June 28, 2013
A new study verified the so-called "nuclear pasta" type of matter arrangements by researching spin rates of neutron star type called pulsars. Inside these very dense objects, the nuclei of atoms are pressed so tightly together that they form shapes resembling lasagna (flat sheets) or fusilli (spirals), hence the name.
Picture of Vela pulsar, some 1000 light-years from Earth.
It has been proposed years ago that matter could form such new shapes, but it was not verified until the team from Alicante University in Spain observed pulsars and found evidence that this so-called nuclear pasta exists in innermost crust, near the core of a neutron star.
Pulsars are dense neutron stars that rotate extremely fast while emitting light in a pair of beams, which resemble a lighthouse. This is why the star, or rather what is left of it, seems to pulse on and off. When massive stars run out of fuel, they explode in supernovae, while their cores collapse into small, very dense objects called neutron stars. This is because these objects are so dense, second only to black holes, that electrons and protons in nuclei of their atoms merge together, producing neutrons and neutrinos. About 90% of neutron star is made of neutrons.
Because pulsars "switch" on and off from our point of view, astronomers can calculate how fast they spin. They have never found a pulsar with spin period longer than 12 seconds. "In principle, that is not expected. You should see some with larger periods," team member José Pons said. It is this pasta-like arrangement of matter that offers an explanation as to why there are no pulsars that spin more slowly.
And this is why – normally, pulsars slow down their rotation by emitting electromagnetic waves (light). When they emit electromagnetic waves, they lose angular momentum. However, if the atomic nuclei inside pulsars form pasta resembling shapes, they would make it harder for electrons to travel through the material, because it is now more electricaly resistant. This causes stars' magnetic fields to dissipate faster, which means they cannot emit electromagnetic waves as strongly, so they don't lose the momentum. This is why pulsars don't have rotation periods longer than 12 seconds.
"Making this connection between the observational astronomical effect, which is the existence of this upper spin period limit, with the need for this layer in the inner crust, is what makes the connection between observations and theory," Pons said. This pasta formation appears in crusts of these stars, which are so dense, a teaspoon of it would weigh 100 million tons. "It's like a huge, gigantic nuclei, a huge continuum."