Dãy chính

Mặt Trời là ví dụ hay gặp nhất của một ngôi sao thuộc dãy chính.

Biểu đồ Hertzsprung–Russell thể hiện độ sáng thực (hay cấp sao tuyệt đối) của ngôi sao so với chỉ mục màu (biểu diễn bằng B-V). Dãy chính thể hiện là một dải chéo rõ rệt chạy từ phía trên bên trái xuống phía dưới bên phải. Biểu đồ vẽ 22.000 sao với dữ liệu từ Danh lục Hipparcos cùng với 1.000 độ sáng thấp (sao lùn trắng và sao lùn đỏ) từ Danh lục Gliese các sao ở gần.

Trong thiên văn học, dãy chính (hoặc dải chính) là một dải hay đường liên tục rõ rệt thể hiện các sao khi vẽ chúng trên biểu đồ chỉ mục màu so với độ sáng. Biểu đồ màu-độ sáng còn gọi là biểu đồ Hertzsprung–Russell mang tên các nhà thiên văn học Ejnar Hertzsprung và Henry Norris Russell. Các ngôi sao trên dãy này được gọi là sao dãy chính hoặc sao "lùn".

After a star has formed, it generates thermal energy in the dense core region through the nuclear fusion of hydrogen atoms into helium. During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. All main-sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward pressure of gravitational collapse from the overlying layers. The strong dependence of the rate of energy generation in the core on the temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the photosphere. The energy is carried by either radiation or convection, with the latter occurring in regions with steeper temperature gradients, higher opacity or both.

The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. Stars below about 1.5 times the mass of the Sun (or 1.5 solar masses (M_☉)) primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton–proton chain. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms. Main-sequence stars with more than two solar masses undergo convection in their core regions, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. Below this mass, stars have cores that are entirely radiative with convective zones near the surface. With decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases, while main-sequence stars below 0.4 M_☉ undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.

In general, the more massive a star is, the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram. The behavior of a star now depends on its mass, with stars below 0.23 M_☉ becoming white dwarfs directly, while stars with up to ten solar masses pass through a red giant stage. More massive stars can explode as a supernova, or collapse directly into a black hole.

Đọc thêm

Tổng quan

Kippenhahn, Rudolf, 100 Billion Suns, Basic Books, New York, 1983.

Chuyên khảo

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12. Hansen, Carl J., Kawaler, Steven D., and Trimble, Virginia Stellar Interiors: Physical Principles, Structure, and Evolution, Second Edition, Springer-Verlag, New York, 2004.

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14. Iben, Icko, Jr, "Stellar Evolution Within and Off the Main Sequence," Annual Review of Astronomy and Astrophysics, 5, 571, 1967.

15. Iglesias, Carlos A, and Rogers, Forrest J., "Updated Opal Opacities," The Astrophysical Journal, 464, 943, 1996.

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Liên kết ngoài

• Charity, Mitchell (4 tháng 6 năm 2001). “What color are the stars?”. Vendian Systems. Truy cập ngày 26 tháng 11 năm 2008.