| Abstract from DBPedia | Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star. Nuclear fusion powers a star for most of its existence. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red-giant phase. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more-massive stars can fuse heavier elements along a series of concentric shells. Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their existence, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs. Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.天体物理学において恒星進化論（こうせいしんかろん、英語：stellar evolution）とは、恒星の誕生から最期までにおこる恒星内の構造の変化を扱う理論である。 恒星進化論においては、恒星を生物になぞらえてその誕生から最期までを恒星の一生とし、幼年期の星、壮年期の星、老年期の星、星の死といった用語を用いる。恒星進化論で用いられている進化も生物になぞらえた言葉であるが、生物の進化とは異なり、世代を超えた変化ではなく1つの恒星の形成から終焉までの変化を表している。 恒星は自分自身の重力があるので常に収縮しようとする。しかし、収縮すると重力によるポテンシャルエネルギーが熱に変わる。また充分に高温高圧になれば核融合反応が起こり熱が発生する。これらの熱によってガスの温度が上昇すればガスは膨張しようとする。このようにして収縮と膨張が釣り合ったところで恒星は安定している。重力と核融合によるエネルギーを使い果たすと、恒星は収縮をとどめることができず最期を迎える。 以下に現在の恒星進化論による恒星の一生を示す。 なお、一般的に恒星は進化の過程で恒星風などの理由により、その質量を徐々に減少させていくため、下記のいずれの過程の太陽質量も「そのイベントが発生した時点の質量」にもとづくものであり、既知の恒星の現時点での質量がそのまま当てはまるものではない。 (Source: http://dbpedia.org/resource/Stellar_evolution) |
