第20章星と星の間――宇宙のガスと塵Between the Stars: Gas and Dust in Space

Interstellar dust can be detected: (1) when it blocks the light of stars behind it, (2) when it scatters the light from nearby stars, and (3) because it makes distant stars look both redder and fainter. These effects are called interstellar extinction and reddening. Dust can also be detected in the infrared because it emits heat radiation. Dust is found throughout the plane of the Milky Way. The dust particles are about the same size as the wavelength of light and consist of rocky cores that are either sootlike (carbon-rich) or sandlike (silicates) with mantles made of ices such as water, ammonia, and methane.

星間塵は、（1）背後の星の光を遮るとき、（2）近くの星の光を散乱するとき、（3）遠くの星を、より赤く暗く見せることによって、検出できる。こうした効果を、星間減光と星間赤化という。塵は熱放射を出すので、赤外線でも検出できる。塵は天の川銀河の円盤面全体に見つかる。塵の粒は光の波長とほぼ同程度の大きさで、炭素に富むすす状のものか、ケイ酸塩からなる砂状の、岩石質の核を持つ。核の周りは、水・アンモニア・メタンなどの氷の被膜で覆われる。