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Adjusting the Clock: Hydrogen Burning in White Dwarfs

White dwarfs have turned out to be more interesting than I had imagined. We know how they form: A star like the Sun exhausts the hydrogen in its core and swells into a red giant, a scenario that is a trope in science fiction, as it posits an Earth of the far-future incinerated by its star. Losing its outer layers near the end of nuclear burning, a red giant ultimately leaves behind an object with much of the mass of the Sun now crammed into a white dwarf that is about the size of the Earth. For years I assumed white dwarfs were dead ends, a terminus for life whose only function seemed to be in binary systems, where they could be the locus, through accretion from the other star, of a stellar explosion in the form of a nova.

Lately we’ve been learning, though, that through analysis of their atmospheres, white dwarfs can yield information about objects that have fallen into them, such as remnants of the original stellar system. Some white dwarfs may have habitable zones lasting several billion years, an interesting thought if surviving planets in the system migrate inward. And now, thanks to Hubble data, we are learning that white dwarfs may have a longer lifetime than previously thought. Continuing to burn hydrogen, some of these stars may be older than they look.

The work was coordinated by Francesco Ferraro (Alma Mater Studiorum Università di Bologna / Italian National Institute for Astrophysics), who points to the useful role of white dwarfs in determining the ages of stars. The white dwarf cooling rate has been used as a natural clock as a way of calibrating the age of star clusters, as their cooling seemed easy to model by following the relationship between age and temperature. But the new study finds that white dwarf aging is nowhere near as simple as this, so an accepted method of measuring cluster ages may have to be re-examined.

To investigate white dwarf aging, the scientists looked at the globular clusters M3 in the constellation Canes Venatici and M13 in Hercules, in which stars in aggregate show common values of metallicity and age. As always in astronomy, the word ‘metals’ refers to elements higher than hydrogen and helium, with the bulk of a star like the Sun being made up of the latter two, while a scant 1.3 percent is given over to metals.

Ferraro’s team compared cooling white dwarfs in the two clusters and found that among the stars that will eventually become white dwarfs, the color of stars in the Horizontal Branch is bluer in M13 than in M3. The Horizontal Branch is a stage in the evolution of a star in which it has begun helium burning in the core, a stage that is flagged by a reduction in luminosity and increases in surface temperature. So the enhanced blue color in M13 is indicative of hotter stars on their way toward white dwarf status.

Image: To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity, but the populations of stars which will eventually give rise to white dwarfs are different. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool. Credit: ESA/Hubble & NASA, G. Piotto et al.

The researchers consider the M3 and M13 clusters to be “a classical horizontal branch (HB) morphology pair” because they share many physical properties including metallicity, making the color difference the salient feature. Homing in on the white dwarfs among this population, the team finds the outer envelope of hydrogen in these bluer white dwarfs allows them to burn for longer and cool more slowly than the standard white dwarf model. Using near-ultraviolet data from Hubble’s Wide Field Camera 3, the team compared more than 700 white dwarfs in the two clusters.

The result: M3’s white dwarfs follow the expected model, consisting of predictably cooling stellar cores and no stable thermonuclear activity. But in M13, two populations of white dwarfs can be found, the second being those that have retained an outer hydrogen envelope, continue thermonuclear burning and therefore cool at a slower pace. In fact, 70 percent of the white dwarfs in M13 appear to be burning hydrogen on their surface. This appears to be the only viable explanation for the ‘blue tail’ found in the Hubble data that distinguishes the two clusters.

From the paper:

At the moment, this [hydrogen burning on the surface] appears to be the most viable and natural explanation, while alternative scenarios should invoke ad hoc and unknown mechanisms able to increase the production or slow down the cooling process of the WDs in M13, and not in M3. The discovery reported in this paper represents the first direct evidence for the occurrence of stable nuclear burning in the residual hydrogen envelope of cooling WDs and offers an empirical measure of the delay in the flow of time marked by the WD clock in the presence of slowly cooling WDs.

The authors believe the road ahead should involve studying other clusters that show differences when at the Horizontal Branch of stellar evolution, while also examining clusters with different degrees of metallicity to determine the role it plays. Clarifying how white dwarfs evolve will force us to adjust the use of these stellar remnants in calibrating age, where current uncertainties can be as large as a billion years.

Adds Ferraro:

“Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old. We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly.”

The paper is Chen et al., “Slowly cooling white dwarfs in M13 from stable hydrogen Burning,” Nature Astronomy 6 September 2021 (abstract).

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{ 15 comments… add one }
  • Robin Datta September 17, 2021, 23:31

    Star lifting has been posited as a means to extend the lifespan of stars. Could “star dumping” be a way to extend the lifespan of white dwarves? Particularly so if the hydrogen might be transferred from other stars in the vicinity from the same cluster?

    And is there a way to detect that hydrogen has been purloined from a star?

    • Alex Tolley September 18, 2021, 12:07

      Isn’t that what happens when a WD pulls material from its companion? (Albeit with nova hiccups to the smooth burning – analogous to a slow, irregular, very inefficient, detonation engine)

      I can imagine an advanced ETI regulating the material flow to ensure a smooth hydrogen burn. Unfortunately, the lifetime of the companion star limits this usefulness, whilst your idea of “star dumping” would extend the life of the energy emissions, if the needed H2 could be stored and directed.

      What about a WD traveling through space with a gravity/magnetic scoop collecting interstellar gas and burning it, yet without using it to propel the star?

      • Robin Datta September 18, 2021, 13:32

        That’s one way to commandeer a star and make it a spaceship.

  • Patient Observer September 19, 2021, 8:38

    The idea of controlled feeding of hydrogen to a white dwarf to harvest energy reminded me of a scheme I have been toying with but using a black hole as the energy source. After some hand waving, a black hole is found or constructed that is the right mass to emit copious amounts of Hawking radiation. The concept is to feed mass to the starving black hole at a rate that is equivalent to the loss of mass from emitted radiation. Thus, we have 100% conversion of mass to energy. A few minor details remain such as getting that mass into the black hole without it being driven back by radiation pressure. There should be a sweet spot between the mass of the back hole, and intensity of radiation emitted A relatively tiny Dyson sphere would collect the energy for some unknown but presumably benign purpose.

    • Andrei September 20, 2021, 9:19

      Having a black hole as an energy source have indeed been a plot device in science fiction for quite a while. The hawking energy omitted for a very small black hole will indeed be substantial, and the idea was used to power space ships of various size. The handwavium you ask for could be used in the construction of a particle accelerator to get the BH core started. Earliest examples: Clarke, Imperial Earth 1975.

    • Jeff Wright September 25, 2021, 23:47

      I hope Planet 9 is a grapefruit sized one.
      The x-Ray jets at the poles used to transmute metals

  • Geoffrey Hillend September 19, 2021, 20:56

    White dwarfs don’t burn hydrogen like stars since there is no fuel or gas left. Their surface is only 10,000 K. Their heat is residual heat left over from the stars main sequence and red giant phase and they are the ash left of from the H, He burning or only carbon and oxygen.

    If there is a companion star binary close enough, the white dwarf will pull the hydrogen gas from it causing slow mass increase. Once it passes the Chandrasekhar limit of 1.44 solar masses nuclear fusion starts and it becomes a type !a supernova. The electron degeneracy is over powered by the runaway fusion hydrogen burning and the temperature and carbon and oxygen are fused into heavier elements, and the whole white dwarf blows apart. Ingles 2015, and Wikipedia.

    • Paul Gilster September 20, 2021, 9:55

      The point of this paper is that the conventional model you cite in your first paragraph needs to be reevaluated.

  • djlactin September 20, 2021, 6:53

    Whence the hydrogen to be burned in M13? My only idea is that it is ‘environmental’; i.e., that M13 has a thicker ‘atmosphere’ than M3. It’s a testable hypothesis.

  • Geoffrey Hillend September 20, 2021, 14:33

    It depends on how one defines hydrogen burning. Fusion requires over 200 billion atmospheres in pressure at ten million kelvin like in the core of a red dwarf star, but 10,000 K , the surface temperature of a white dwarf, possibly could ionize hydrogen. Hydrogen II or ionized hydrogen begins at over 10,000 K.

    • Michael Million September 21, 2021, 15:11

      At over 300 000 g hydrogen would rapidly fuse at sufficient thickness on the WD’s surface. A sufficiently clever alien species could drop hydrogen packets onto the surface to keep the star warm or perhaps encloses it to keep the heat in.

  • Geoffrey Hillend September 24, 2021, 15:29

    Hydrogen would not fuse on the surface of a WD until it passed 1.44 solar masses after which it immediately turns into a type one A supernova, and after that I don’t think anyone would want to hang around the vicinity of a WD or even be still inside it’s solar system unless you like being vaporized.

  • Passing Starship September 25, 2021, 15:28

    Greetings, Mr Hillend,

    white Dwarfs do not need to exceed 1.44 Solar Masses to fuse hydrogen on or near their surfaces.

    Please do look at the Wikipedia page on “Nova” (https://en.wikipedia.org/wiki/Nova). Here you will see that white dwarves accreting mass from nearby orbiting red dwarves eventually have that accumulated hydrogen fuse. The resulting explosion throws most of the accumulated hydrogen off, but some remains. This residue itself accumulates bringing the mass of the white dwarf up to 1.44 solar masses.

    Hydrogen fusing on or near the surface of a white dwarf that is under 1.44 solar masses causes the cataclysmic event that brought about the orginal term ‘nova’ (meaning ‘new star’), when a previously unobservable white dwarf became bright enough to see. That’s why the cataclysmic event when the whole 1.44 solar mass white dwarf explodes is called a *super*nova.

    Incidently, beware of confusing a dwarf nova with a nova. While a supernova can only occur once and may take millions of years to come about, the same red-white dwarf binary system can produce a nova every few thousand years, but a dwarf nova roughly monthly. That’s when the white dwarf’s accretion disc collapses onto the white dwarf.

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