Interstellar extinction

Interstellar extinction is the reduction and wavelength-dependent alteration of starlight as it travels through interstellar matter, chiefly dust that absorbs and scatters light.

Interstellar dust: Solid grains suspended in the space between stars, composed mainly of silicates, carbonaceous material, and ices. These grains cause most visible and ultraviolet extinction by absorbing and scattering starlight.
Interstellar reddening: The increase in a star’s observed redness caused by wavelength-dependent extinction along the line of sight. It is the color change paired with extinction’s reduction of total brightness.
Electromagnetic spectrum: The complete range of electromagnetic radiation, ordered by frequency or wavelength. Extinction changes across this spectrum, with dust affecting ultraviolet, visible, and infrared light differently.
Interstellar absorption lines: Narrow spectral features produced when atoms or molecules in interstellar gas absorb specific wavelengths. These discrete spectral imprints differ from dust’s broad, continuous attenuation.
Robert Julius Trumpler: A Swiss-American astronomer who established the importance of interstellar absorption through studies of open clusters. His cluster measurements provided early evidence that interstellar material dims distant starlight.
Mie scattering: Scattering of electromagnetic waves by particles whose sizes are comparable to their wavelengths. Its wavelength-dependent scattering helps shape how dust removes light from a stellar beam.
Color excess: The difference between an object’s observed color index and its intrinsic color index. It provides an observational measure of the reddening caused by extinction.
Apparent magnitude: A logarithmic measure of an astronomical object’s brightness as observed from Earth. Extinction increases a star’s apparent magnitude by making it look fainter.
Circumstellar extinction: Dimming and reddening caused by material surrounding an individual star rather than distributed between stars. Its local origin distinguishes it from extinction along the general interstellar path.
Trumpler's observations: Early twentieth-century measurements showing that distant open clusters appeared fainter and larger than expected. They helped demonstrate that interstellar extinction alters inferred stellar distances.

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