High-velocity star

A star whose spatial velocity relative to the Sun is about 65 kilometers per second or more. Most stars in the Sun's vicinity have a relative spatial velocity of about 20 kilometers per second. Depending on the cause of their formation, they are considered to be runaway stars, halo stars, and hypervelocity stars. Runaway stars are stars that are thought to have been ejected at high speeds during the evolution of a binary star system, or that were ejected by a supernova explosion of the other star that formed a binary system. Halo stars are stars that were born in the halo outside the galactic disk and happened to cross the galactic disk near the Sun. Hypervelocity stars, unlike the former two, have an extremely high speed of 1,000 kilometers per second, and are thought to have been accelerated by a collision with a massive black hole believed to be at the center of the galaxy, or by a collision between our galaxy and a nearby dwarf galaxy. Many high-velocity stars have a different elemental composition from standard stars such as the Sun, and show a lack of heavy elements. High velocity dwarfs, which are highly deficient, are also called subdwarfs after their position on the H-R diagram. Reflecting their high relative velocities, they are distributed widely throughout the Galaxy, extending beyond the lenticular disk into the ellipsoidal region known as the halo. They are typical Population II stars, and are old, generally over 10 billion years old, and include small to medium mass dwarfs, giants, and horizontal branch stars.

[Keiichi Kodaira and Hiroyasu Ando]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

恒星のうち太陽に対する空間運動速度が秒速約65キロメートル以上の星。太陽周辺の大部分の恒星は、相対空間速度が秒速20キロメートル程度である。生成原因の違いによって、逃走星（runaway star）、ハロー星（halo star）、極高速度星（hypervelocity star）が考えられている。逃走星は、連星系の進化の過程で高速度で飛び出したか、連星を形成していた相手方の星が超新星爆発して放出されたと考えられる星。ハロー星は、銀河円盤外のハローで誕生した星がたまたま太陽近傍の銀河円盤を横切ったもの。極高速度星は、前者二つと違い秒速1000キロメートルと桁違いに大きい速度を示し、銀河中心にあるとされる巨大ブラック・ホールとの衝突で加速されたか、わが銀河系と近傍の矮小銀河との衝突で加速されたか、どちらかの原因による星と考えられている。高速度星の多くはその元素組成が太陽など標準の星と異なり、重元素の欠乏を示す。欠乏度の大きな高速度矮星(わいせい)は、HR図上の位置にちなんで準矮星ともよばれる。高い相対速度を反映して、銀河系内の分布も広範囲にわたり、レンズ状の円盤部を越えてハローとよばれる楕円(だえん)体領域に及ぶ。代表的な種族Ⅱの星で、年齢は古く、一般に100億年以上であって、中小質量の矮星、巨星、水平枝星を含む。

［小平桂一・安藤裕康］

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