Interstellar Reddening, Extinction, and Red Sunsets
- Extinction is the technical term for the dimming
of starlight by interstellar dust.
- Dust grains are very effective at absorbing/scattering light
which has a wavelength about the size of the dust grain.
- Short wavelengths are absorbed/scattered.
- Long wavelengths pass unhindered.
- Dust grains in the interstellar medium are about 1 micron
in diameter or less.
- Interstellar grains block blue light (wavelength=0.4µm) effectively, but red
light (wavelength=0.7µm) can get through more easily.
- Because blue light is blocked more effectively than red, interstellar
clouds containing dust tend to make distant stars appear redder than they actually
are.
- This effect is called interstellar reddening.
- The Earth's atmosphere contains small dust particles as
well which make sunsets appear red (particularly after volcanic eruptions add
dust to the atmosphere).
- Because infrared and radio light have much longer wavelengths
than
visible light interstellar clouds are nearly transparent to these
types of light.
- We can use infrared and radio observations to
study distant regions of our galaxy which are hidden
from view at visible wavelengths.
- Similarly, Titan's atmosphere is filled with obscuring particulate
grains. The visible-wavelength images
of Titan show only the haze while infrared-wavelength images penetrate the haze to show surface features.
Revised April 22, 2002
