星系

浏览

星系(galaxy)

恒星和星际物质组成的系统。几十亿个这样的系统构成了宇宙。在大小、组成和结构等方面，彼此差别很大，但它们几乎全都排布成群或团，其每一个的成员星系，少者几个，多者可达万个。每个星系均由为数众多（大多是从几亿个到一万亿个以上）的恒星组成。在许多星系（如银河系）中，都能检测到由星际气体和尘粒组成的星云。星空中，约70%的亮星系都是旋涡星系。旋涡星系有一个恒星主盘，沈陷在盘结构中的是旋臂，从中心向外旋伸。在旋臂中，最大量地集聚着旋涡星系的星际气体和尘埃。旋涡星系的中央核周围是一个巨大的核心隆起结构，大多数情况下均接近於球体。在核球和盘结构之外是由星团、单个恒星或许还有其他物质构成的一个稀疏的、或多或少为球状的星系晕。椭圆星系是一个由对称分布的恒星构成的球形或椭球形，这种星系的大小范围相差很大。虽然没有一个矮椭圆星系在天上引人注目，但它们却是星系中最常见的一类。不规则星系（如麦哲伦星云）是相当罕见的。无线电星系是无线电波很强的来源。塞佛特星系（Seyfert galaxy）有非常亮的核，常放出很强的无线电波，可能与类星体有关。

English version：

galaxy

Any of the billions of systems of stars and interstellar matter that make up the universe. Galaxies vary considerably in size, composition, and structure, but nearly all are arranged in groups, or clusters, of from a few to as many as 10,000. Each is composed of millions to trillions of stars; in many, as in the Milky Way galaxy, nebulae (see nebula) can be detected. Roughly 70% of the bright galaxies in the sky are spiral galaxies, with a main disk in which spiral arms wind out from the center. The arms contain the greatest concentration of a spiral galaxy's interstellar gas and dust, where stars can form. Surrounding the center (nucleus) is a large, usually nearly spherical nuclear bulge. Outside this and the disk is a sparse, more or less spherical galactic halo. In elliptical galaxies, which vary greatly in size, stars are distributed symmetrically in a spherical or spheroidal shape. Dwarf ellipticals (with only a few million stars) are by far the most common kind of galaxy, though none is conspicuous in the sky. Irregular galaxies, such as the Magellanic Clouds, are relatively rare. Radio galaxies are very strong sources of radio waves. Seyfert galaxies, with extremely bright nuclei, often emit radio waves and may be related to quasars.

参考文章

阅读全文