Finally we are also learning the secrets of neutron stars

Colliding neutron stars, as shown here in an artist’s image, could send a quick radio burst through the universe. Image: ESO / L. Calçada / M. Kornmesser

Are the neutron stars, the incredibly dense, burned-out cores of dead stars, about to reveal their secrets? Yes, let’s say astronomers carefully scrutinize the findings of new research that suggests there’s more to the death of a star than the telescope.

The life of a typical star is as fascinating as its death. It shines by burning its nuclear fuel, converting hydrogen to helium to hold itself up against the force of gravity for billions of years. But when the fuel runs out, gravity wins the long battle and causes the star debris to collapse. New nuclear reactions begin to convert helium to carbon, releasing more gravitational energy.

The tremendous heat that is produced bursts forth into the outer layers of the star, a destiny that also awaits the Sun some five billion years from now, when it expands to become a red giant, twitching all the planets to Mars. . When all the helium in a star is converted to carbon, the nucleus becomes more compact and even hotter, as nuclear fusion converts carbon to oxygen. Ultimately, most of the nucleus material becomes an iron-rich nucleus, at which point the addition of more protons and neutrons from the reaction does not release more energy.

As the heat source disappears, the larger stars simply collapse, the mass of their outer layers falls inward under the force of gravity, and becomes very hot as gravitational energy is released. Given enough mass, under these conditions, there is a sudden surge in activity when the protons and electrons of hydrogen and helium in the star’s atmosphere merge into neutrons and compress the nucleus explosively. The explosion takes place in a shell around the core, like an orange peel, and the explosion travels outward, blasting the rest of the star’s atmosphere in a flash as bright as a galaxy to form an expanding nebula made of gas. ionized and dust.

It also travels inward, squeezing the nucleus and producing a number of elements heavier than iron, some of which can be thrown into the nebula. This “supernova” explosion leaves a rapidly spinning neutron star known as the pulsar: the smallest and denser known entity in the universe. The incomparable power of a supernova can be seen in the Crab Nebula, which is 6,500 light years away from us. The gases from that stellar explosion eons ago are still moving outward at 1,300 km per second!

So it’s a matter of mass, so to speak, since stars larger than the Sun explode as supernovae. While scientists have been able to uncover this much of a star’s story, no one really knows what will become of a neutron star or pulsar after this. What happens to your super dense iron-filled core? Is there a stage beyond that where the neutrons are further reduced to their final components, the quarks, and the ghost star has a new avatar made of some kind of quark soup? And perhaps most importantly, how do stars many times larger than the Sun continue to collapse beyond the neutron star and pulsar stages, their implosion bending the very structure of space and time to form black holes?

Fortunately, the very nature of neutron stars as the densest objects in the universe that have not yet turned into black holes makes it possible for scientists to discover what goes on inside them. As long as they can accurately measure the width of the neutron stars, from which their density can be determined.

Enter NASA’s Neutron Star Inner Composition Explorer (NICER), a large telescope on the International Space Station in orbit, which is helping astronomers do exactly that. NICER’s sensors are more accurate than atomic clocks and can capture X-rays thrown into space by pulsars. In December 2019, NICER released data so precise that astronomers were able to measure two crucial aspects of neutron stars: their rotational speed and how much the photons (light particles) from pulsars are doubled by gravity. The results, when combined with the stellar mass (the masses of various neutron stars are already known), produce the radius of the star.

In fact, an international team of scientists from the Max Planck Institute for Gravitational Physics in Germany has measured the size of a neutron star with unprecedented precision. Using various radio telescopes around the world, the researchers discovered that a “typical” neutron star has a radius of “between 10.4 and 11.9 km,” about 5 km less than previously believed.

These new measurements, along with data collected by Earth’s gravitational wave telescopes on how neutron stars warp space and time by colliding and merging with each other, will help scientists look to the depths of a dead star. No wonder theoretical physicists call it “the golden age of neutron star physics.”