nebula in Orion and the Coalsack in the southern Milky Way. Infrared telescopes can see through them to the young stars inside, see how stars form.
Planetary nebulae
Despite the name, they have nothing to do with planets. The term dates from the 18th century, when they looked like small disks in telescopes. They are shells of gas cast off by Sun-like stars at the end of their lives, lit by the hot core left behind, which becomes a white dwarf, see white dwarfs. Examples: the Ring Nebula (M57) and the Helix Nebula. They last only tens of thousands of years.
Supernova remnants
Expanding debris from exploded stars. The Crab Nebula (M1) is the remnant of a supernova recorded by Chinese astronomers in 1054, with a pulsar spinning at its center, see supernovae and pulsars.
Seeing nebulae yourself
- The Orion Nebula is the easiest target: visible in binoculars from November to March in the Northern Hemisphere.
- Dark skies matter more than telescope size. A “nebula” or OIII filter helps on emission and planetary nebulae.
- Colorful photos come from long exposures. Through an eyepiece, most nebulae look gray.
See how telescopes work.
