Scitech | Life after a star’s death

The extraordinary world of neutron stars

If you have trillions of tonnes of fuel burning at millions of degrees for billions of years and you suddenly run out, what happens?

Stars in the twilight of their lives are doomed to different fates depending on exactly how much stuff is in them. In the case of sun-like stars, they will die by puffing up into red giants (with a radius bigger than earth’s orbit around the sun) and eventually ejecting much of their stardust to form so-called “planetary nebulae” that can span dozens of light-years across.

If the star in question is bigger and hotter than the sun, it performs the much more dramatic exit from the cosmic stage known as a supernova. When the last of the available fuel runs out, the fusion reaction providing the counter-force to the ridiculously immense gravitational forces at the core of these stars is cut off. Without any counterbalance to gravity, the entire star falls in on itself, compressing the core and bouncing back out – tearing the entire star apart in a colossal shock wave explosion that can be seen clear across the universe.

The energy provided by supernovae is sufficient for the creation of heavy elements such as the ones that make up rocky planets and the little ape-things that live on them. In the words of Carl Sagan: “We are all stardust.” Not all of the stardust is ejected, though, the compressed cores of the dead stars – which can become either black holes or neutron stars – are among the strangest and most inexplicable objects in the entire universe.

Black holes are what happen when there is simply too much mass in that compressed core – at least ten times the mass of the sun. The remnant of the core collapses into a single point of infinite density, warping space-time so severely that not even light can escape from the area. If the core is below that critical mass, the object that forms may be a neutron star. A neutron star contains about 1.5 times the mass of the sun (about 500,000 Earth masses) compressed into a ball with a radius of just 12 kilometres. That is about the same density as the entire human population squished into the volume of a sugar cube. The gravitational pull is so intense that the electromagnetic repulsion between protons and electrons is not sufficient to keep them apart, which forces them together and creates neutrons.

Neutron stars conserve the rotational energy of their progenitor stars in the same way that the speed of a spinning figure skater increases as they bring their arms to their body. With the extreme difference in radii, neutron stars have been spotted rotating more than 1,000 times a second! In these conditions of extreme gravity, heat, and spin, astronomers have been stymied in their attempts to adequately model their interior workings.

In 2009, theoretical astrophysicist Dany Page at the National Autonomous University of Mexico championed a new, if bizarre model to explain the inner workings of neutron stars. He predicted that the conditions within the interior would cause the neutrons present to collapse together into the lowest possible quantum energy state, called a Cooper pair. This process would result in the formation of neutrinos (particles that are virtually without mass and can freely pass through just about anything) that would carry energy away from the star, thereby lowering its temperature. Matter bound up in Cooper pairs behaves essentially as a macroscopic quantum particle, conventionally known as a “superfluid.” Flowing superfluids perform as superconductors, meaning electricity flows through them without energy loss, and superfluids flow without friction. If superfluid was put into a glass back here on Earth, it would climb up over the walls of the glass and escape.

Page’s model was awaiting evidence to support its strange physical predictions. Luckily for him, astrophysicist Craig Heinke at the University of Alberta has found compelling new evidence in data collected from the Chandra X-Ray telescope. They observed a supernovae remnant named Cassiopeia A (Cas A), located a mere 11,000 light years away, over a period of years. The first light from the supernova reached Earth 330 years ago, which is exceedingly young in terms of stellar evolution. According to Heinke, we’ve got “ringside seats to studying the life cycle of a neutron star from its collapse to its present, cooling off state.”

Heinke’s team has found that Cas A has cooled off by 800,000 degrees in just ten years. This cooling rate was impossible to account for without including the mechanism first proposed by Page, which was the conclusion that both Heinke and independent researchers at the University of Southampton in the U.K. came to. The two teams jointly announced their findings in papers that will appear in Monthly Notices of the Royal Astronomical Society.


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