Khác biệt giữa bản sửa đổi của “Sao lùn đen”
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Dòng 20:
'''Sao lùn đen''' là một loại [[sao đặc]] giả thiết, mà cụ thể là [[sao lùn trắng]] đã nguội đến mức không còn phát ra đáng kể bức xạ [[nhiệt]] hoặc [[ánh sáng]]. Bởi vì thời gian cần thiết để một sao lùn trắng đạt tới trạng thái này được tính toán là lâu hơn cả [[tuổi của vũ trụ]] (13,8 tỷ năm), do vậy không có một sao lùn đen nào được cho là tồn tại trong [[Vũ trụ]], và nhiệt độ của sao lùn trắng lạnh nhất là một trong những giới hạn quan sát về tuổi của vũ trụ.<ref name="2003ApJ...591..288H"/>
==Sự hình thành==
A white dwarf is what remains of a [[main sequence|main-sequence]] star of low or medium mass (below approximately 9 to 10 [[solar mass]]es ({{Solar mass|link=y}})) after it has either expelled or [[nuclear fusion|fused]] all the [[chemical element|element]]s for which it has sufficient temperature to fuse.<ref name="2003ApJ...591..288H">§3, {{cite journal
|author1=Heger, A. |author2=Fryer, C. L. |author3=Woosley, S. E. |author4=Langer, N. |author5=Hartmann, D. H. | title=How Massive Single Stars End Their Life
| journal=Astrophysical Journal
| year=2003 | volume=591 | issue=1 | pages=288–300
| bibcode=2003ApJ...591..288H | doi = 10.1086/375341
|arxiv = astro-ph/0212469 }}</ref> What is left is then a dense sphere of [[electron-degenerate matter]] that cools slowly by [[thermal radiation]], eventually becoming a black dwarf.<ref name="osln">{{cite web|url=http://www.astronomy.ohio-state.edu/~jaj/Ast162/lectures/notesWL22.pdf|format=PDF|title=Extreme Stars: White Dwarfs & Neutron Stars|first=Jennifer|last=Johnson|publisher=[[Ohio State University]]|accessdate=2007-05-03}}</ref><ref>{{cite web | last = Richmond | first = Michael | url = http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html | title = Late stages of evolution for low-mass stars | publisher = Rochester Institute of Technology | accessdate = 2006-08-04 }}</ref> If black dwarfs were to exist, they would be extremely difficult to detect, because, by definition, they would emit very little radiation. They would, however, be detectable through their [[gravity|gravitational]] influence.<ref>{{cite journal|title=Baryonic Dark Matter: The Results from Microlensing Surveys|author1=Charles Alcock |author2=Robyn A. Allsman |author3=David Alves |author4=Tim S. Axelrod |author5=Andrew C. Becker |author6=David Bennett |author7=Kem H. Cook |author8=Andrew J. Drake |author9=Ken C. Freeman |author10=Kim Griest |author11=Matt Lehner |author12=Stuart Marshall |author13=Dante Minniti |author14=Bruce Peterson |author15=Mark Pratt |author16=Peter Quinn |author17=Alex Rodgers |author18=Chris Stubbs |author19=Will Sutherland |author20=Austin Tomaney |author21=Thor Vandehei |author22=Doug L. Welch |year=1999|bibcode=1999ASPC..165..362A|volume=165|pages=362|journal=In the Third Stromlo Symposium: the Galactic Halo}}</ref>
Various white dwarfs cooled below 3900 K (M0 [[Stellar classification|spectral class]]) were found in 2012 by astronomers using [[MDM Observatory]]'s 2.4-meter telescope. They are estimated to be 11 to 12 billion years old.<ref name=2examples>http://www.spacedaily.com/reports/12_Billion_Year_Old_White_Dwarf_Stars_Only_100_Light_Years_Away_999.html</ref>
Because the far-future evolution of stars depends on physical questions which are poorly understood, such as the nature of [[dark matter]] and the possibility and rate of [[proton decay]], it is not known precisely how long it will take white dwarfs to cool to blackness.<ref name="adams">{{Cite web|url=http://xxx.lanl.gov/abs/astro-ph/9701131v1 |title=A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects| doi=10.1103/RevModPhys.69.337|author1=Fred C. Adams |author2=Gregory Laughlin |lastauthoramp=yes |arxiv = astro-ph/9701131 |bibcode = 1997RvMP...69..337A }}</ref><sup>, § IIIE, IVA.</sup> Barrow and Tipler estimate that it would take 10<sup>15</sup> years for a white dwarf to cool to 5 K;<ref>Table 10.2, {{BarrowTipler1986}}</ref> however, if [[weakly interacting massive particles]](WIMPs) exist, it is possible that interactions with these particles will keep some white dwarfs much warmer than this for approximately 10<sup>25</sup> years.<ref name="adams" /><sup>, § IIIE.</sup> If protons are not stable, white dwarfs will also be kept warm by energy released from proton decay. For a hypothetical proton lifetime of 10<sup>37</sup> years, Adams and Laughlin calculate that proton decay will raise the [[effective temperature|effective surface temperature]] of an old one-[[solar mass|solar-mass]] white dwarf to approximately 0.06 K. Although cold, this is thought to be hotter than the [[cosmic background radiation]] temperature 10<sup>37</sup> years in the future.<ref name="adams" /><sup>, §IVB.</sup>
==Xem thêm==
{{Wiktionary}}
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