Close Look at Cas A Reveals Bizarre ‘Superfluid’


NASA’s Chandra X-ray Observatory has discovered the first direct evidence for a superfluid, a bizarre, friction-free state of matter, at the core of a neutron star.

The image above, released today, shows X-rays from Chandra (red, green, and blue) and optical data from Hubble (gold) of Cassiopeia A, the remains of a massive star that exploded in a supernova. The evidence for superfluid has been found in the dense core of the star left behind, a so-called neutron star. The artist’s illustration in the inset shows a cut-out of the interior of the neutron star, where densities increase from the orange crust to the red core and finally to the inner red ball, the region where the superfluid exists.

Superfluids created in laboratories on Earth exhibit remarkable properties, such as the ability to climb upward and escape airtight containers. When they’re made of charged particles, superfluids are also superconductors, and they allow electric current to flow with no resistance. Such materials on Earth have widespread technological applications like producing the superconducting magnets used for magnetic resonance imaging [MRI].

Two independent research teams have used Chandra data to show that the interior of a neutron star contains superfluid and superconducting matter, a conclusion with important implications for understanding nuclear interactions in matter at the highest known densities. The teams publish their research separately in the journals Monthly Notices of the Royal Astronomical Society Letters and Physical Review Letters.

Cas A (RA 23h 23m 26.7s | Dec +58° 49′ 03.00) lies about 11,000 light-years away. Its star exploded about 330 years ago in Earth’s time-frame. A sequence of Chandra observations of the neutron star shows that the now compact object has cooled by about 4 percent over a ten-year period.

“This drop in temperature, although it sounds small, was really dramatic and surprising to see,” said Dany Page of the National Autonomous University in Mexico, leader of one of the two teams. “This means that something unusual is happening within this neutron star.”

Neutron stars contain the densest known matter that is directly observable; one teaspoon of neutron star material weighs six billion tons. The pressure in the star’s core is so high that most of the charged particles, electrons and protons, merge — resulting in a star composed mostly of neutrons.

The new results strongly suggest that the remaining protons in the star’s core are in a superfluid state and, because they carry a charge, also form a superconductor.

Both teams show that the rapid cooling in Cas A is explained by the formation of a neutron superfluid in the core of the neutron star within about the last 100 years as seen from Earth. The rapid cooling is expected to continue for a few decades, and then it should slow down.

“It turns out that Cas A may be a gift from the Universe because we would have to catch a very young neutron star at just the right point in time,” said Page’s co-author Madappa Prakash, from Ohio University. “Sometimes a little good fortune can go a long way in science.”

The onset of superfluidity in materials on Earth occurs at extremely low temperatures near absolute zero, but in neutron stars, it can occur at temperatures near a billion degrees Celsius. Until now there was a very large uncertainty in estimates of this critical temperature. This new research constrains the critical temperature to between one half a billion to just under a billion degrees.

Cas A will allow researchers to test models of how the strong nuclear force, which binds subatomic particles, behaves in ultradense matter. These results are also important for understanding a range of behavior in neutron stars, including “glitches,” neutron star precession and pulsation, magnetar outbursts and the evolution of neutron star magnetic fields.

Sources: Press releases from the Royal Astronomical Society and Harvard. See additional multimedia at NASA’s Chandra page, and the two studies in MNRAS and Phys. Rev. Letters.



10 Replies to “Close Look at Cas A Reveals Bizarre ‘Superfluid’”

  1. Interesting. Would Neutron stars represent the hottest possible state of matter at a billion & 1/2 degrees? What is the hottest an object can get?

    1. I once asked a chemistry professor ‘ how hot can matter get’? His answer, ‘There is no maximum’. So then I asked ‘what if the matter got so hot that the atoms reached the speed of light’? That was over forty years ago, and he had no answer. I wonder what the answer would be now?

      1. He should have had an answer for you!

        First of all, atoms themselves will break down into a plasma as they go up in temp. But aside from this point, even particles with a near infinite amount of kinetic energy (temperature) will never reach the speed of light.

        The relativistic equation for the kinetic energy of an object is

        Ek = mc^2 / (sqrt(1-(v^2/c^2))) – mc^2

        So as v approaches the speed of light c, the kinetic energy of an object diverges, effectively meaning that an object can have any kinetic energy up to infinity, and yet still not travel faster than light. Putting it another way – if you rearrange the above eqn to give v in terms of the other quantities, then you can plug in an arbitrarily high kinetic energy, and still not have v exceed the speed of light.

        Since kinetic energy is basically what we are referring to when we talk about the temperature of subatomic particles, temp can rise without limit given the assumption that particles don’t completely dissolve due to some as yet not understood physics…

      2. That wasn’t worded well. I meant something along the lines of:

        So as v approaches the speed of light c, the kinetic energy of an object diverges, meaning that even an object traveling slower than light speed can have an arbitrarily high energy. Flipping it on its head – if you rearrange the above eqn to give v in terms of the other quantities, then you can plug in an arbitrarily high kinetic energy, and yet v will still not exceed the speed of light.

    2. “Interesting. Would Neutron stars represent the hottest possible state of matter at a billion & 1/2 degrees? What is the hottest an object can get?”

      1) No, and 2) it depends what you mean by ‘an object’.

      In terms of matter itself, presumably things can get hotter and hotter without limit, breaking down into more and more fundamental particles as you go up in temperature (this assumes that there are truly fundamental, ‘indestructible’ particles). The hottest matter that we’ve ever measured/created would be a quark-gluon plasma. For example, there is one group at the Relativistic Heavy Ion Collider claiming an effective temperature of 4 Trillion deg. C. A billion and a half degrees isn’t even 1/thousandth of the way from absolute zero to this temperature!


      1. I think more interesting would be some kind of self-sustained object with defined boudaries…that exists on it’s own… like a neutron star 🙂 but is there something hotter?

  2. The maximum temperature is the Planck temperature. The equipartition theorem for a thermal system is where the energy equals the Boltzmann constant times the temperature E = kT, and the E = m_pc^2. Here m_p = sqrt{hbar c/G} = 2.17x 10^{-8} kg. The temperature is then a whopping 1.4×10^{32}K. This makes the temperature of a neutron star seem down right frigid by comparison. This is the temperature one gets at the singularity of a black hole. Also a quantum black hole that is around 10^2 to 10^4 Planck masses would have a temperature near this.

    This inference about the state of matter in a neutron star is interesting, even if it is somewhat indirect. Experiments at RHIC and some preliminary heavy ion Pb-Pb collisions also give superfluid type of results. In these experiments high atomic number ions scatter and form a quark-gluon plasma for a period of about 10^{-16} seconds. The properties are deduced by the scattered products. These fluid-like results are expected if gluon chains have correspondences with the graviton particle — the quanta of the gravitational interaction.


  3. the interior of a neutron star contains superfluid and superconducting matter

    Ah, so that is why neutron stars magnetic fields can freeze, it is superconducting rather than (only) neutron spin aligning.

    As for the temperature, one must remember that a) it is the average velocity of a distribution when considering a relativistic high T gas b) Planck temperature is merely the energy where current physics of known quantum physics and general relativity breaks down, not a maximum (or special relativity would break).

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