Peculiar star

Japanese: 特異星 - とくいせい(英語表記)peculiar star
Peculiar star

A general term for stars that exhibit spectra significantly different from those of normal stars. They are broadly divided into stars with abnormalities in the chemical composition of their outer atmosphere and stars with abnormalities in their atmosphere or internal structure.

A-type peculiar stars are a typical example of the former, and are stars in which the absorption lines of rare earth elements such as Mn (manganese), Cr (chromium), Sr (strontium) and Eu (europium) are superimposed on the spectrum of a standard star of type A (the type in the spectral series in which hydrogen absorption lines are strongest). This type of star has a strong magnetic field of up to 10,000 Gauss, which suppresses the movement of gas in the star's atmosphere, and while the above elements that absorb light well are pushed up from the star's interior to the outer atmosphere by light pressure, elements that do not absorb light sink instead, which is thought to have caused an abnormality in the chemical composition of the atmosphere.

Carbon stars are M-type giants, and have strong molecular spectra of CN (cyan), C2 (carbon molecules), CH (hydrocarbons), etc. It is believed that carbon produced by thermonuclear reactions of helium in the center of the star is carried to the atmosphere on the surface by deep convection in the star, resulting in an abnormal increase in carbon compared to normal stars. This carbon is released from the surface of the star, cools, and becomes solid dust, which surrounds the star. Therefore, carbon stars emit strong infrared radiation and are also a source of molecules in space. In addition, because they are bright and easy to detect with a distinctive spectrum, they are essential celestial objects for investigating the structure of our galaxy.

A representative example of the latter is the Wolf-Rayet star, which has a surface temperature of 100,000 K and an absolute luminosity of -6.5, hundreds of thousands of times brighter than the Sun. In Wolf-Rayet stars, emission lines of highly ionized ions such as He (helium), C (carbon), N (nitrogen), and O (oxygen) are observed. Their atmospheres expand at speeds of up to 2,000 kilometers per second, continuously releasing large amounts of gas into interstellar space. Since many of them form double stars, it is believed that when a star with a mass several tens of times that of the Sun evolves into a giant star, most of its mass is stripped away by the other star, leaving the helium core at the center of the star exposed. Wolf-Rayet stars play an extremely important role in the formation of elements such as C, N, O, and Ne (neon) in the universe. This type of peculiar star is hundreds of thousands of times brighter than the Sun, and in recent years it has been possible to detect it in galaxies outside the Milky Way, such as the Magellanic Clouds and the Andromeda Galaxy.

[Kenichi Wakamatsu]

[References] | Wolf-Rayet stars | Giant stars | Stars | Multiple stars | Spectral types
Wolf-Rayet stars
A hot star in the constellation of Venus (center of photo). It is surrounded by the M1-67 nebula. Photographed by the Hubble Space Telescope ©ESA/Hubble&NASA, Acknowledgement: Judy Schmidt ">

Wolf-Rayet stars


Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

通常の星とは著しく異なるスペクトルを示す星の総称。その外層大気の化学元素組成に異常をきたした星と、その大気や内部の構造に異常をきたした星とに大別される。

 A型特異星は前者の代表例で、A型(スペクトル型系列のうち、水素の吸収線がもっとも強くなる型)の標準星のスペクトルの上にMn(マンガン)、Cr(クロム)、Sr(ストロンチウム)や、Eu(ユウロピウム)などの希土類元素の吸収線が重なった星である。この種の星は1万ガウスにも達する強い磁場をもっているため、星の大気中のガスの運動が抑えられ、光をよく吸収する前記の元素が星の内部から外層大気へと光圧で押し上げられるのに対し、光を吸収しない元素は逆に沈んでしまい、大気の化学組成に異常が生じたと考えられている。

 炭素星はM型巨星で、CN(シアン)、C2(炭素分子)、CH(炭化水素)などの分子スペクトルが強く現れている。星の中心部でのヘリウムの熱核反応によってつくられた炭素が星の内部の深い対流によって表面の大気層まで運ばれてしまい、炭素が通常の星に比べて異常に増加したと考えられている。これらの炭素は星の表面から放出され、冷えて固体のダストとなり、星を取り囲んでしまう。そのため、炭素星は強い赤外線を放出すると同時に、宇宙空間での分子の供給源にもなっている。また、際だった特徴あるスペクトルで明るく検出しやすい星のため、わが銀河系の構造を調べるのには欠かせない天体である。

 後者の代表例としては、表面温度が10万Kにも達し、絶対光度がマイナス6.5等級と太陽の数十万倍も明るいウォルフ‐ライエ星があげられる。ウォルフ‐ライエ星にはHe(ヘリウム)、C(炭素)、N(窒素)、O(酸素)などの高い電離状態にあるイオンの輝線が観測される。大気は秒速2000キロメートルにも達する速さで膨張し、星間空間へ多量のガスを放出し続けている。二重星を形成しているものが多いことから、太陽の数十倍の質量をもつ星が巨星に進化したとき、その質量の大半が相手の星によってはぎとられてしまい、星の中心部のヘリウムのコア(核)がむき出しになった状態と考えられている。ウォルフ‐ライエ星は宇宙におけるC、N、OやNe(ネオン)などの元素の形成にきわめて重要な役割を担っている。この種の特異星は太陽の数十万倍の明るさがあり、そのため近年では、マゼラン星雲やアンドロメダ銀河などの銀河系外の銀河でも検出することができるようになっている。

[若松謙一]

[参照項目] | ウォルフ‐ライエ星 | 巨星 | 恒星 | 重星 | スペクトル型
ウォルフ‐ライエ星
や座に位置する高温度の恒星(写真中央)。周囲をM1-67星雲が取り囲む。ハッブル宇宙望遠鏡により撮影©ESA/Hubble&NASA,Acknowledgement:Judy Schmidt">

ウォルフ‐ライエ星


出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例

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