At the core of a star

ASTROPHYSICS

Craig Heinke and others at the University of Alberta and Wynn Ho and team at the University of Southampton have taken a peek into the interior of Cassiopeia A, the remains of a supernova seen 300 years ago. The peek confirms a bizarre conjecture, that the centre of such massive and super-dense remnants of supernovae harbour a superfluid core.

This will be reported in the Monthly Notices of the Royal Astronomical Society. The same results will also appear in Physical Review Letters, reported by Dany Page at the National Autonomous University of Mexico and colleagues in the US. Identical results were also arrived at by a group headed by Dima Yakovlev from the Ioffe Physical Technical Institute in St Petersburg.

Neutron stars

The end of a star’s lifeline is when it runs out of fuel and collapses under its own weight. This happens many times in the star’s lifetime, when it superheats and expands, to cool by expansion and then collapse. The collapse heats the star and reignites the nuclei of atoms in its core to fuse, or merge to form atoms of other elements, emitting fabulous energy in the process. The heating causes expansion, resulting in cooling and collapse.

But when the atoms have all fused and formed metals that can no longer form stable combinations, usually after the final explosion as a supernova, there can be no expansion phase and the collapse continues.

If the star was massive enough to start with, the collapse can strip atoms of their electron exterior, to create a gas of charged particles and with even greater pressures, the positively charged protons in the nuclei of atoms combine with electrons to become neutrons. Neutrons, which are uncharged and do not mutually repel, can be crushed further together, till the matter of the star is tightly packed and cannot get any smaller only because of other effects, which prevent matter from getting completely closer together.

This core of neutrons, which is then at a tremendous temperature, also expresses any rotation momentum of the original star, in the form of extremely rapid rotation. This is much like circus artistes, once set spinning with their arms outstretched, and begin to spin suddenly faster just by drawing their arms inwards. The reason is that momentum of rotation depends on how far apart the parts of the rotating body are. If the same momentum has to show in parts that are closer together, then the body needs to spin faster. A star consists mainly of gases spread across hundreds of light years of space.

Even if the star were to have the slightest rotation, when all its mass is compressed within a few kilometers, in a neutron star, we can imagine that the speed of rotation would become rapid! With charged particles in its insides and tremendous heat, there is a raging X ray maelstrom in the neutron star’s vicinity, which flashes like a lighthouse.

Superfluids

There is friction or resistance to relative motion in the case of all matter in contact. There is some viscosity or resistance, in the case of fluids, which is a burden when driving liquids through pipelines or even our blood through our veins and arteries. At extremely low temperatures, substances like liquid helium, which are normally regular liquids, seem to lose all viscosity or resistance to flow.

The reason for this, superfluidity, is now understood as occurring because of the helium nucleus consisting of four nuclear particles, two protons and two neutrons. In any material, the particles are distributed at various energies, according to the temperature of the material. But as the material is cooled, the bulk of the particles come down to the lowest energy levels, with very few at higher levels. Now, there is a fundamental law of physics that in the case of nuclear particles, which have a quality called spin, of ½, no two particles in a system can occupy the same energy level. But if the spin is 1 or 2, in place of 3/2, 5/2 etc, which are odd multiples of ½, there is no restriction.

When such a material is cooled, then, although the particles take the lowest energy levels, only one can be at each level and the particles, in principle, are distinct. This is unlike a material where the rule does not apply and all the particles can bunk down at the same energy level and become identical. When this happens, the mass of identical particles, by the rules of mechanics for very small particles, begin not to interact but to behave in cohesion, as if they were all one extended particle.

Now neutrons and protons have spin of ½ each and cannot form superfluids. This is also true of most materials, which cannot become superfluids for reasons that prevent them. But the helium nucleus, which has two protons and two neutrons, has spin of 4x ½ = 2, and such particles in a group are allowed to occupy the same energy level, and it is in helium that superfluidity was first observed.

Neutron stars and superfluids

The idea that has been around since 1959 is that at the pressures in the core of neutron stars, helium nucleus-like particles of whole number spin, and hence superfluidity, can arise. Last year, Craig Hienke and Wynn Ho noticed something surprising about Cassiopeia A, a neutron star in the Cassiopeia constellation and 11,000 light years from the earth.

At over two mn degrees centigrade, data from the Chandra X Ray telescope showed that the neutron star was cooling a whole four per cent in just 10 years, faster than it should have been. The theory of how neutron stars cool is that the neutrons decay into protons and electrons, giving off nearly massless particles and very weakly interacting particles called neutrinos, which escape and carry energy away. Neutron decay is slow and cannot account for so much heat loss.

If the neutrons formed a superfluid, then there were conditions that allowed massive neutrino production and sizeable energy drain.

Rapid cooling is then a pointer to the presence of superfluids in neutron stars. If Cassiopeia A continues to cool at this rate for the next 10 years, this will confirm the presence of superfluids in its core.