Don’t you love the way the cosmos keeps us from getting too comfortable with our ideas? The Hubble Constant (H0), which tells us about the rate of expansion of the universe, is still a hot issue because observations from both the Hubble Space Telescope and JWST don’t tally with what the European Space Agency’s Planck mission concluded from its data on the Cosmic Microwave Background (CMB).
How exactly do we fine tune the standard model of cosmology to make sense of this? The so-called Hubble Tension is hardly the only issue raised by the acquisition of new and better data, although it may be the biggest. All kinds of questions linger about what dark energy is, not to mention dark matter. Of course, challenging observations are hardly limited to cosmology. Dialing down to the stellar level, new work has emerged challenging the way white dwarf stars evolve. Contrary to all expectation, some white dwarfs seem to stop cooling, and can indeed live to a satisfying old age.
A white dwarf is what is left after a star goes through its red giant phase and sheds its outer layers. After all, most stars (over 95 percent) don’t have the mass to become a neutron star or black hole. A star like the Sun will one day enlarge, then contract, casting off its outer layers to form a planetary nebula and leaving a white dwarf behind. Planets in our inner system would likely be engulfed in the red giant phase, but get out around the asteroid belt and the chances of survival are high, with subsequent outward migration in the white dwarf period because of the star losing mass.
Sirius B is a white dwarf 8.6 light years away, and there are eight of the objects among the 100 nearest star systems. They should be cooling down because fusion no longer occurs, with the dense plasma in the star’s interior freezing so that the star solidifies from the center out. The cooling process itself can take billions of years, which is why I find these objects so appealing. They’re another example of an exotic place where planets can orbit and conceivably produce some kind of life, and recent studies have uncovered that many of them show signs of atmospheric ‘pollution,’ meaning they’ve ingested materials near them. As many as 50 percent show metals in their spectra.
Image: This is an artist’s impression of a white dwarf (burned-out) star accreting rocky debris left behind by the star’s surviving planetary system. It was observed by Hubble in the Hyades star cluster. At lower right, an asteroid can be seen falling toward a Saturn-like disk of dust that is encircling the dead star. Infalling asteroids pollute the white dwarf’s atmosphere with silicon. These dead stars are located 150 light-years from Earth in a relatively young star cluster, Hyades, in the constellation Taurus. The star cluster is only 625 million years old. The white dwarfs are being polluted by asteroid-like debris falling onto them. Credit: NASA, ESA, and G. Bacon (STScI).
Finding a debris disk around a white dwarf is one thing, but planets are another matter. Only a few have so far been identified. There have been white dwarf surveys to look for surviving gas giants in such systems but their numbers are low. What we have found, though, is intriguing. WD 1856+534 b, for instance, orbits in a remarkably tight orbit and raises further questions about orbital evolution. A planetesimal designated SDSS J1228+1040 b likewise appears in a tight orbit within a white dwarf debris disk.
Because the contrast between star and planet in a white dwarf system could be as low as 1:200, according to a new paper, JWST gives us significant capabilities for imaging planets around white dwarfs. Susan Mullally (Space Telescope Science Institute) is lead author of the paper, which appears in The Astrophysical Journal Letters. The researchers describe two giant planet candidates orbiting the white dwarfs WD 1202−232 and WD 2105−82. The projected separations are 11.47 and 34.6 AU respectively, and the best take on mass for each is in the range of 1 to 7 Jupiter masses. “If confirmed,” note the authors, “using common proper motion, these giant planets will represent the first directly imaged planets that are similar in age, mass, and orbital separation as the giant planets in our own solar system.”
So we’re getting there as we slowly build our exoplanet catalog around such stars. Even so, the questions seem to be multiplying because of the fact mentioned above: There is a population of white dwarfs along the so-called Q Branch of the Hertzsprung–Russell (H–R) diagram that is maintaining a constant luminosity over billions of years. This would mean there must be a source of energy inhibiting the cooling process.
Here we’re dealing with data from the Gaia satellite, cited in 2019 to announce the discovery of this population of white dwarfs. White dwarf cooling is thought to involve crystallization of core materials into a solid phase. Sihao Cheng (Institute for Advanced Study, Princeton) and colleagues point to this transition in the 2019 paper:
As a liquid-to-solid phase transition in the white dwarf core, crystallization releases energy through latent heat (e.g., van Horn 1968) and phase separation (e.g., Garcia-Berro et al. 1988; Segretain et al. 1994; Isern et al. 1997), which can indeed create a cooling delay. However, the observed pile-up on the Q branch is higher and narrower than expected from the standard crystallization model (Tremblay et al. 2019, Figure 4), suggesting that there exists a cooling anomaly, i.e., an extra cooling delay in addition to crystallization.
Explaining this phenomenon is the task of a just published paper in Nature from Antoine Bédard (University of Warwick), with Cheng as a co-author. Its hypothesis is that freezing of the interior into a solid state does not produce the expected result, the solidification of the star from the inside out. Instead, crystals formed upon freezing displace heavier liquids downward, a mechanism that releases gravitational energy. This, then, would be the source of the persistent star’s energy, and would constitute, according to Bédard, “a whole new astrophysical phenomenon.” This delay in cooling could mean that we are underestimating the age of some dwarfs by billions of years.
Cheng sees the question of stellar age as central, even if we’re not sure why some white dwarfs take this path and others do not:
“One fascinating aspect of this discovery is that the physics involved is similar to something we observe in daily life: the frozen crystals within the white dwarf star float instead of sink. We might compare their behavior to ice cubes floating in water. Our work will necessitate updates to astronomy textbooks. We hope that it will also prompt astronomers to reassess the methods employed to calculate the age of stellar populations.”
It’s always satisfying to think that our textbooks will need to be updated on a regular basis, for the pace of discovery is accelerating. In this case, something as fundamental as stellar age is up for grabs. Indeed, as the Bédard and Cheng paper notes, this “population of freezing white dwarfs maintains a constant luminosity for a duration comparable with the age of the universe.” These white dwarfs, at least, fall entirely out of the category of ‘dead stars’ and force a healthy re-thinking of our assumptions.
The Mullally paper is “JWST Directly Images Giant Planet Candidates Around Two Metal-polluted White Dwarf Stars,” Astrophysical Journal Letters 962 (15 February 2020), L32 (full text). The 2019 paper discussing the cooling issues in white dwarfs is Cheng et al., “A Cooling Anomaly of High-mass White Dwarfs,” The Astrophysical Journal Vol. 886, No. 2 (25 November 2019), 100 (full text). The new paper on white dwarf cooling is Bédard et al., “Buoyant crystals halt the cooling of white dwarf stars,” Nature 627 (06 March 2024), pp. 286-288 (abstract).
The death of our Sun will be greatly exaggerated !
Hundreds of thousands of earth masses of material will be ejected, enough to build a substantial number of water worlds and a fusion centre at its heart ready to be kept warm by merely dropping material on it or have a mini Dyson sphere around it.
I love using this simulator to see what different masses do to the lifespan of stars.
http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.html
I don’t think we can use a view limited to classical physics in the laboratory to understand degenerate matter, the electron degeneracy in white dwarfs. An idea came to me while reading this paper with the help of google references. There is not water or no crystals in the center of a white dwarf. Diamond is the crystalline form of carbon. A diamond would be crushed to carbon atoms in the center of a white dwarf because of the pressure in the center where all of the electron shells are completely due to the electron degeneracy pressure and Pauli exclusion principle. Gravitational energy is the same as pressure and degenerate matter inside a white dwarf must have a high gravitational energy due to the mass of a white dwarf being close to our Sun or a little greater. Therefore it is the gravitational energy that causes the vibration and rotation of atoms. According to Chat GPT AI There are not electron jumps vibration or rotation of atoms in the degenerate matter of white dwarves, but there is quantum tunneling which is responsible for some of the heat motion of atoms which is emitted in the visabile light and black body ultra violet light. It gives the explanation that most of the radiation in the visible light is from the stored remnant energy from the stars nuclear reactions before it died and became a white dwarf which is the generally accepted view in astrophysics. I don’t like that idea, but I can’t invalidate it. My main reason is that the gravitational energy of a white dwarf will remain stable over a trillion years and always be there. AI admits that the idea of the remnant heat from the stars life radiated from a white dwarf can’t be proven in a laboratory. There still can be human error in this. White dwarfs could live much longer if our observations of them are wrong like two white dwarfs with the same mass but with different luminosity. I like the idea that the gravitational energy causing a high temperature in the center of a white dwarf might be responsible for most of the visible light emitted from white dwarfs which would go against generally accepted views. I don’t think I am right about it, but this is just an idea, a testable hypothesis. The EMR would escape at the surface of the White dwarf. AI has not convinced with the remnant heat from the nuclear energy of a star causes most of the visible light of white dwarfs. In my opinion, we can still challenge that idea. Also the fact that we have a good idea about how long it takes for white dwarfs to cool off which could be wrong due to human error. I am an advocate for new ideas and progress if we stick to first principles.
Quote from Another Conundrum: How Long Do White Dwarfs Live? “Indeed, as the Bédard and Cheng paper notes, this “population of freezing white dwarfs maintains a constant luminosity for a duration comparable with the age of the universe.” I like this idea. I’ve always accepted the idea that the life of a white dwarf star is based on it’s limited time of luminosity over time. The generally accepted view is that white dwarf’s light is mostly from the remnant heat energy saved and stored from the life of the star’s nuclear reactions before it became a white dwarf. I think that idea is difficult to prove, and I have never thought before challenging it until I read this paper. I do think though we have to come up with another explanation to make this generally accepted view invalid like gravitational energy at the center of a white dwarf which must last a very long time. I do think that we should stick to first principles and it is not a new idea that these have made past views obsolete. Also observations are anecdotal and therefore would be wrong due to human error and assumptions. Consequently, I do like new ideas like this one.
Today I asked AI some other questions about heat capacity and degenerate matter versus ordinary matter and thermodynamics of atoms and black body radiation wavelength and it looks like my judgment about this article is premature. The fact that we can have two white dwarfs with the same mass but different brightness tells us their age and life. Consequently, I the idea that white dwarfs loose energy from their remnant heat they retained from their life on the main sequence and red giant phase matches observations. Our observations would have to be wrong. I still like the idea that white dwarfs have a much longer life, but it does not support astronomical observations according to AI.
The problem with my idea about internal gravitational energy being responsible for their loss brightness is that I don’t think white dwarfs would loose enough mass to account for their loss in brightness over time especially due to the high energy necessary to break apart degenerate matter. It can be easy to jump to conclusions especially if I thought I stumped AI which is possible.
AI also agreed that there is the vibration and rotation of atoms for being responsible for the temperature in a white dwarf and degenerate matter and also due to the wavelike nature of atoms, and electrons.
What I find interesting about the cited study is that they only looked at 4 white dwarfs, and found 2 systems with candidate planets. Yes, small number stats but who knows, there might be an avalanche of these discoveries as direct detection methods improve.
Phil
When lower main sequence stars collapse to white dwarf size and equilibrium, they are likely to release some surface layers a fraction of their mass. White dwarfs can be as massive as 1.4 solar but probably somewhere near the K and M boundary would be a minimum mass to start the process. And given the long lives around there – well maybe it’s a case of “stay tuned”… Though having got past the fusion shutoff, the collapse state results in a radius about that of the Earth.
OK, so even on the MS, not all luminosity is directly related to H fusion. But say it is = and you have a sun that shrinks from 100x the width of the Earth down to Earth size and it is sustaining 5800 Kelvin, it’s got 10,000th area the next day to radiate all that heat off. The internal structure collapses to a non gaseous state and is conductive; so the heat transfer is not based on PV= nRT internally. Early white dwarfs can have temperatures at the surface of tens of thousand degrees K, but they also go down to substantially lower – over a long period of time. I think the squeeze in surface radius and area sets the stage. But saying this so far, it should be possible to distinguish between an old red dwarf and a white dwarf based on an order of magnitude difference in size, The RD equilibrium is a star 10x wider and set to a lower thermostat to begin with.
But on the other hand, exploring the local neighborhood, have to wonder about major and minor dog star binaries. Procyon and Sirius. The two larger elements in each of these binaries have to be younger, brighter and more massive than the sun. So how did they start off or get acquainted with white dwarfs?
The only thing I can think of is that Sirius A and Procyon A started out as some sort of close binaries – and then somehow merged. But the white dwarf nearby each never noticed anything strange? No release of outer layers into something like
a nova? With two star systems in our vicinity with such similar traits, it would seem that a wider search would provide evidence of the transition going on elsewhere.
Newly formed white dwarfs can be around 100 000 K and at a fourth power emission that is a hugmungas amount of energy emission. In the case of Sirius b it was around 5 solar masses and looks to have polluted Sirius A with metals during its red giant stage. They were also much closer with a period suspected of been around 9 years as well.
The scenario you mention of a possible earlier state for Sirius B as a 5 solar mass star is intriguing. And I hope that the origin of white dwarfs like these two is re-examined from time to time.
At the very least, Sirius B was not of its near solar mass now before its transformation to a white dwarf, but I suspect that a range of masses and corresponding mass ejections could be possible. As with the case of Procyon B.
Attached below is some additional information from on line (Wikipedia) sources (adapted for Comments) with which to mull further origin of local white dwarfs, particularly those in binary systems:
If the mass of a main-sequence star is between 0.5 and 8 solar, like the Sun, its core will become sufficiently hot to fuse helium into carbon and oxygen via the triple-alpha process, but it will never become sufficiently hot to fuse carbon into neon. Near the end of the period in which it undergoes fusion reactions, such a star will have a carbon–oxygen core which does not undergo fusion reactions, surrounded by an inner helium-burning shell and an outer hydrogen-burning shell. On the Hertzsprung–Russell diagram, it will be found on the asymptotic giant branch. It will then expel most of its outer material, creating a planetary nebula, until only the carbon–oxygen core is left. This process is responsible for the carbon–oxygen white dwarfs which form the vast majority of observed white dwarfs.
Spectral Types ( preceded by D as in DA, etc.
A H lines present
B He I lines
C Continuous spectrum; no lines
O He II lines, accompanied by He I or H lines
Z Metal lines
Q Carbon lines present
X Unclear or unclassifiable spectrum
Name distance spect abs mass luminosity giga-year
light yrs type mag age
Sirius B 8.66 DA 11.18 0.98 .0295 0.10
Procyon B 11.46 DQZ 13.20 0.63 .00049 1.37
VanM 2 14.07 DZ 14.09 0.68 .00017 3.30
LP 145-141 15.12 DQ 12.77 0.61 .00054 1.29
40 Eridani B 16.39 DA 11.27 0.59 .0141 0.12
Stein 2051 B 17.99 DC 13.43 0.69 .00030 2.02
G 240-72 20.26 DQ 15.23 0.81 .000085 5.69
Gliese 223.2 21.01 DZ 15.29 0.82 .000062 7.89
Gliese 3991 B 24.23 D? >15 0.5 6
Hot white dwarfs cool down mostly by neutrino emission from plasmons. https://arxiv.org/pdf/2109.06158.pdf I wonder if it is possible for high-energy forms of life to manipulate this process, using the low-pressure environment that neutrinos enter into as the cold reservoir for a heat engine to generate free energy.
That would be difficult as they interact very rarely, I often see a light year of lead phrase used to absorb one neutrino. It may be possible to slow one down if say a beta decay atom is send to near light speed and then decays the neutrino backwards in effect slowing it down but that would depend on if it has mass or not.
OT
Nice to see progress from firstlight fusion reaching a new pressure limit of near 2 Tpa. The reason I like the first light design is the linear drive process, no need for compression from all directions as the amplifier carrys this out for us. Also the z pinch can use both processes to compress the fuel. They will run another shot in December using FLF’s own amplifier, hoping for a 8 to 10 Tpa.
https://m.youtube.com/watch?v=8XRSBA9elm8
In the spirit of looking at white dwarf long times, a couple more observations.
Rounding things off somewhat, if you decrease the dimensions of a G star like the sun to about the dimensions of the earth, the density has increased consider in inverse accordance on a cubic scale. If solar density is about 1 g/cc ( akin to Jupiter). and this matter is packed in a 1/100th the radius, then density is increased a million times.
Element mercury about 13 x the density of water, move over for white dwarf material on order of a a million g/ cm3. Every now and then I would read about that in the astronomy books when I was a kid. So given the white dwarf radius and the concentration of matter, the internal heat content is very dense too and has little means to radiate away. Conductivity in this configuration is high, but since the surface area is limited and interior temperature is rather uniform, the thermal energy is going to be stuck in there for a long time.
The smallest red dwarfs are about a tens the radius of the sun and about a tenth the mass, volume has decreased to a 1000th of the sun’s. Density here is higher than with the sun or Jupiter: 100 times. An order of magnitude more dense than
liquid mercury. Yet one region red dwarfs stay on the main sequence so long is that in their interiors there is more convection than there is associated with the sun – in the sense that more hydrogen can be churned to regions where nuclear fusion can occur. The white dwarf is static resembling a heat conducting metal with a solid grid like structure.
Now I suppose that one reason to contemplate red and white dwarf life times is to consider them as safe harbors for life over vast periods of time. Both type objects would experience thermal flux decays over long periods of time- and they would also have different nominal habitable zones. A newly hatched white dwarf emerging out a planetary nebula might be 100,000 kelvin and objects in our vicinity have temperatures under 10,000 K ( e.g., 7,740 ± 50 K: Age: 1.37 Gyr; Sirius b about 25,000 K, suggesting a more recent origin). In terms of a habitable zone for a planet, and the white dwarf is about the radius of the earth, by the time it cools to 5800 K, for equivalent temperature at radius, about 1/100th of an AU.
Hi wdk
Interesting thing with White Dwarfs is that there’s a mostly isothermal interior of degenerate matter and a very insulating crust of regular matter, thus a high temperature gradient. So while they glow at thousands of K, the insides can be in the millions.