reddening

Reddening is the phenomena regarding the color of a visible astronomical object (e.g., star): it can appear more red from a long distance than from nearer. Within the visible wavelength range, the absorption of a given distance through interstellar medium (ISM) rises with frequency, absorbing more of the blue light than red. The red light is also undergoing absorption, so reddening is a hint that the objects are generally dimmer than they otherwise would be, and the term reddening is often used to mean this overall dimming effect. Where reddening is observed, lower-luminosity objects such as red dwarfs may be obscured entirely, which is termed extinction. Measurements of the amount of reddening reveal information regarding the dust (e.g., how much) along the line-of-sight, one measure being color excess (E_B-V, a particular color index), defined as:

EB-V = AB-AV
   where
 AV = V - V0
   and
 AB = B - B0

and

• B and V are its B and V apparent magnitudes.
• B₀ and V₀ are those that would be observed if there were no reddening.

The ratio of total to selective visual extinction (R_V) can be used to characterize dust in galaxies:

      AV
RV = ————
     EB-V

Differential reddening refers to a varying amount of reddening for different portions of a group of stars with basically similar reddening, such as a stellar cluster. If a cluster's reddening is consistent, a "straight" color-magnitude diagram reveals information on the stars, e.g., their age groupings. This is less valid if there is differential reddening, and techniques have been developed to work around this.

The basis of reddening, i.e., of the ISM's characteristic of absorbing more blue light than red, is not fully understood. Clearly any material's absorption and scattering will vary by wavelength and some affect on color is unsurprising. The observed affects on visible light are part of a general trend of absorption rising with frequency over the course of visible and much of ultraviolet (UV), leading to an absorption peak at around 800 angstroms (800 angstrom feature). A second contributor is a lesser absorption feature peaking at 2175 angstroms (2175 angstrom feature, aka 220 nm feature). The cause of the latter feature is still of research interest, a possibility being an effect of polycyclic aromatic hydrocarbons (PAHs).