About Galaxy (銀河) (15)

2006-10-20 5:27 am
About Galaxy (銀河)
In english and Chinese

回答 (4)

2006-10-28 8:54 am
✔ 最佳答案
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.

Although theoretical dark matter appears to account for around 90% of the mass of most galaxies, the nature of these unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies.

Intergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than a hundred billion (1011) galaxies in our observable universe.

The method of galactic formation is a major open question in astronomy. Theories may be divided into two categories: top-down and bottom-up. In top-down theories such as the Eggen–Lynden-Bell–Sandage (ELS) model, protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.[2] In bottom-up theories such as the Searle-Zinn (SZ) model, globular clusters form first, and then a number of such bodies accrete to form a larger galaxy. Modern theories must be modified to account for the probable presence of large dark matter halos. A sketch of a galactic formation model follows.

Shortly after recombination, baryonic matter begins to condense around cold dark matter halos. Zero-metal high-velocity halo stars (called Population III stars) are the first to develop around a protogalaxy as it starts to contract. These huge stars quickly supernova, releasing heavy elements into the interstellar medium. Within the next billion years, globular clusters, the central supermassive black hole and galactic bulge of metal-poor Population II stars form. Within two billion years, the remaining material settles into a galactic disk. The galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life; the cycle of stellar birth and death will increase the abundance of heavy elements, eventually allowing the formation of planets.
參考: Internet
2006-10-29 5:07 am
The galaxy or the milky way is a cloud of stars, planets and star dusts.
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.

Although theoretical dark matter appears to account for around 90% of the mass of most galaxies, the nature of these unseen components is not well understood. There is evidence that supermassive black holes exist at the center of all galaxies.

Intergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than a hundred billion (1011) galaxies in our observable universe.
2006-10-21 9:07 pm
銀河系是太陽系所處的星系。它是與同處於本星系團的仙女座大星系一樣,都是旋渦星系,因為它像一條流淌在天上閃閃發光的河流一樣,古稱銀河、天河,北半球來說夏季最明顯看到銀河(在天蠍座、人馬座延伸至夏季大三角,甚至仙后座),冬季的那邊銀河很黯淡(在獵戶座與大犬座)。估計約1000多億顆恆星組成。


對銀河系的探索
雖然從非常久遠的古代,人們就認識了銀河系。但是對銀河系的真正認識還是從近代開始的。

1750年,英國天文學家賴特(Wright Thomas)認為銀河系是扁平的。1755年,德國哲學家康德提出了恆星和銀河之間可能會組成一個巨大的天體系統;隨後的德國數學家郎伯特(Lambert Johann heinrich)也提出了類似的假設。到1785年,英國天文學家威廉·赫歇耳繪出了銀河系的扁平形體,並認為太陽系位於銀河的中心。

1918年,美國天文學家沙普利(Harlow Shapley)經過4年的觀測,提出太陽系應該位於銀河系的邊緣。1926年,瑞典天文學家林得布拉德(Lindblad Bertil)分析出銀河系也在自轉。




特徵
銀河系是一個中間厚,邊緣薄的扁平盤狀體。他的主要部分稱為銀盤,是一個漩渦狀。

它的總質量約有太陽的一萬億倍,直徑約為十萬光年,中央厚約1萬光年,邊緣厚約3000-6000光年。太陽約處於與銀河系中心距離約27,700光年的位置。

銀盤外面是由稀疏的恆星和星際物質組成的球狀體,稱為銀暈,直徑約10萬光年。

仔細觀察銀河系,可以發現有4條旋臂,分別是人馬臂,獵戶臂,英仙臂,3000秒差距臂。太陽位於獵戶臂內側。旋臂主要由星際物質構成。

銀河系也有自轉。太陽系以每秒250千米速度圍繞銀河中心旋轉,旋轉一周約2.5億年。

銀河系有兩個伴星系:大麥哲倫星系和小麥哲倫星系。

與銀河系相對的稱之為河外星系。



年齡
依據歐洲的南方天文臺(ESO)的研究報告,估計銀河系的年齡約為136億歲(109年),幾乎與宇宙一樣老。 [1]

由天文學家Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton.所組成的團隊在2004年使用非常巨大望遠鏡(VLT)的紫外線視覺矩陣光譜儀進行的研究,首度在球狀星團NGC 6397的兩顆恆星內發現了鈹元素。這個發現讓他們將第一代恆星與第二代恆星交替的時間往前推進了2至3億年,因而估計球狀星團的年齡在134±8億歲,因此銀河系的年齡不會低於136±8億歲。


在2005年,銀河系被發現以哈柏分類來區分應該是一個巨大的棒旋星系SBbc(懸臂寬鬆的棒旋星系),總質量大約是太陽質量的6,000至30,000倍。[1][2], 有大約1,000億顆恆星。[3]

從80年代開始,天文學家才懷疑銀河是一個棒旋星系而不是一個普通的旋渦星系。在2005年,史匹哲太空望遠鏡証實了這項懷疑,還確認了在銀河的核心的棒狀結構與預期的還大。[4]

銀河的盤面估計直徑為100,000光年,太陽至銀河中心的距離大約是26,000光年,盤面在中心向外凸起。

銀河的中心有巨大的質量和緊密的結構,因此被強烈的懷疑有超重質量黑洞,因為已經有許多星系被相信有超重質量黑洞在核心。

就像許多典型的星系一樣,環繞銀河系中心的天體,在軌道上的速度並不由與中心的距離和銀河質量的分佈來決定。在離開了核心凸起或是在外圍,恆星的典型速度是每秒鐘210~240公里之間。[5]因此這星恆星繞行銀河的週期只與軌道的長度有關,這與太陽系不同,在太陽系,距離不同就有不同的軌道速度對應著。
2006-10-20 6:21 am
位於荷蘭的Lieden Observatory 天文觀測所的物理學家相信他們已經解開一個謎: 為何年輕的天體奔馳於銀河中.

飛馳的星體並不像大部分天體如此安份地遵循銀河系的轉動, 而是以每秒200公里的速度橫越星空. 大約有十分之一年輕的天體( 年齡約5仟萬年 ), 以顏色區分為 'O' 和 'B' 兩類型行為異常的天體.

有一解釋此種天體古怪的行為: 許多天體會成對而形成一雙星系統, 靠著重力彼此吸引, 互相繞著對方而旋轉. 倘若有一伴星以星爆方式結束其生命, 即很難再約束另一天體在軌道上運行, 其現象好比一端綑綁著線而繞轉的石頭, 突然將線剪斷的情況類似, 伴星亦會沿著切線方向離去.

另一解釋則是兩個雙星系統太過於靠近, 彼此相互作用拉扯, 而使得某一雙星系統掙脫彼此束縛飛離去.

Ronnie Hoogerwerf 及其同事於Leiden利用於1989年發射升空, 可精確定出天體位置的人造衛星 Hipparcos, 重新追蹤若干此種飛馳天體的路徑, 並將觀測結果發表於 Astrophysical Journal. 在觀測結果中, 分別各有一案例支持著上述可能的兩種解釋.


收錄日期: 2021-04-12 20:10:38
原文連結 [永久失效]:
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