- Reflection nebula
Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby staror stars. The energy from the nearby star, or stars, is insufficient to ionizethe gas of the nebula to create an emission nebula, but is enough to give sufficient scatteringto make the dust visible. Thus, the frequency spectrumshown 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 Hubbledistinguished between the emission and reflection nebulae in 1922.
Reflection nebulae are usually blue because the
scatteringis 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
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::5 log("R") = -"m" + "k"where "k" is a constant that depends on the sensitivity of the measurement.
* James B. Kaler (1998). "Kosmische Wolken. Materie-Kreisläufe in der Milchstraße." Spektrum Akademischer Verlag.
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