Why does interstellar dust make stars look redder?

Dust scatters and absorbs blue light more strongly than red, so starlight passing through dust loses proportionally more of its blue component. The transmitted light is therefore both dimmer and shifted toward red, an effect called reddening.

What is interstellar dust made of?

It is composed mainly of microscopic grains of silicate minerals and carbon-rich material, with a range of sizes. These grains form in the outflows of evolved stars and supernovae and are dispersed into the interstellar medium.

Interstellar Dust and Extinction

Tiny solid grains dispersed through interstellar space absorb and scatter starlight, dimming and reddening distant objects and shaping the chemistry of the gas.

Definition

Interstellar dust consists of micron-and-smaller solid grains of silicate and carbonaceous material mixed with the interstellar gas; extinction is the dimming and reddening of starlight caused by absorption and scattering by these grains.

Scope

This topic covers the composition and size distribution of interstellar dust grains, the wavelength dependence of extinction and reddening, the prominent extinction features and emission bands, the role of dust in heating and cooling and in catalyzing molecule formation, and the methods used to correct observations for dust.

Core questions

What are interstellar dust grains made of, and what sizes do they span?
How does extinction vary with wavelength, and what is reddening?
What spectral features and emission reveal the nature of dust?
How do astronomers correct observations for dust extinction?

Key theories

Grain size distribution: The classic MRN model describes interstellar grains as a power-law distribution of silicate and graphite sizes, reproducing the observed extinction across wavelengths.
The extinction curve: Extinction rises toward shorter wavelengths and shows a prominent ultraviolet bump, and its shape can be parameterized by a single quantity, providing standardized corrections for reddening.
Dust as a chemical catalyst: Grain surfaces enable reactions, most importantly the formation of molecular hydrogen, that cannot proceed efficiently in the gas phase, making dust central to interstellar chemistry.

Clinical relevance

Extinction must be corrected for in almost every observation of stars and galaxies, the dust reddening it produces both complicates and enables distance and reddening measurements, and dust grains drive the chemistry that builds molecules and eventually planets.

History

Robert Trumpler demonstrated interstellar absorption in 1930 by noting that distant clusters appeared too faint and reddened. The 1977 MRN grain-size model and the 1989 Cardelli, Clayton, and Mathis extinction parameterization became standard tools, refined by infrared and ultraviolet spectroscopy of dust features.

Key figures

Bruce Draine
John Mathis
Jason Cardelli
Geoffrey Clayton

Seminal works

mathis1977
cardelli1989
draine2003

Frequently asked questions

Why does interstellar dust make stars look redder?: Dust scatters and absorbs blue light more strongly than red, so starlight passing through dust loses proportionally more of its blue component. The transmitted light is therefore both dimmer and shifted toward red, an effect called reddening.
What is interstellar dust made of?: It is composed mainly of microscopic grains of silicate minerals and carbon-rich material, with a range of sizes. These grains form in the outflows of evolved stars and supernovae and are dispersed into the interstellar medium.