thumb|200px|The [[Witch Head Nebula|Witch Head reflection nebula (IC2118), about 1000 light years from earth, is associated with the bright star Rigel in the constellation Orion. The nebula glows primarily by light reflected from Rigel, located just outside the top right corner of the image. Fine dust in the nebula reflects the light. The blue color is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red.]]
In
Astronomy,
reflection nebulae are
clouds of dust which are simply reflecting the light of a nearby
star or stars. The energy from the nearby star, or stars, is insufficient to
ionize the gas of the nebula to create an
emission nebula, but is enough to give sufficient
scattering to make the dust visible. Thus, the
frequency 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 the galactic magnetic field and cause the scattered light to be slightly
polarized (Kaler, 1998).
Edwin Hubble distinguished between the emission and reflection nebulae in 1922.
Reflection nebulae 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 as
diffuse nebulae. An example of this is the
Orion Nebula.
Some 500 reflection nebulae are known. Among the nicest of the reflection nebulae are those surrounding the stars of the
Pleiades. A blue reflection nebula can also be seen in the same area of the sky as the
Trifid Nebula. The
giant star Antares, which is very red (
spectral class M1), is surrounded by a large, red reflection nebula.
Reflection nebulae may also be the site of
star formation.
In 1922, Hubble published the result of his investigations on
bright nebulae. One part of this work is the Hubble luminosity law for reflection nebulae which make a relationship between the
angular size (
R) of the nebula and the
apparent magnitude (
m) of the associated star:
where
k is a constant that depends on the sensitivity of the measurement.
See also
