What Is A Nebula?
QUICK ANSWER
A nebula is a vast cloud of gas and dust in space. Some are stellar nurseries where new stars form; others are remnants of dying or exploded stars. The most famous include the Orion Nebula (a stellar nursery) and the Crab Nebula (a supernova remnant). Nebulae produce some of the most photogenic images in astronomy.
Nebulae are clouds of gas and dust in space, often spanning many light years. Some are stellar nurseries actively forming new stars; others are the remains of dying or exploded stars. Nebulae are also some of the most visually stunning objects in the night sky, producing the iconic colorful images that astronomy is famous for.
What are nebulae made of?
Mostly hydrogen gas, with some helium and traces of other elements, plus dust. According to NASA, nebulae are vast clouds of gas and dust in space, ranging from a few light years across to hundreds of light years. They can be quite tenuous (denser than empty space but much thinner than Earth's atmosphere) yet still glow brightly when illuminated by nearby stars. The dust in nebulae often scatters or absorbs light, creating the dark patches sometimes seen in nebula images.
What types of nebulae are there?
Several main types. Emission nebulae glow because their gas is ionized by hot nearby stars (the Orion Nebula is a famous example). Reflection nebulae glow because their dust reflects starlight (the Pleiades are surrounded by one). Dark nebulae appear black because they block light from objects behind them (the Horsehead Nebula is iconic). Planetary nebulae are shells of gas thrown off by dying stars (despite the name, they have nothing to do with planets). Supernova remnants are the expanding gas from exploded stars.
Where do nebulae come from?
Either star formation or stellar death. Stellar nursery nebulae form in regions where interstellar gas is dense enough to start collapsing under gravity. Once stars form, their radiation and stellar winds shape the surrounding gas into the dramatic structures we see. At the other end of stellar life, dying stars eject their outer layers, creating planetary nebulae. Exploding supernovae create supernova remnants that can persist for thousands of years. Nebulae are essentially the beginning and end states of stars.
Which are the most famous nebulae?
Several are well-known. The Orion Nebula, visible in Orion's sword, is the closest stellar nursery and visible to the naked eye. The Crab Nebula is the remnant of a supernova observed by Chinese astronomers in 1054 CE. The Eagle Nebula's Pillars of Creation became iconic after Hubble photographed them in 1995. The Ring Nebula is a textbook planetary nebula visible through small telescopes. The Helix Nebula and the Carina Nebula are also popular targets for amateur astronomers and stunning Hubble images.
A nebula is a vast cloud of gas and dust in space, with several distinct types based on origin and behavior. Some nebulae are stellar nurseries forming new stars; others are remnants of dying or exploded stars. The Orion Nebula, Crab Nebula, and Pillars of Creation are among the most famous. Nebulae represent both the birth and the death of stars in the cosmic cycle.
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