Dwarf star

A star with a mass similar to the Sun or less that undergoes hydrogen fusion reactions at its center. In a broader sense, it refers to all main sequence stars undergoing hydrogen reactions. It is contrasted with "giant stars" that have a noticeable outer expansion caused by the reaction of the central contraction after the hydrogen reaction at the center ends. It is named as such because it is small and relatively dense. It is sometimes called a red dwarf to distinguish it from a white dwarf. The rate at which red dwarfs are born is higher than that of high-mass stars, and they have a long lifespan, so they make up the overwhelming majority of existing stars. The Sun is one of the most representative dwarfs, and the nearest neighboring star to the solar system, Alpha Centauri, is also a very similar dwarf. Observations have shown that M-type dwarfs with a mass of about one-tenth of the Sun's mass have been known, but even smaller brown dwarfs have also been discovered. These are stars with a mass of 0.07 times the Sun's mass or less that cannot emit light through nuclear fusion reactions, and they slowly contract due to gravity and cool down. One type of dwarf star is a "subdwarf" star, which lacks heavy elements. This lack of heavy elements causes abnormalities in their spectral type and color index, and they are so named because they form a sequence slightly below the main sequence on the H-R diagram. Many of these stars are "high-velocity stars."

[Keiichi Kodaira and Hiroyasu Ando]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

中心で水素核融合反応を行っている太陽程度または、それよりも小質量の星。広義には水素反応を行っているすべての主系列星をさす。中心での水素反応終了後の中心部収縮の反動として生じる外側の膨張が目だつ「巨星」に対置される。小型で比較的密度が高いのでこのように命名された。白色矮星と区別して赤色矮星とよぶこともある。誕生する割合も大質量星に比べて多く、また寿命も長いので、存在している恒星の圧倒的多数を占めている。太陽は代表的な矮星の一つで、太陽系にもっとも近い隣の恒星であるケンタウルス座α(アルファ)星もよく似た矮星である。観測的には太陽質量の10分の1程度のM型矮星までが知られていたが、さらに質量の小さい褐色矮星も発見された。これは0.07倍太陽質量以下の質量の星で、核融合反応により光ることができず、ゆっくりと重力収縮して冷えていく。矮星の一種に、重元素の欠乏している「準矮星」がある。重元素が少ないためにスペクトル型や色指数に異常が生じ、HR図上で主系列のわずか下方に系列をなすのでこの名があり、その多くは「高速度星」である。

［小平桂一・安藤裕康］

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