- 小李飛刀Lv 61 十年 前最佳解答
From Wikipedia, the free encyclopedia
NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant.
A galaxy is a huge gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain ten million to one trillion (107 to 1012) stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years.
In 1610, Galileo Galilei used a telescope to study the bright band on the night sky known as the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) that the Galaxy might be a rotating body of a huge number of stars, held together by gravitational forces akin to the solar system but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate galaxies.
Sketch of the Whirlpool Galaxy by Lord Rosse in 1845
In 1944, Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm, resulting from interstellar atomic hydrogen gas; this radiation was observed in 1951. This radiation allowed for much improved study of the Galaxy, since it is not affected by dust absorption and its doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating bar structure in the center of the Galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s it was discovered in Vera Rubin's study of the rotation speed of gas in galaxies that the total visible mass (from stars and gas) does not properly account for the speed of the rotating gas. This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.
Types of galaxies
Types of galaxies
Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) or activity in the core (in active galaxies).
Galaxy Abell 1835 IR1916 is seen as a tiny dot in this photo of distant galaxies. Image courtesy of ESO.
- TimmyLv 61 十年 前
銀河系呈漩渦狀 包含大約2000億顆顏色 大小 亮度和年齡各不相同的恆星 這些恆星大多會互相聚集成星團 此外 星和星之間還有許多由灰塵 氣體組成的各種形狀的星雲 因為我們地球本身就位於銀河系中 所以只能看到銀河系較靠近地球的部份 這部份組成一條星帶環繞地球 看起來就像一條閃閃發光的銀色河流 因此俗稱天河 西方人稱為牛奶道 若從正面看銀河系是一個圓盤狀的大漩渦直徑長達100000光年 若從側面看 卻像一面凸透鏡 中心部份特別厚 大約有15000光年 而我們的太陽系就位於距銀河系中心約30000光年的地方 以每秒250公里的速度環繞中心旋轉 每轉一周約需2億年的時間
銀河是一座大星域 也是宇宙中的一座星島 由無數的星星構成 太陽和九大行星恆星 星雲 星團 宇宙氣體及宇宙塵都是銀河的一部份 銀河大約有100億顆星星 根據銀河的外形 可分為三大類 一 不規則銀河 二 漩渦銀河 三 橢圓銀河科學家相信它大致與仙女座大星雲的形狀相同 沒有人確實知道宇宙中有多少個銀河 而越來越大的望遠鏡使我們看到更多的銀河 但有的距離太遠了 甚至於要花5億光年的時間才能把光射到地球 在仲夏或沒有月亮的晚上 較易看到銀河 在墨西哥文中 銀河的意思是色彩繽紛彩虹中的白姑娘 而古希臘人的傳說則認為銀河是英雄波西亞士在殺死頭上有蛇的米都薩後 捲起的塵埃衝上天空所造成的 其他還有許多有關銀河的有趣傳說 就不再介紹了