Recently, two teams of scientists announced they had direct evidence that the core of neutron stars contain a bizarre type of matter.
Neutron stars are of course the corpses of massive stars as big as a city yet weighing as much as two of our suns.
When the core of certain exploding supernovas collapse, the protons and electrons overcome degeneracy pressure and squeeze together to form neutrons. What you end up with then is a ball of almost pure neutrons that weighs millions of tons per proverbial table-spoon.
We’ve learned a lot about these stellar remnants over the decades but the cores of these objects have always been a mystery.
Astrophysicist Dany Page of the National Autonomous University and lead author of the paper that prompted this news item said recently:
“The interior of neutron stars is one of the best kept secrets of the universe……It looks like we broke one of them.”
What two separate teams of researchers discovered recently was solid evidence that the core of a neutron star called Cassiopeia A, 11,000 light-years from Earth and the youngest known neutron star in the Milky Way, probably contains a neutron superfluid.
The research described in a recent issue of the Physical Review Letters said the following:
“This is the first direct evidence that superfluidity and superconductivity occur at supranuclear densities within neutron stars”
So, what the hell is a superfluid?
Superfluids are so cool (so to speak). They’re an example of one of those fascinating and rare instances in which the bizarre counterintuitive behavior we see in the atomic/quantum realm, pokes its head out into the macroscopic world so we can see with it our naked eyes.
Superfluids are a phase of matter that behaves like a fluid but without viscosity and with infinite thermal conductivity.
It therefore flows with no friction which means that when you look at it’s behaviour for the first time you’re genetically programmed to say “What the hell is that”!
For example: If you cool liquid helium to the superfluid state and put it in a container, it will literally climb the walls and get out. If you swish it around in a closed container and come back in a million years it will still be swishing around. If you instead spin the container it’s in, the fluid will rest motionless within it as the container spins around it.
Superfluids are so awesome that 3 Nobel prizes have been won by scientists messsing around with it.
So how does this super-cold helium in a lab relate to super dense neutrons in a collapsed star?
Wow, great question Bob….
It has to do with Cooper pairs…
For superfluids and superconductivity to happen, you need to have cooper pairs form. So for example…when superconduction happens (electricity flowing without any resistance), electrons need to pair-up into special low-energy bound states with other electrons. The same is true for a lab-created superfluid made of super-cooled helium atoms. The helium atoms pair up producing its bizarre behaviour. It turns out then that neutrons can form into these cooper pairs as well creating a superfluid in the crazy-dense core of a neutron star.
So how can scientists notice these cooper pairs inside a stellar remnant from light years away??
It has to do with how rapidly the temperature of the neutron star changes. Conventional theory states that coolling should be a slow process in a neutron star as neutrons decay back into electrons and protons plus our little nearly-massless neutrino friends that then fly away.
When they looked at the data for Cassiopeia A though, they discovered that it was cooling at an incredible rate…from 2.12 million K to 2.04 million K, or 4%, in 10 years (85 thousand degrees) That may not sound like much but it is far beyond what conventional cooling can explain.
What they think is happening is this. Neutrons are joining with other neutrons (or maybe an occasional proton) and forming cooper pairs which form the basis of a superfluid. With each cooper pair creation though, neutrinos are produced which fly away at near the speed of light carrying away some energy which cools the neutron star.
It all makes sense now, right?
Ultimately, these scientists got very lucky. They initially wanted to study lots of these stars to create a profile of their age and temperatures to support the superfluid idea. This should have been a long drawn-out process but they were lucky enough to find one during the relatively brief window in its life when the temperature changes very fast. In fact, they expect the rapid cooling to end in a decade or so.
Considering Neutrons stars can live for 10 billion years, that one tiny window.