View source for Reflection nebula

 {{Short description|Clouds of interstellar dust which reflect the light of nearby stars}}
[[File:reflection.nebula.arp.750pix.jpg|thumb|200px|       The [[IC 2118|Witch Head reflection nebula]] (IC2118), about 900 light years from Earth, is associated with the bright star Rigel in the constellation [[Orion (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 [[interstellar cloud|clouds]] of [[Cosmic dust|interstellar dust]] which might reflect the light of a nearby [[star]] or stars. The energy from the nearby stars is insufficient to [[Ionization|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.&nbsp;g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with the [[Galaxy#Magnetic fields|galactic magnetic field]] and cause the scattered light to be slightly [[Polarization (waves)|polarized]].<ref>Kaler, 1997.</ref>

==Discovery==
[[File:Young star lights up reflection nebula IC 2631.jpg|left|thumb|Reflection nebula [[IC 2631]].<ref>{{cite web|title=A Star's Moment in the Spotlight|url=http://www.eso.org/public/news/eso1605/|access-date=10 February 2016}}</ref>]]
[[File:VdB1.jpg|thumb|200 px|left|Reflection nebula vdB1]]
Analyzing the spectrum of the nebula associated with the star [[Merope (star)|Merope]] in the [[Pleiades]], [[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 star [[Alcyone (star)|Alcyone]]).<ref>{{cite journal|bibcode=1912LowOB...2...26S|title=On the spectrum of the nebula in the Pleiades |last1=Slipher|first1=Vesto M.|volume=2|date=1922|pages=26–27|journal=Lowell Observatory Bulletin}}</ref> Calculations by [[Ejnar Hertzsprung]] in 1913 lend credence to that hypothesis.<ref>{{cite journal|bibcode=1913AN....195..449H|title=Über die Helligkeit der Plejadennebel|last1=Hertzsprung|first1=E.|volume=195|issue=23|date=1913|pages=449–452|journal=Astronomische Nachrichten|doi = 10.1002/asna.19131952302 |url=https://zenodo.org/record/1424888}}</ref> [[Edwin Hubble]] further distinguished between the emission and reflection nebulae in 1922.<ref>{{cite journal|bibcode=1922ApJ....56..400H|title=The source of luminosity in galactic nebulae|last1=Hubble|first1=E. P.|volume=56|date=1922|pages=400|journal=Astrophysical Journal|doi=10.1086/142713|doi-access=free}}</ref>

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 as [[diffuse nebula]]e.

Some 500 reflection nebulae are known.  A blue reflection nebula can also be seen in the same area of the sky as the [[Trifid Nebula]]. The [[Red supergiant star|supergiant star]] [[Antares]], which is very red ([[Stellar classification|spectral class]] M1), is surrounded by a large, yellow reflection nebula.

Reflection nebulae may also be the site of [[star formation]].

==Luminosity law==
[[File:Cosmic dust clouds in reflection nebula Messier 78.jpg|thumb|200 px|Cosmic dust clouds in [[Messier 78]].<ref>{{cite news|title=Sifting through Dust near Orion's Belt|url=http://www.eso.org/public/news/eso1219/|access-date=2 May 2012|newspaper=ESO Press Release}}</ref> ]]
In 1922, [[Edwin Hubble]] published the result of his investigations on [[bright nebula]]e. One part of this work is the Hubble luminosity law for reflection nebulae, which makes a relationship between the [[angular size]] (''R'') of the nebula and the [[apparent magnitude]] (''m'') of the associated star:
{{block indent|1={{math|1=5 [[logarithm|log]]({{var|R}}) = &minus;{{var|m}} + {{var|k}}}}}}
where {{mvar|k}} is a constant that depends on the sensitivity of the measurement.

== See also ==
* [[Variable nebula]]
* [[List of reflected light sources]]

== References ==
{{reflist}}

==Bibliography==
* James B. Kaler (1997). ''Cosmic Clouds -- Birth, Death, and Recycling in the Galaxy'', Scientific American Library, Freeman, New York, 1998.

{{Commons category|Reflection nebulae}}

{{nebula}}

[[Category:Reflection nebulae| ]]
[[Category:Nebulae]]