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

Interstellar matter is constantly flowing through the Galaxy and changing from one phase to another. At the same time, gas is constantly being added to the Galaxy by accretion from extragalactic space, while mass is removed from the interstellar medium by being locked in stars. Some of the mass in stars is, in turn, returned to the interstellar medium when those stars evolve and die. In particular, the heavy elements in interstellar space were all produced inside stars, while the dust grains are made in the outer regions of stars that have swelled to be giants. These elements and grains, in turn, can then be incorporated into new stars and planetary systems that form out of the interstellar medium.

星間物質は、銀河内を絶えず流れ、一つの状態から別の状態へ変わる。同時に、銀河外空間からの降着によって、ガスが絶えず銀河へ加わり、星の中に閉じ込められることで、星間物質から質量が取り去られる。星の質量の一部は、その星が進化して死ぬと、今度は星間物質へ戻る。特に、星間空間の重元素は、すべて星の内部で作られたものであり、塵の粒は、巨星へ膨らんだ星の外側の領域で作られる。これらの元素や粒は、さらに星間物質から生まれる、新しい星や惑星系へ取り込まれうる。