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A galaxy is a huge aggregate
in which tremendous stars and a lot of interstellar matter gather.
The spectrum of galaxies is a composite of spectra
of stars and gas in the galaxy.
In the spectrum of the disk part of the galaxy,
typical absorption lines of stars appear;
e.g., H, K lines of CaII, G bands of molecules,
the Ballmer lines of hydrogen, and so on.
In the HII region, however, the spectrum shows emission lines and
resembles that of gaseous nebulae.
It was believed that normal galaxies hardly emit an X-rays and a radio wave several decades ago. In fact, however, X-rays are detected in various galaxies. Even if it looks like a normal galaxy, the enormous activity takes place in the galaxy. Galaxies are morphologically classified into Elliptical Galaxies, Spiral Galaxies, Barred Spiral Galaxies, and Irregular Galaxies. Such a classification is known as Hubble Sequence (Classification).
銀河は，多数の星や星間物質が集まった巨大な集合体です．
そのスペクトルには，構成している星やガスのスペクトルが合わさって見えます． かつては，通常の銀河はあまりX線や電波は出さないと思われていましたが， 実際にはさまざまな銀河からX線が検出されています． そして，見かけは普通に見える銀河でも， 高温のプラズマや激しい活動が起こっていることがわかってきました． 銀河は，その形状から， 楕円銀河，渦状銀河，棒渦状銀河，不規則銀河に大別されます （ハッブル分類）． |
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M49
M31
Large Magellanic Cloud