Inastronomy,reflection nebulae areclouds ofinterstellar dust which might reflect the light of a nearbystar or stars. The energy from the nearby stars is insufficient toionize the gas of the nebula to create anemission nebula, but is enough to give sufficientscattering to make the dust visible. Thus, thefrequency spectrum shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with thegalactic magnetic field and cause the scattered light to be slightlypolarized.[1]
Analyzing the spectrum of the nebula associated with the starMerope in thePleiades,Vesto Slipher concluded in 1912 that the source of its light is most likely the star itself, and that the nebula reflects light from the star (and that of the starAlcyone).[3] Calculations byEjnar Hertzsprung in 1913 lend credence to that hypothesis.[4]Edwin Hubble further distinguished between the emission and reflection nebulae in 1922.[5]
Reflection nebula are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets).
Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to asdiffuse nebulae.
Some 500 reflection nebulae are known. A blue reflection nebula can also be seen in the same area of the sky as theTrifid Nebula. Thesupergiant starAntares, which is very red (spectral class M1), is surrounded by a large, yellow reflection nebula.
Reflection nebulae may also be the site ofstar formation.
In 1922,Edwin Hubble published the result of his investigations onbright nebulae. One part of this work is the Hubble luminosity law for reflection nebulae, which makes a relationship between theangular size (R) of the nebula and theapparent magnitude (m) of the associated star:
wherek is a constant that depends on the sensitivity of the measurement.
| Visible nebula | |
|---|---|
| Pre-stellar nebulae | |
| Stellar nebula | |
| Post-stellar nebulae | |
| Clouds | |
| Morphology | |
| Intergalactic blobs | |
 Lists |
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| Related | |
