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Neutron Stars shown to be probably pretty normal, for neutron stars

When I was a kid, I thought that I was going to become a theoretical particle physicist. Then I learned enough to know that that thought is hilarious.

But, you know, some people actually do have to grow up to be theoretical particle physicists. I've never met any of them, but I know they exist, because I see their theories that they make about the physics of particles.

One set of theories is about the behavior of matter that has been condensed so far that I doubt even the people who made the theories can truly have a mental model for how dense this denseness is. Does extremely dense matter decompose itself into free quarks? Does it become more like a kaon condensate, or a happy set of hyperons? Or is the result less exotic than that--do the atoms' protons and electrons, when pushed up against each other, simply fuse to become plain old neutrons and nothing else? Or is it a combination of these scenarios? How will we ever know, and how will we live without knowing?

We here on planet Earth do not have the ability to prove any of the theories right or wrong, because how are we supposed to crush matter to densities extreme enough to condense atomic nuclei? We can't. So we have to look elsewhere.

Luckily, astronomers have found us these dudes called 'neutron stars,' which get their name from the idea that proton+electron+pressssssssssssssssssssssssssssssssssssure=(just) neutron. But that idea has been challenged, a lot, by theorists who say that more exotic states of matter will appear.

Well, since we can't schlep to a bunch of neutron stars, and even if we could, taking close-up measurements would kill us dead, how are we going to figure out what they're made of? How are we going to cross some of these competing theories off the List of Stuff with Scientific Validity (a real list)?

The trick lies in 'constraining' the equations of state predicted by the theories. Equations of state (EOS) describe relationships between the important parameters within a system, basically telling you how all the significant aspects of an object or substance behave together. They, like all good equations, can be plotted.

If you take measurements of the parameters described in the EOS, you can put your measurements on the plot and see how the reality and the theory mesh or don't mesh.

So guess what? I'm not the only one who knows that. Some astronomers have heard of this technique too. In particular, here, I'm talking about Demorest, Pennucci, Ransom, Roberts & Hessels and their ApJ Letter "A two-solar-mass neutron star measured using Shapiro delay," a title that kind of steals the thunder of the disclosures that follow.

Neutron star background in one minute or less: Massive star goes supernova; remnant of star, material that hasn't been blown away, is compressed and retains much of the magnetic field and angular momentum of the original star, although this remnant is only the size of Washington, D.C. This D.C., though, weighs more than the Sun.

a) it's very dense ('very' being the understatement of 2011 so far)
b) if you think about how 'conservation of angular momentum' means that when an ice skater is spinning and then pulls her arms in and then she (or he, or he) spins faster, and then you think about how that would apply to a star that used to be massive star-sized and is now city-sized...that's one fast rotater (sometimes more than 600 rotations a second)
c) the intense magnetic field causes intense radiation to be beamed from the poles, which is how we typically find neutron stars. When we can see a beam of radiation from a neutron star (which we see once per rotation, when it's pointed at us), the neutron star is also a 'pulsar.'

Sometimes pulsars are in binary systems. Sometimes they are in binary systems with other massive objects. Those other massive objects affect the spacetime around them, curving it. The pulses from the pulsar must travel along this curve, which takes longer than traveling across a straight line. Because of this time difference, the pulses are delayed, an effect that is shown in this graphic (credit: Bill Saxton, NRAO).

Though the delay, called the Shapiro delay, is not large, it is large enough: pulsars' signals are so regular and we are able to measure them so accurately that we can latch on to very small deviations.

Once the delay is measured, astronomers can calculate how much mass is required to create the spacetime dip that would cause that delay. Then, based on the orbital parameters of the system, they can use regular-old Keplerian dynamics to determine, given one known mass, what the other mass must be. In this case, the companion was a white dwarf half the mass of the sun, while the neutron star/pulsar was 1.97 times the mass of the sun. Turns out this is by far the most massive neutron star known and BAM (roundhouse kick) defeats some proposed equations of state.

On this graph (from the paper) of radius versus mass, the EOSs are the ones that look like snakes; each snake represents the predictions of a different theory of what is inside a superdense object.

We only know the mass--not the radius--of the heaviest neutron star, so we don't actually know its density. However, the simple statement "A neutron star can be up to 1.97 solar masses" results in a horizontal line (red), which results in a lot of the snakes being trapped underneath. These trapped snakes represent theories that can never produce 1.97-solar-mass neutron stars.

Turns out that these low-lying snakes are the ones that predicted more crazy interior states, such as the aforementioned kaon condensates, which I just like saying.

Isn't it strange that finding something so strange (even for a neutron star) resulted in the ruling out of the strangest possibilities for its interior state?

And isn't it reassuring that observations still have the power to confirm or deny theories, even if we can never create the necessary observational conditions in a lab? It is a relief to me. I sleep better at night...whereas (to make this a neatly circular blog post) if I had become a theoretical particle physicist, I would probably never sleep, especially not if my EOS was a buried snake.

Demorest, et al. Nature, 467: 1081–1083, 28 October 2010)

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Reader Comments (3)

How do you pronounce kaon? Also what is a kaon condensate?

February 16, 2011 | Unregistered CommenterBrooke N.

I think kaon condensate is an ingredient in my Kroger Fat-Free Half-and-Half.

February 16, 2011 | Unregistered CommenterTripp

I was pronouncing it like crayon without the "r" & I believe it is a colorful condensation on my 50ml falcon tubes.

February 17, 2011 | Unregistered CommenterBrooke N.

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