Colliding neutron stars, as depicted here in an artist’s image, could send a fast radio burst coursing through the universe. Image: ESO/L. Calçada/M. Kornmesser.
Are neutron stars – the incredibly dense, burnt-out cores of dead stars – about to yield their secrets? Yes, say astronomers poring over the findings of new research that suggests there is more to a star’s death than meets the telescope.
The life of a typical star is as fascinating as its death. It shines by burning its nuclear fuel, converting hydrogen into helium to hold itself up against the pull of gravity for billions of years. But when the fuel is exhausted, gravity wins the long drawn out battle and causes the stellar remnants to collapse. New nuclear reactions then begin to convert the helium into carbon, releasing more gravitational energy.
The tremendous heat produced puffs out the outer layers of the star – a fate that awaits the Sun, too, in about five billion years from now when it expands to become a red giant, crisping all planets up to Mars. When all the helium in a star is converted to carbon, the core becomes more compact and hotter still, as nuclear fusion converts the carbon into oxygen. Eventually, most of the core material is converted into an iron-rich nucleus, at which point the addition of more protons and neutrons from the reaction does not release any more energy.
With the source of heat gone, larger stars simply collapse, the mass of their outer layers falling inwards under the pull of gravity and getting very hot as gravitational energy is released. Given enough mass, in these conditions, there is a sudden flareup of activity as protons and electrons of hydrogen and helium from the star’s atmosphere fuse into neutrons and compress the core explosively. The explosion takes place in a shell around the core – like an orange peel – and the blast travels outwards, ejecting the rest of the star’s atmosphere in a flash as bright as a galaxy to form an expanding nebula made of ionised gas and dust.
It also travels inwards, squeezing the core tight and producing a smattering of elements heavier than iron, some of which may get thrown out into the nebula. This ‘supernova’ explosion leaves behind a rapidly spinning neutron star known as a pulsar: the smallest and densest known entity in the universe. The incomparable power of a supernova can be observed in the Crab Nebula, which is 6,500 light years away from us. Gases from that stellar explosion eons ago are still moving outwards at 1,300 km per second!
So, it is a matter of mass, so to speak, as stars larger than the Sun explode as supernovae. While scientists have been able to figure out this much of a star’s story, nobody really knows what becomes of a neutron star or a pulsar after this. What happens to its iron-filled super-dense core? Is there a stage beyond that in which the neutrons are further reduced to their ultimate components – quarks – and the ghost star has a new avatar made up of a sort of quark soup? And perhaps most importantly, how do stars many times bigger than the Sun continue to collapse past the neutron star and pulsar stages, their implosion bending the very fabric of space and time to form black holes?
Fortunately, the very nature of neutron stars as the densest objects in the universe that have yet to become black holes makes it possible for scientists to figure out what goes on inside them. So long as they can measure accurately the width of neutron stars, from which its density can be determined.
Enter NASA’s Neutron Star Interior Composition Explorer (NICER), a large telescope on the orbiting International Space Station, which is helping astronomers do just that. NICER’s sensors are more precise than atomic clocks and can pick up X-rays spewed into space by pulsars. In December 2019, NICER turned in data so precise that astronomers could measure two crucial aspects of neutron stars: their speed of rotation and how much the photons (light particles) from pulsars are bent by gravity. The results, when combined with the stellar mass (the masses of several neutrons stars are already known), yield the star’s radius.
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 accuracy. Using several radio telescopes across 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 what was previously believed.
These new measurements, along with data collected by terrestrial gravitational wave telescopes on how neutron stars warp space and time by colliding and merging with each other, will help scientists peer into the depths of a dead star. No wonder theoretical physicists are hailing this as “the golden age of neutron-star physics”.
Prakash Chandra is a science writer.