Topics · Space Codex
What is a nebula?
Nebulae are clouds of gas and dust.
Clouds where stars are born or die
At a glance
| Measure | Value | Source |
|---|---|---|
| Famous nursery | Orion Nebula, M42, about 1,300–1,350 lyNASA Science: Hubble Messier catalog: M42 (opens in a new tab) | NASA Science: Hubble Messier catalog: M42 (opens in a new tab) |
| Hydrogen glow | Deep red H-alpha at 656 nmESA: ESA/Webb weic2216 (opens in a new tab) | ESA: ESA/Webb weic2216 (opens in a new tab) |
| Dying stars | Ring, Helix and Crab nebulaeESA: ESA/Webb weic2320 (opens in a new tab) | ESA: ESA/Webb weic2320 (opens in a new tab) |
| First nebula spectrum | 1864, William Huggins: bright lines, so glowing gasHuggins & Miller 1864, Phil. Trans. R. Soc. 154:437 (opens in a new tab) | Huggins & Miller 1864, Phil. Trans. R. Soc. 154:437 (opens in a new tab) |
| 'Nebulium' explained | 1927: the green lines are doubly ionised oxygenBowen 1927, Nature 120:473 (opens in a new tab) | Bowen 1927, Nature 120:473 (opens in a new tab) |
Key points
Emission nebulae glow when young stars ionise hydrogen; planetary nebulae and supernova remnants are the debris of dying stars. Orion (M42) is a nearby stellar nursery visible to the naked eye.
- Webb sees through dust with infrared light
- Heavy elements in your body were recycled through nebulae
- Webb and Hubble colours are mapped to filters, not what an eye would see
How we learned they are gas
For centuries nobody could tell whether nebulae were gas or swarms of distant stars. In 1864 William Huggins pointed a spectroscope at the Cat's Eye Nebula and saw bright emission lines instead of the continuous rainbow of starlight: it had to be glowing gas. Its brightest green lines matched no known element and were blamed on a new one, 'nebulium', until Ira Bowen showed in 1927 that they come from doubly ionised oxygen in gas far thinner than anything on Earth.
Kinds of nebula
Emission nebulae such as Orion and Carina glow because ultraviolet light from hot young stars ionises their hydrogen. Reflection nebulae shine blue by scattering the light of nearby stars, as around the Pleiades. Dark nebulae, like the Horsehead, are dense dust clouds seen in silhouette. Planetary nebulae such as the Ring and the Helix are shells cast off by dying Sun-like stars, and supernova remnants such as the Crab are the debris of exploded massive stars.
A sense of scale
Planetary nebulae are small and brief: the Ring is about a light-year across and the Helix's bright ring nearly three, and each fades within tens of thousands of years. Star-forming clouds are far larger and longer-lived; the Carina complex spans on the order of a hundred light-years or more. Even the bright parts of a nebula are vastly thinner than Earth's air. They shine because they are enormous, so a long sightline adds up a great deal of faint glow.
Common misconceptions
Planetary nebulae have nothing to do with planets; William Herschel named them in the 1780s because their round discs looked planet-like in his telescope. Nebulae also look far less colourful through an eyepiece than in photographs, because night vision is poor at seeing colour: most appear grey, with a green tint at best. And nebulae are not permanent. Starlight, stellar winds and supernovae disperse them, recycling the gas into later generations of stars.
Related entries
- Orion NebulaM42 · the nearest large stellar nursery
- Pillars of CreationStar-forming columns in the Eagle Nebula
- Crab NebulaM1 · the wreck of the supernova of 1054
- Ring NebulaM57 · a dying Sun-like star
Sources
- NASA Science: Hubble Messier catalog: M42 (opens in a new tab)
- ESA: ESA/Webb weic2216 (opens in a new tab)
- ESA: ESA/Webb weic2320 (opens in a new tab)
- Huggins & Miller 1864, Phil. Trans. R. Soc. 154:437 (opens in a new tab)
- Bowen 1927, Nature 120:473 (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.