Nebulae
Interstellar clouds of gas and dust — stellar nurseries and remnants.
Overview
A nebula is an interstellar cloud of gas and dust. The word covers several physically distinct classes: regions where stars are forming, shells ejected by dying stars, debris from explosions, and cold clouds that simply block the light behind them.
- The single word 'nebula' hides at least four unrelated physical situations.
- Emission nebulae glow because nearby hot stars ionise them; reflection nebulae only scatter light.
- Planetary nebulae have nothing to do with planets — the name is an eighteenth-century telescopic misimpression.
- The colours in published nebula images usually map specific emission lines, not what the eye would see.
Emission nebulae
An emission nebula glows by its own light. Ultraviolet radiation from nearby hot, young, massive stars ionises the surrounding hydrogen, and when electrons recombine with protons they emit at specific wavelengths — most prominently the red hydrogen-alpha line at 656.3 nanometres, which dominates the appearance of these objects in images.
Such regions are called H II regions, and they mark active star formation because the ionising stars are short-lived and cannot have travelled far from where they formed. The Orion Nebula, about 1,300 light-years away, is the nearest large example and is visible to the unaided eye as a fuzzy patch below Orion's belt.
Reflection and dark nebulae
A reflection nebula contains no ionising source. Dust grains simply scatter light from nearby stars, and because scattering is more efficient at shorter wavelengths the result is characteristically blue — the same physics that makes Earth's sky blue. The nebulosity around the Pleiades is a well-known example.
Dark nebulae are the same cold dusty material seen without illumination behind or within it. They are visible only as silhouettes against a brighter background: the Horsehead Nebula is a dark cloud outlined against an emission region, and the dark lanes splitting the Milky Way are dust clouds in the Galactic disc, not gaps between stars.
Planetary nebulae
A planetary nebula is the ejected outer envelope of a dying low- or intermediate-mass star, ionised by the exposed hot stellar core left behind — which is on its way to becoming a white dwarf. The name comes from William Herschel's era, when small round telescopic images resembled planetary discs; it is entirely unrelated to planets and is retained only by convention.
These are short-lived objects, dispersing over tens of thousands of years, which is why relatively few are known despite the process being the normal fate of most stars. They are a principal route by which carbon and nitrogen produced inside stars are returned to the interstellar medium.
Supernova remnants
When a massive star explodes, or a white dwarf detonates, the ejected material expands into the surrounding medium at thousands of kilometres per second, shock-heating gas and generating emission across the spectrum from radio to X-rays.
The Crab Nebula is the remnant of the supernova recorded by Chinese and other observers in 1054 CE, and it contains a pulsar — the neutron star left by the collapsed core. Remnants are the primary mechanism by which heavy elements synthesised in massive stars are dispersed, and their shocks are believed to accelerate a large share of Galactic cosmic rays.
Why nebula images look the way they do
Published nebula images are usually composites of narrowband exposures, each isolating a specific emission line, assigned to display colours. The widely used Hubble palette maps sulphur, hydrogen and oxygen lines to red, green and blue respectively — which is why familiar objects appear in colours quite unlike their visual appearance.
This is a legitimate scientific visualisation choice, not decoration: it separates physically distinct emitting species that would otherwise blend. It is worth knowing that the eye at the eyepiece sees mostly faint grey, because at low light levels human colour vision barely operates.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why are planetary nebulae called that if they are not planets?
- Because of how they looked through eighteenth-century telescopes. Their small, round, greenish discs resembled the appearance of planets such as Uranus, and the name stuck. They are actually the ejected outer layers of dying low- and intermediate-mass stars, ionised by the exposed stellar core.
- What makes emission nebulae red?
- Hydrogen-alpha emission at 656.3 nanometres, in the red part of the spectrum. Ultraviolet light from hot nearby stars strips electrons from hydrogen atoms, and when the electrons recombine and cascade down energy levels they emit at fixed wavelengths, with H-alpha usually the strongest in the visible range.
- Why do nebulae look grey through a telescope but colourful in photographs?
- Because human colour vision requires more light than these faint extended objects provide, so at the eyepiece the rod cells dominate and the view is essentially monochrome. A camera accumulates light over long exposures and registers colour that the eye cannot. Many published images additionally use narrowband filters mapped to display colours to separate different emitting elements.