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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.
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)
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)
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:
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)
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)