Talk:White dwarf

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WikiProject Echo has identified White dwarf as a foreign language featured article. You may be able to improve this article with information from the Russian language Wikipedia.

Hmm, this article could use some headers and better structure. Fredrik 16:59, 23 Feb 2004 (UTC)

You think so? I am currently doing a paper of the ending of stars lives, and found this to be actually one of the clearer, more structured articles. As I am just being introduced to these things (first course this semester), clarity and order is key, and I found it quite easy to follow, and appreciate, this article.

In the article it states that a white dwarf of a certain mass undergoes a supernova, it is said the mass is usually gained from a companion star. When this occurs to a white dwarf, is it not a nova instead of a supernova? Harley peters 01:10, 4 Feb 2005 (UTC)

No, the article is correct. A core-collapse supernova results (in theory) when the mass of the core exceeds the limit that electron degeneracy can support. It destroys the star. A nova is a lesser event where hydrogen from the companion star ignites (nuclearly) near the surface and blows away some fraction of its mass, but it lives to explode another day. Novae can reccur; supernovae cannot.

Contents 1 Unanswered Question 2 White dwarfs don't fuse anything 3 Weight of a white dwarf 4 Helium white dwarf 5 The maximum mass of ideal white dwarfs

[edit] Unanswered Question

I read thru the article and couldn't figure out whether white dwarfs more often contained helium, carbon, or whether other elements were found in them. If there is any information then perhaps the article could be clearer which elements could be found in these starts and how that would depend on their ealier mass.

As an addition, I looked at the Russian version of the article (translated by www.translate.ru) and it seems to have a lot more information, but the translator couldn't get all the words in the article so I didn't totally get what it was talking about. Anyway, if anybody knows russian they could probably bring a lot more information just from reading the russian version.

HI there. Here are your answers. White dwarfs are observed in greater number to be what we call "Hydrogen white dwarfs", and relatively fewer are "helium white Dwarfs". The difference in composition between the two is only in the outer atmosphere of the star where the spectral lines are formed. Small changes here result in big changes in what we observe. All white dwarfs contain Carbon in large amounts in their cores (So theory dictates) and also in small amounts in their atmospheres (which we detect by observing carbon absorption lines in the far ultraviolet). Other elements are seen in the white dwarfs atmospheres and the amounts of these roughly follow universal element abundances. Again we see these elements as absorption lines in the FUV. In fact nearly all elements have been spotted in the spectra from FUSE up to Iron in atomic mass. We havn't seen much (though with some exceptions) heavier than this as nobody has calculated the lab wavelengths for these spectral lines. As heavier and heavier atoms are considered you have to consider more and more electrons (whos change in energy creates the lines in the spectra) and so much beyond Iron and it starts getting messy with 20,000 possible lines in the FUSE wavelength reagion.

Any more questions would be helpful to me, as I am doing a phd on white dwarfs and it is helpful to experience questions cheers Soloist 20:48, 9 November 2006 (UTC)

I keep seeing references in other articles to different types of white dwarves, namely oxygen/carbon and neon/oxygen types ; but the wiki article doesn't seem to go into these - maybe someone more knowledgable can add these details and how they differ, etc. ? The Yeti 13:10, 28 January 2007 (UTC)

[edit] White dwarfs don't fuse anything

The Formation section claims that white dwarfs avoid collaps by having fuel... They generally need not have any fuel, nor burn any fuel to avoid collaps – they simply are not massive enough to "reach the Chandrasekhar limit", kind of... Rursus 22:52, 8 January 2007 (UTC)

You are wrong when you say White dwarfs don't fuse anything. Our Sun is a white dwarf and it certainly does fuse hydrogen into helium. Otherwise we would not exist. Maybe you confuse white dwarfs with brown dwarfs (sub-stellar objects too small to support hydrogen fusion)? --Friendly Neighbour 06:36, 9 January 2007 (UTC)

Friendly Neighbour! Sol (our sun) is a "yellow dwarf", which is an alias for main sequence star of class G. "White dwarfs" aren't stars in the ordinary sense, especially they aren't class A main sequence stars. "White dwarfs" and "dwarfs" are separated by some 7-8 magnitudes, where the "white dwarfs" are the fainter ones. Astronomers are known to be sloppy with terms and definitions, so "white dwarf" are real dwarfs in comparison to "dwarfs". White dwarfs are upholding inner pressure by electron degeneracy, not fusion - this approximatelly means that the matter is so compressed that it's impossible to compress more electrons into it, while "paradoxically" the more heavy atom nuclei move freely within this electron degenerate matter. I've never seen it by myself, only heard it spoken of. Rursus 18:51, 30 March 2007 (UTC)

He is not wrong (*). White dwarf is the endpoint of a low-mass star, supported by electron degeneracy pressure. Our Sun is a yellow dwarf, or a yellow main sequence star. Sirius B, Procyon B, and Van Maanen's star are white dwarfs. One could call Sirius A also a "white dwarf" as it is a a class A main sequence star, but that is misleading.

(*) Except that white dwarfs occasionally do fuse hydrogen. The event is explosive and is called a nova.--JyriL ^talk 11:26, 9 January 2007 (UTC)

[edit] Weight of a white dwarf

I remember being told back in high school that "one teaspoonful" of a white dwarf would way about the same as our Earth. Is this anywhere where near true or was it just my science teacher exaggerating again?? --Ukdan999 17:55, 10 January 2007 (UTC)

That is a huge exaggeration. But still would weight many tons.--JyriL ^talk 17:59, 10 January 2007 (UTC)

Speaking of odd fantasies, I used to read that if Saturn would be dropped in a huge ocean of water, then it would float. I could never properly imagine such an ocean of water. Rursus 18:59, 30 March 2007 (UTC)

[edit] Helium white dwarf

This article doesn't cover helium white dwarfs. That is, a white dwarf with a helium core, formed when the outer envelope of a red giant is stripped away,[1][2][3] or from a precursor star with less than 0.3 solar masses.[4] Thanks. — RJH (talk) 18:48, 2 March 2007 (UTC)

Fixed, in the lead section. — RJH (talk) 16:21, 5 March 2007 (UTC)

[edit] The maximum mass of ideal white dwarfs

The text reads:

S. Chandrasekhar discovered in 1930 (Astroph. J. 1931, vol. 74, p. 81-82 [5]) in an article called "The maximum mass of ideal white dwarfs" that no white dwarf can be more massive than about 1.4 solar masses.

If you read the paper, however, it says 0.91 solar masses. — RJH (talk) 19:38, 3 March 2007 (UTC)

As written in the text, "Chandrasekhar solved the hydrostatic equation together with the nonrelativistic Fermi gas equation of state,[8] and also treated the case of a relativistic Fermi gas, giving rise to the value of the limit shown above [0.91]." --Gspinoza 16:00, 20 March 2007 (UTC)

Yes, but that would be my point. The paper says 0.91 solar masses. So there must be a later paper with the currently accepted value of 1.44, making the current reference outdated. — RJH (talk) 16:18, 20 March 2007 (UTC)

The limit is a function of the average atomic weight per electron, μ, of the star. In the 1931 Astrophysical Journal paper, μ is taken to be 2.5. In other papers by Chandrasekhar (e.g., Monthly Notices of the Royal Astronomical Society, vol. 95, pp. 207–225) the limit is computed as a function of μ. When this is done the limit is approximately 5.7/μ² solar masses. Taking μ=2 gives the modern value.

The discrepancy in μ arises from changing beliefs about the composition of the stars. In the past it was believed that stars were composed of heavy atoms, for which μ=2.5 would be approximately correct. For a carbon-oxygen white dwarf however one should clearly take μ=2. Spacepotato 20:06, 20 March 2007 (UTC)

Thanks. So it might make sense to clarify this in the history section. — RJH (talk) 20:43, 20 March 2007 (UTC)