Talk:White dwarf

Computational relations for white dwarfs

Given a mass (M/M๏) and an effective temperature (T), it is possible to determine the radius (R/R๏), the luminosity (L/L๏), and the age (t) for a carbon-oxygen core white dwarf.

The two quadratic equations and the line below comprise a good curve-fit for the white dwarf mass-radius relation from 0.25 to 1.41 solar masses.

If 0.25 ≤ M/M๏ < 0.45, then

R/R๏ = 0.07279307 (M/M๏)² − 0.0752974 (M/M๏) + 0.03327478

If 0.45 ≤ M/M๏ ≤ 1.2, then

R/R๏ = −0.010421 (M/M๏) + 0.018821

If 1.2 < M/M๏ ≤ 1.41, then

R/R๏ = −0.0814246 (M/M๏)² + 0.1899852 (M/M๏) − 0.1044496

Next, the luminosity of the white dwarf is found from the Stefan-Boltzmann law.

T๏ = 5784K

L/L๏ = (R/R๏)² (T/T๏)⁴

Finally, the age of the white dwarf, t, in years.

t = 10^[6.7 − (5/7) log(L/L๏)]

Observed white dwarf color temperatures are slightly affected by the gravitational red shift, which can be corrected spectroscopically. Also, if the apparent magnitude and the parallax (distance) of the white dwarf are accurately determined, the luminosity can be calculated without reference to the temperature.

It has been asked whether white dwarfs could be used as suns for artificial colonies in space. The answer is yes, barely. The problem is that the distance at which an orbiting space station would receive the same intensity of radiation that the Earth gets from the sun is usually very close to, or even within, the Roche limit of the white dwarf with respect to the space station (nominally assumed to have an average density of 100 kg m⁻³).

Beginning with the Stefan-Boltzman law again, and making the necessary substitutions, while assuming a subsolar temperature equal to that of Earth (393.6K), we find the nominal distance of the habitable zone:

rᵤ = 48977 t^(−0.7)

And for the Roche limit:

rᵥ = (5.22495e-12 AU/m) (M/ρᵥ)^(1/3)

where ρᵥ is the effective density of the space station, in kg m⁻³. M is the white dwarf's mass in kilograms. However, rᵥ and rᵤ are both returned in astronomical units.

An 0.5 solar mass white dwarf having an effective temperature of 6000K will have a radius of 0.0136105 solar radii, a luminosity of 2.1451e-4 L๏, and an age of 2.0912 billion years. It's habitable radius will be 0.014646 AU. It's Roche limit with respect to a (ρᵥ = 100 kg m⁻³) space station will be 0.011236 AU.

Most white dwarfs will be too massive or too old (i.e., too cool) for the habitable zone to exist in the conventional sense because it would occur inside the Roche limit.

versus Red and Brown flavors

I was curious as to the difference between Red dwarfs, Brown dwarfs and White dwarfs. Each of the three articles rarely or never mention the others, although it's natural to assume there's a similarity. From what I've read of the three:

• Red dwarfs are full-on stars, just small, maybe can't do helium fusion.

• Brown dwarfs are smaller still, can't even fuse hydrogen, but can participate in some lame fusion reactions, if they're lucky. Their surface temperatures range down to room temperature! Their sizes range down to gas giant planets.

• White dwarfs are supernova remnants; totally different from the other two. They often have surface temperatures comparable to stars (hence 'white') from residual heat; can't do fusion. If they get bigger, 1.44M_☉, they become neutron stars (after maybe a supernova). (And neutron stars similarly become black holes beyond 3M_☉.)

I'd prefer if a professional could supply and correct these guesses of mine in the article, maybe a separate section. Also the other two articles. OsamaBinLogin (talk) 20:35, 11 May 2022 (UTC)

White dwarfs are not supernova remnants, just the remnants of ordinary stars, although possibly supernova progenitors. See also black dwarf and blue dwarf. Not sure we can explain every type of stellar object with "dwarf" in the name in every other article with "dwarf" in the name. So long as each article is clear about what it is, that would hopefully be enough. Perhaps best not to assume there is a similarity, certainly no similarity to green dwarf. However, for an overview of the term "dwarf" when applied to stars and star-like objects, see dwarf star, perhaps something that could be linked from other articles more often. Lithopsian (talk) 20:48, 11 May 2022 (UTC)

Couldn't find reference in Wikipedia to "green dwarf" referenced in your answer. Are you messing with us? If so, carry on...more power and so on....Makermark (talk) 21:44, 20 August 2022 (UTC)

Densest collections of matter

Aren't atomic nuclei also collections of matter surpassing the density of white dwarfs? 174.103.211.189 (talk) 23:08, 26 December 2023 (UTC)

The article doesn't say that white dwarfs are the densest collections of matter. The only reference to the word 'densest' is this sentence: 'White dwarfs are composed of one of the densest forms of matter known, surpassed only by other compact stars such as neutron stars, quark stars (hypothetical), and black holes.' And I think it's fine the way it is. Nuclei could arguably be considered a form of matter, but I think it's clear from the way it's phrased that they mean matter in the macroscopic sense of the word. MathewMunro (talk) 03:43, 7 January 2024 (UTC)

Thanks. That's what I figured. 74.135.194.87 (talk) 18:28, 19 January 2024 (UTC)

WP:URFA/2020

I am reviewing this article as part of WP:URFA/2020, and initiative to evaluate older featured articles to ensure that they still meet the FA criteria. I have some concerns with this article:

• There are numerous uncited passages, including whole sections at the end of the article.
• The gallery at the end of the article should be evaluated. Can these been used in other sections of the article instead, or perhaps removed?
• There are references that need to be properly formatted, especially the last three. Has anyone checked to ensure that they are high-quality sources?

Is anyone intersted in fixing up the article? If not, I will nominate this to WP:FAR in a couple of weeks. Z1720 (talk) 15:10, 26 January 2024 (UTC)

Radiation and cooling:

The paragraph says that radiative heat transfer is low "because any absorption of a photon requires that an electron must transition to a higher empty state, which may not be possible as the energy of the photon may not be a match for the possible quantum states available to that electron" I don't think the lack of absorption is important, but the lack of emission is. If thermal radiation wouldbe emitted, the thermal energy would go somewhere. It would probably be absorbed in the outer layers, or be emitted, leading to fast cooling of the core. But I think in the same way as the lack of empty states prevents absorption, it is also preventing thermal excitation of electrons to energy levels from where they could emit radiation. Emilo Alberto (talk) 11:21, 8 March 2024 (UTC)

Contradiction with article on SN 1181

The article on SN 1181 says:

With a temperature near 200,000 K, WD J005311 is among the hottest stars known. The extreme properties of the central star are being powered by the residual radioactive decay of ⁵⁶Ni, where the usual half-life of 6.0 days from electron capture is increased to many centuries due to the nickel being completely ionized.

This contradicts the present article, whose lead paragraph twice states that a white dwarf "has no source of energy"; the article doesn't mention that white dwarfs may be powered by radioactive decay. Joriki (talk) 07:55, 25 October 2024 (UTC)