恆星形成
维基百科,自由的百科全书
恆星形成 |
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天體分類 |
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理論的觀念 |
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相關學門 |
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恆星形成是分子雲的高密度區崩潰成為球形的電漿形成恒星的過程。作為天文物理的一個分支,恆星形成的研究包括作為前導的星際物質和巨大分子雲,到恆星形成過程,早期型恆星和行星形成則是直接的成果。恆星形成的理論,不僅是一顆單獨恆星的形成,還必須統計聯星和起始質量函數。
目录 |
[编辑] 理論概說
依據目前的恆星形成理論,分子雲的核心(特別是高密度區)會因為重力不穩定,由片段的碎片開始崩潰(一般稱為自然的恆星形成,參考金斯不穩定性 ),或是因為來自超新星的衝激波,或是在附近的其他能量充沛的天文學過程觸發分子雲中的恆星形成(一般稱為觸發的恆星形成)。部分的重力能量在崩潰的過程中會以紅外線的形式損失掉,其餘的則會用於增加天體核心的溫度。 累積的部份物質將會形成拱星盤,當溫度和密度夠高時,氘的核融合將會被引發,並產生向外的壓力,結果將使崩潰減緩(但不會停止),而由雲氣組合成的物質仍繼續如雨般的落在原恆星上。在這個階段,或許是由落入物質的角動量造成的,將會產生雙極噴流。最後,在核心的氫開始融合成為恆星,這時,還環繞在周圍的物質將開始被驅離。
原恆星的發展在赫羅圖上會遵循林軌跡,[1]原恆星會繼續收縮,直到到達林邊界,然後收縮會以穩定的溫度繼續下去直到凱爾文-亥姆霍茲時標。質量低於0.5太陽質量的恆星將進入主序帶,稍重的原恆星,在林軌跡的終點仍將緩慢的塌縮,追隨著亨耶跡,以接近流體靜力平衡。[2]
這種活動形式會使恆星的質量在大約一個太陽質量的附近。高質量的恆星形成過程,也有類似的演化(發展)時程表,但時間會短許多,而且也還未清楚的被定義出來。恆星後期的發展屬於恆星演化研究的範疇。
[编辑] 觀測
The formation of individual stars can only be directly observed in our Galaxy, but in distant galaxies star formation has been detected through its unique spectral signature.
[编辑] 值得注意的導引天體
- MWC 349 was first discovered in 1978, and is estimated to be only 1,000 years old. Since the object is located at a distance of 10,000 lightyears, it actually is now 11,000 years old.
- VLA 1623 -- The first exemplar Class 0 protostar, a type of embedded protostar that has yet to accrete the majority of its mass. Found in 1993, is possibly younger than 10,000 years [1].
- L1014 -- An incredibly faint embedded object representative of a new class of sources that are only now being detected with the newest telescopes. Their status is still undetermined, they could be the youngest low-mass Class 0 protostars yet seen or even very low-mass evolved objects (like a brown dwarf or even an Interstellar planet). [2].
- IRS 8* -- The youngest known main sequence star, discovered in August 2006. It is estimated to be 3.5 million years old [3].
[编辑] 低質量對高質量的恒星誕生
Stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by a plethora of observations, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. As described above, the collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channeled onto a central protostar. For stars with masses higher than about 8 solar masses, however, the mechanism of star formation is not well understood.
Massive stars emit copious quantities of radiation which pushes against infalling material. In the past, it was thought that this radiation pressure might be substantial enough to halt accretion onto the massive protostar and prevent the formation of stars with masses more than a few tens of solar masses. Recent theoretical work has shown that the production of a jet and outflow clears a cavity through which much of the raditation from a massive protostar can escape without hindering accretion through the disk and onto the protostar. Present thinking is that massive stars may therefore be able to form by a mechanism similar to that by which low mass stars form.
There is mounting evidence that at least some massive protostars are indeed surrounded by accretion disks. Several other theories of massive star formation remain to be tested observationally. Of these, perhaps the most prominent is the theory of competetive accretion, which suggests that massive protostars are "seeded" by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region. Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.
[编辑] 外部鏈結
- 恆星形成(英文)
[编辑] 資料來源
General subfields within 天文學 |
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Astrometry | 宇宙學 | Galactic astronomy | Extragalactic astronomy | 星系的形成和演進 Planetology | Stellar astronomy | Stellar evolution | 恒星的形成 |