IC 4592 (also known as the Blue Horsehead Nebula) is a reflection nebula in the Scorpius constellation that is lit by Nu Scorpii.
What is a reflection nebula?
In astronomy, reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars. The energy from the nearby 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.
Analyzing the spectrum of the nebula associated with the 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). Calculations by Ejnar Hertzsprung in 1913 lend credence to that hypothesis. Edwin Hubble further distinguished between the emission and reflection nebulae in 1922.
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, is surrounded by a large, red reflection nebula. Reflection nebulae may also be the site of star formation.
Image: Rogelio Bernal Andreo
Text: Wikipedia contributors. “IC 4592.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 19 Mar. 2017. Web. 9 Aug. 2017. & Wikipedia contributors. “Reflection nebula.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 18 Jan. 2017. Web. 9 Aug. 2017.