Reflection nebula

In astronomy, reflection nebulae are clouds (nebulae) of interstellar dust that "reflect" the light of neighboring stars, but more precisely, scatter it.

Unlike emission nebulae, the stars in reflection nebulae are not hot enough to ionize the material, so they do not cause the nebula to glow on its own. Instead, the starlight is scattered by the microscopic nebula particles, which is what makes the nebula visible in the first place. The spectrum of the reflection nebula therefore resembles that of the incoming stars. Reflection nebulae usually appear blue because blue light is scattered more than red light; a similar effect provides the blue color of the sky.

Among the nebular particles are many carbon compounds (including diamond dust), but also those of other elements, especially iron and nickel. The latter often align themselves with the galactic magnetic field and thus cause a slight polarization of the scattered light (James B. Kaler, 1998).

About 500 reflection nebulae are known. Especially splendid are the bluish reflection nebulae around the stars of the Pleiades M 45. Also blue is the reflection nebula at the northern edge of the Trifid Nebula M 20. The red supergiant Antares is surrounded by the large red reflection nebula Ced 132.

The distinction between reflection and emission nebulae goes back to Edwin Hubble, who published the result of his investigations of astronomical nebulae (English bright nebulae) in 1922. The two types of nebulae are now sometimes called diffuse nebulae and can often be observed together. Part of Hubble's work is the Hubble luminosity law for reflection nebulae, which establishes a relationship between the apparent magnitude of the nebula and the apparent magnitude of the associated star:

where is a constant that depends on the sensitivity of the measuring instruments.