Cosmic dust makes distant stars look redder, like a smoky sunset
Starlight crossing the galaxy runs a gauntlet of dust. The grains scatter and absorb blue light more readily than red, so faraway stars arrive dimmer and ruddier than they really are. It is the same physics that paints sunsets orange here on Earth, and astronomers must undo it before they can trust a star's true colour or distance.
Astronomers call this dimming extinction. Friedrich Georg Wilhelm von Struve noticed its effects in 1847, and several observers saw stars' colours shifted without realising dust was spread throughout the galaxy. Robert Julius Trumpler finally documented interstellar extinction as such in 1930. Reddening is not redshift: a redshift slides a whole spectrum to longer wavelengths, whereas dust simply strips out more of the short-wavelength light and leaves spectral lines where they were.
Near the Sun, visible light loses roughly 0.7 to 1.0 magnitudes per kiloparsec, meaning a star's brightness roughly halves for every 3,260 light years of distance. Toward the galactic centre the dust is so thick that extinction exceeds 30 magnitudes, letting through fewer than one optical photon in a trillion. That murk creates a zone of avoidance across the sky, hiding galaxies such as Dwingeloo 1, which turned up only recently through radio and infrared searches. Because dust interferes far less at longer wavelengths, measuring stars in the near infrared helps pin down cosmic distances.
To gauge the effect, astronomers compare a star's spectrum with that of a similar star known to be unreddened. Glowing gas clouds offer a neat shortcut: the ratio of two hydrogen emission lines should sit near 2.85 almost regardless of conditions, so any departure reveals the dust in between. Most dust follows the spiral arms, as it does in other spiral galaxies. A broad dip in ultraviolet light near 2175 angstroms, first seen in the 1960s, remains poorly explained, though carbon-rich grains are suspects. It is weak or missing in parts of the Magellanic Clouds and in starburst galaxies, hinting that intense star formation reshapes the grains.
Earth's own atmosphere adds extinction too, through scattering by air molecules and particles and absorption by oxygen, ozone and water. Some wavelengths never reach the ground at all, which is why X-ray, ultraviolet and much infrared astronomy happens from satellites.
Source: Extinction (astronomy)
