Chapter 18 The Interstellar Medium

18.1 Interstellar Matter

• A wide-angle view of the Milky Way— the dark regions are dust clouds, blocking light from the stars beyond

• The interstellar medium consists of gas and dust

• Gas is atoms and small molecules, mostly hydrogen and helium

• Dust is more like soot or smoke; larger clumps of particles

• Dust grains: ~0.1µm

• Transparent to long-wavelength (radio waves); opaque to short-wavelengths (visible light)

• Extinction: dimming of star light by dust

• Reddening: can interfere with blackbody temperature measurement, but spectral lines do not shift

• Reddening: Blue light (high frequency) is greatly reduced while red light (low frequency) is slightly reduced

• The shape of the dust grains is elongated, rather than spherical. Because dust grains polarize light passing through them