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Current Affairs

Secrets of Neutron Stars

Date: 23 April 2020 Tags: Space

Issue

Scientists studying life of dead stars have been able to measure the radius of the neutron star that allows them to study various aspects of the life of the star after it undergoes supernova explosion.

 

Background

The 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. 

 

Details

  • 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.

  • 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 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.

  • 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. 

  • Fortunately, the nature of neutron stars as the densest objects in the universe makes it possible for scientists to figure out what goes on inside them as long as they can measure accurately the width of neutron stars, from which its density can be determined.

  • NASA’s Neutron Star Interior Composition Explorer (NICER), a large telescope on the orbiting International Space Station, 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.

  • 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.