Spinning white dwarf bombs may lurk the galaxy

By Phil Plait | September 20, 2011 6:30 am

Some new research just released asks a question near and dear to me: are there thousands of spinning white dwarfs in our galaxy, just waiting to explode as they gradually slow their rotation?

The answer is very probably yes. Let me be clear, as I always must be when covering topics like this: we’re not in any real danger from these things. Space is vast, and supernovae are few. If these things were that volatile we wouldn’t be here to talk about them in the first place.

But it’s still a very cool scientific question, and actually a fairly simple concept. Here’s how it works.

Imagine a binary system of two stars like the Sun, orbiting each other. One star nears the end of its life, swells up into a red giant, and blows off its outer layers. After a few millions years, all that’s left is its core: a dense, hot ball called a white dwarf. The size of the Earth but with the mass of a star, white dwarfs are pretty weird. They have incredibly strong gravity, which wants to crush them down even further, but they are supported by the electric repulsion of electrons, which is a pretty mighty force. It’s an uneasy truce.

It’s made even uneasier by the other star. It too eventually swells up, and can start to dump matter onto the dwarf (like in the picture above). If enough mass piles up, the immense gravity of the dwarf can induce nuclear fusion. Sometimes the material explodes, flaring in brightness, and we get a nova. Other times, if enough matter piles up — making the total mass of the white dwarf a bit more than 1.4 times that of the Sun — the ignition of fusion can cause a runaway reaction in the star, disrupting it entirely. The white dwarf tears itself apart, and you get one of the biggest and most violent explosions in the Universe: a supernova.

But there’s a hitch. As the material falls from the red giant onto the white dwarf, it tends to spiral in due to angular momentum — the same idea of how an ice skater spins faster when they bring their arms in. This infalling matter can then make the white dwarf spin faster. But if it spins really fast, then the centrifugal force acts against the force of gravity, supporting the material*.

So what you get is a white dwarf with more than enough mass to explode, but its spin prevents the supernova from occurring. For a while, that is. Various factors slow the star down over time (for example, a magnetic field will accelerate particles in the stellar wind, acting a bit like a parachute dragging on the white dwarf). At some point — and this may take a billion years — the white dwarf slows to the point where centrifugal force can no longer win the fight against gravity. Fusion of the material begins, and BANG! Supernova.

The research I mentioned at the top of this post was theoretical — it’s hard to get a white dwarf into the lab — but it does explain a pesky problem we’ve been having. The explodey white dwarf supernova is characterized by a lack of hydrogen in it (the other kind of supernova, when the core collapses in a massive star, is lousy with hydrogen since the star’s outer layers are loaded with it). But we should see some hydrogen, since the other star is dumping it onto the white dwarf.

But the delayed time bomb scenario may fix that; if it takes a billion years for the dwarf to slow its spin, then by that time the other star may also have expelled all its outer layers, evolving into a white dwarf itself. The stream of hydrogen onto the first white dwarf would’ve cut off long before, so we don’t detect it.

If this idea is correct, we might be able to find such stars. Because they’re in binaries, we can use the orbital period to get the masses of the two stars (using math Newton invented four centuries ago!). If one of them is 1.5 times the mass of the Sun — more than enough to explode — we have a winner. It may also be possible to measure how rapidly the star is rotating by looking for a Doppler shift in its spectrum; the shift happens as one side of the star spins toward us and the other spins away. The hypothesis predicts any supercritical white dwarfs must be spinning pretty dang fast, which would be detectable.

According to the paper authors (PDF), there may be thousands of these systems in our Milky Way alone. The nearest would still be hundreds of light years away on average, way too far to hurt us (they’d have to be very roughly a hundred light years or closer before the explosion would affect us), but close enough to spot in surveys.

It may not take long, either: several surveys exist or will soon which could spot these ticking bombs. That’s exciting! We know a lot about supernovae, but there’s still a lot to learn (which is why everyone is studying the new and relatively close one in M101 right now) about the exact process. And since we gauge the measure of the size of the entire Universe on these types of supernovae, the more we know, the more we can learn about the Universe itself.


* Yes, I meant centrifugal. It’s the same thing as centripetal, just as seen in the frame of reference of the object spinning, so it makes more logical sense to use it here. Read that link before you leave a nerdrage comment, please.


Related posts:

Supernova update: it’s peaking now
Dwarf merging makes for an explosive combo
The Universe is expanding at 73.8 +/- 2.4 km/sec/megaparsec. So there.

CATEGORIZED UNDER: Astronomy, DeathfromtheSkies!

Comments (37)

  1. gski

    Phil,

    Could you add something about the lack of centrifugal force at the white dwarfs poles and what if the axis of rotation is not perpendicular to the plane of the two stars?

    Thanks.

  2. (Sotto voce) This post showed up in my newsreader untruncated, as in the good old days. I really hope this practice continues.

  3. Pete Jackson

    Reminds me to check my wife’s purse. I don’t normally dare to investigate it too closely, but as she puts more and more things into it, the mass may be getting dangerously close to critical.

  4. ceramicfundamentalist

    could this same phenomenon work to keep a neutron star or pulsar whose mass is just a tiny bit beyond the normal black hole limit from becoming a black hole? could there be fast spinning neutron stars just waiting to collapse into black holes as their spin slows down?

  5. Chris

    So how much more massive can a spinning white dwarf get?

  6. Nigel Depledge

    @ Chris (5) –

    Sorry, I can’t resist.

    None. None more massive.

  7. Very interesting!

    So the centrifugal force decreases the pressure that needs to be supplied by electron degeneracy.

    It appears that Phil allowed one small error to slip into his excellently cool post:

    In the theory as I understand it, the pressure support in a white dwarf does not come from “electric repulsion of the electrons”. Rather, the pressure comes from the Pauli exclusion principle for the electrons. The bulk matter in the white dwarf is electrically neutral, and the nuclei are presumably distributed evenly among the electrons, but, as the density goes up, the electrons run out of low-momentum quantum states before the nuclei do. So some electrons are forced into states that have more momentum than can be accounted for by the temperature. That’s the electron degeneracy pressure, which overtakes the thermal pressure at sufficiently high density, and this degeneracy pressure is what stops the collapse of a white dwarf.

    That is, until the mass limit is reached.

    It’s cool to think that the mass limit might be extended upward by spinning, but I imagine that the calculation of the behavior is a bear because it involves general relativity. This spinning part makes it sound as though something like the Kerr metric is involved, not the simple Schwarzchild (or Schwarzchild-like?) metric presumably used by Chandrasekhar.

  8. Desmond Qaffi

    GM/r^2 = (omega)^2R, P = 2(pi)/(omega), minimum period for binding
    P(min) ~ sqrt [3(pi)/G(rho)], rho = MKS density

    Earth balances at P = 1.4 hrs, a white dwarf around P = 9 seconds, a neutron star around P = 0.5 msec. An Earth-sized white dwarf wth a 9-second period would have an equatorial velocity around 2800 miles/second – not close to being relativistic. It would still squeeze out some nice geodetic and Lens-Thirring effects.

    Now, pile on more mass. We see that the density determines the minimum spin period, proportional to sqrt[K/(rho)]. Lowering the average density (barely bound surface layers) INCREASES the minimim spin period for a gravitationally bound white dwarf. One then suspects progressive equilibrium puts limits on eventual catastrophe magnitude by bounding mass accretion.

  9. James

    Great story. White dwarves are my favorite type of star.

  10. Poul-Henning Kamp

    Wouldn’t ESA’s GAIA mission be the perfect tool for finding these in practice ?

  11. Hey, Phil, I understood almost everything you said this time! Words like “explodey” and “nerdrage” are very helpful; “centrifugal” and “centripetal” not so much. Fascinating topic!

  12. Mike

    Something I’ve been wondering about ever since I heard of this. Wasn’t the rate of expansion of the universe measured by looking at type 1a supernovae, which was possible because they all had exactly the same absolute brightness due to all of them having the same mass? If so, now that their mass can vary, wouldn’t this throw those measurements into question or were they verified by some other method?

  13. TStein

    Naively, I would think that allowing the star to pack on more weight then the limit would increase the variance in the luminosity. Some stars could spin up faster packing up more material and have a longer delay to they explode. Others would have less material to pack on spin up slower and explode faster. Therefore the first stars to explode this way will be possibly be dimmer since they have less material to explode. (The more massive ones take longer to spin down)

    Obviously, this is way too simplistic of a model. I am curious, though, if there is enough information to try to model this better and if there might be someone out there doing it.

  14. DukeTG

    I believe any mention of centrifugal vs centripetal is required by law to include a link to this XKCD comic:

    http://xkcd.com/123/

  15. Chris

    @7 Thomas
    You are right. It’s the electron degeneracy pressure.

  16. amphiox

    re @TStein #13;

    I’m also curious how this potential variance in luminosity affects the use of these supernovae as standard candles.

  17. X

    I find it pretty hard to believe that the negligible amounts of hydrogen being accreted onto a WD SN progenitor would be observable. They would be instantly wiped out during the detonation phase. The paper doesn’t mention any such mechanism, although it does suggest looking for the SN shock interacting with the planetary nebula of the binary partner.

  18. Erika

    This is just too cool.

  19. CraterJoe

    So, what happens to the other star when the first one goes boom?

  20. Wzrd1

    @ Ceramicfundamentalist #4, nope. Those would become Kerr black holes.
    http://en.wikipedia.org/wiki/Kerr_black_hole
    Once an event horizon forms, it’s a black hole.

  21. Wzrd1

    @ TStein #13, you forget the root cause of the type 1a supernova, it is a core explosion, not the outer layers fusing. The core is compressed sufficiently for the temperatures and pressures to reach the carbon fusing point, causing a runaway fusion reaction of carbon and other elements present in the core in a matter of seconds. That releases such a great impulse of thermal energy that it unbinds the star totally.
    Hence, the same amount of energy is released, regardless of the outer layer being more massive by a modest amount, though more gas would be accelerated out, the larger amount would not be that easily detected. You’re talking the difference between 1.38 solar mass and maybe, to be generous, 1.5 solar masses. To be honest, I doubt one would get as high as 1.5 solar masses though.

  22. Wzrd1

    @Larian Le Quella #22:
    Referenced off that site is a nice one showing evidence for the survival of the companion star.
    http://blogs.discovermagazine.com/80beats/2011/04/28/tychos-supernova-went-boom-after-slurping-up-some-of-its-neighbor/

  23. Dragonchild

    What I’m wondering is what happens to the supernova shock wave if the stars are close together. I’d think in most of these systems the younger star is sufficiently far away to have a negligible effect on the supernova, but if they’re really close together, would the gravity of the younger star make a “dent” in the shock wave?

  24. Wzrd1

    See the above link. They talk about a “hole” knocked in the wave and nebula from the companion star.

  25. Paul

    I think there’s a “your mom” joke in here somewhere.

  26. Phil, four centuries ago, Newton had to wait over thirty years to get born. Even 350 years ago was only 1661, well before everything fell together for him. So you have to say, “ using math Newton invented three plus centuries ago”.

  27. Grand Lunar

    This may add a twist to “Death from the Skies”!
    Perhaps a sequel may be in store?

    “The research I mentioned at the top of this post was theoretical — it’s hard to get a white dwarf into the lab”

    Not so hard for the Time Lords! :D

  28. Gary Ansorge

    Seems an elegant solution to the problem of no observed hydrogen.

    I’ve been wondering where all the ejected stellar planets are and if there are enough of them to account for part of our dark matter problem. They’d be really hard to spot, even if within a light year of us(very cold). It seems to be a common process as solar systems form, as in this article,,,

    http://content.usatoday.com/communities/sciencefair/post/2011/09/study-solar-system-ejected-giant-planet-early-on/1?csp=Tech

    Gary 7

  29. Ain’t the universe cool? We just keep discovering new stuff like this. The more we know, the faster we learn new stuff.

  30. ophu

    Explodey? Is that highly technacious termage?

  31. TMB

    @12, 13, 16: The uniformity of SNIa is mainly empirical, not theoretical – for every one where we already know the distance, they lie in a narrow range. Moreover, what variation there is correlates tightly with the overall timescale on which the SN fades, so it can be corrected for to make an even narrower range. Which brings up the point that we already know that there is some variable that can change from SN to SN – whatever it is modulates both the luminosity and the fading time. The general hypothesis has been that it is stochastic variations in the amount of 56Ni that ends up getting fused during the explosion, but maybe it is in fact the mass of the progenitor.

  32. Mike Torr

    Hooray! Entire posts in my RSS feed! Now I can actually start reading this blog again.
    So… did I miss anything during the dark ages of truncation? :-

  33. CraterJoe

    @Mike Torr. Truncation? Whatever do you mean…I can’t see how…you…have experienced…of the sort?

  34. Ken

    Phil, I have to wonder, where did that 1.5 solar masses number come from? Did it come simply from observing white dwarves and noting that there were none with over 1.5 solar masses? If so, is it now possible the number is overly high, and that smaller stars could also go supernova? Like, say, Sirius B with 0.98 solar masses, well within 100ly of Earth?

  35. Having just given a talk on supernovae types , historical examples and Milky Way candidates earlier tonight .. I wish this had been posted earlier!

    Still great write up and interesting theory. :-)

    @33. Mike Torr asks : “Now I can actually start reading this blog again.
    So… did I miss anything during the dark ages of truncation?”

    You do know you can just keep paging down through the ” Older Entries” button don’t you? ;-)

    @35. Ken : “Phil, I have to wonder, where did that 1.5 solar masses number come from?”

    I think you’ll find it came from the calculations of a bloke named Subrahmanyan Chandrasekhar, who worked out the limits of what mass a white dwarf can carry and had the Chandrasekhar limit named in his honour for it.

    (Click on my name for more info. via the usual fountain of all wisdom.)

    @30. VinceRN :

    “Ain’t the universe cool? We just keep discovering new stuff like this. The more we know, the faster we learn new stuff.”

    Yup indeed. But also the faster we learn that what we thought we knew is more complicaticated than that and learn that we not have had a full understanding of the things we *thought* we knew too. ;-)

  36. Joseph G

    @29 Gary Ansorge: I’ve been wondering where all the ejected stellar planets are and if there are enough of them to account for part of our dark matter problem. They’d be really hard to spot, even if within a light year of us(very cold). It seems to be a common process as solar systems form, as in this article,,,

    Now there’s an interesting question – how far above absolute zero (well, the apparent microwave background temp, which is pretty darn close – what is it, like 4 or 5 kelvin?) would the average temperature of the surface of a planet like Earth (with a molten mantle and volcanism) be in interstellar space? And how far away could such a planet be detected?

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