When viewed or photographed with a large telescope, only the nearest galaxies exhibit individual stars. For most galaxies, only the combined light of all the stars is detected. Galaxies exhibit a variety of forms. Some have an overall globular shape, with a bright nucleus. Such galaxies, called ellipticals, contain a population of old stars, usually with little apparent gas or dust, and few newly formed stars. Elliptical galaxies come in a vast range of sizes, from giant to dwarf. In contrast, spiral galaxies are flattened disk systems containing not only some old stars but also large populations of young stars, much gas and dust, and molecular clouds that are the birthplace of stars. Often the regions containing bright young stars and gas clouds are arranged in long spiral arms that can be observed to wind around the galaxy. Generally a halo of faint older stars surrounds the disk; a smaller nuclear bulge often exists, emitting two jets of energetic matter in opposite directions.
- exhibit [iɡˈzibit] v. 呈现
- detect [diˈtekt] v. 探测
- globular shape [ˈɡlɔbjulə - ʃeip] n.球形
- nucleus [ˈnju:kliəs] n. 核子
- elliptical [iˈliptikəl] n. 椭圆星系
- giant [ˈdʒaiənt] adj. 巨大的
- dwarf [dwɔ:f] adj. 矮小的
- flattened disk system [ˈflætn - disk - ˈsistəm] n. 平底圆盘状系统
- population [ˌpɔpjuˈleiʃən] n. 个数
- molecular cloud [məuˈlekjulə - klaud] n.分子云
- spiral arm [ˈspaiərəl - a:m] n. 旋臂
- halo [ˈheiləu] n.光环
- bulge [bʌldʒ] n. 凸出部分
- emit [iˈmit] v. 喷射
- jet [dʒet] n. 喷射物
