nebula

Nebula is a term for a large cloud of gas and dust in interstellar space. They are key components of the interstellar medium and can be both the sites of star formation and the remnants of stellar death.

The main types include emission nebulae, which glow from ionized gas around hot young stars; reflection nebulae, which shine by reflected starlight; and dark nebulae, which obscure background light. A planetary nebula is a shell ejected by a dying star, and a supernova remnant is the expanding debris of an exploded star.

Nebulae are composed primarily of hydrogen, helium, and dust. Their sizes range from a few light-years to hundreds of light-years, with masses from thousands to millions of solar masses in star-forming regions. They are observed across many wavelengths, from radio to X-ray.

They form when regions in giant molecular clouds collapse under gravity, often triggered by turbulence, stellar winds, or nearby supernovae. Within many nebulae, gravity concentrates material into protostars that may ignite as new stars.

Observations rely on optical telescopes to see glow in visible light, while infrared and radio observations penetrate dust to reveal embedded stars. Famous examples include the Orion Nebula, the Eagle Nebula's Pillars of Creation, the Helix Nebula, and the Crab Nebula.

The term nebula has historical roots in astronomy to describe fuzzy celestial objects; later distinctions recognized as gas clouds and later by composition and emission processes. Modern study uses spectroscopy and multiwavelength imaging to analyze physical conditions.