European Space Agency’s XMM-Newton

Neutron stars are weighty matter, both physically and intellectually. A neutron star cannot be more than 20-30 km in diameter but at the same time it can be more massive than the sun, so much so that a spoonful of this tiny giant can weigh more than 100 million tonnes on earth! Neutron stars are mysterious as well. Looking at them from the earth — even through the best optical telescopes — is like searching for a speck of dust on Pluto. In fact, it can be worse.

Scientists need to know about neutron stars because they could harbour some exotic matter not easy to create on earth. But probing them is difficult because they are so small and far away. You can easily locate them because they pull nearby stars with great force, but you cannot see inside them or estimate their size as they are too small to see from the earth. There are, however, some indirect methods to measure them. Recently scientists added one more valuable technique, providing, futher evidence to corroborate Einstein’s two theories of relativity.

Sudip Bhattacharyya (a graduate from Calcutta’s Jadavpur University) and Tod Strohmayer of the Goddard Space Flight Centre of Nasa in Maryland, US, have found a new way of looking at neutron stars: analysing the colourful lines made by light from the star. The scientists used the European Space Agency’s XMM-Newton telescope and looked at a neutron star with a companion 26,000 light years away. It all seems simple, but much depends on where and how you look. “No technique of measuring a neutron star is perfect, so we need to have multiple methods,” says Bhattacharyya.

Neutron stars are remnants of large stars. A star’s life story is determined by its size — the larger the star, the shorter is its life. A large star, much bigger than our sun, generally ends its life in a gigantic explosion called the supernova. The outer core of the star blasts away into space, crunching the inner parts into a super-dense sphere called the neutron star. Its density is unimaginable. In normal matter, every atom has a nucleus at the centre with protons and neutrons and then electrons and empty space around it. If the atom were as big as a football field, you would be hardly able see the nucleus. In fact, more than 99 per cent of the atom — actually all atoms — is empty space.

In a neutron star, all the nuclei are crunched and stand shoulder to shoulder. In fact they are even closer, so close that neutron stars have been described as one giant neutron. The crust is solid and as you go inside it becomes liquid and gets denser, and at the core is a matter that defies description. Scientists call it neutron degenerate, and it has several exotic names in popular literature. “They could be quarks, they could be Bose-Einstein condensates, they could be anything. The only way to understand them is to study them in nature,” says Bhattacharyya.

Bhattacharyya and Strohmayer looked at a neutron star called Serpens X-1. It has a normal companion star, but the gravity of the neutron star is so strong that it is ripping apart matter from the companion star. Matter thus breaks out of the companion star and whirls around the neutron star before finally falling inside and becoming a part of it. This whirlpool of matter rotates extremely fast, at as much as 40 per cent the speed of light.

Einstein’s special theory of relativity tells us that in such circumstances, matter in the disc that is coming toward us would emit more radiation than the one that is moving away. Moreover, the light coming from there would strive hard to escape the neutron star’s gravity and lose energy, making its colour shift toward the red part of the spectrum. This shift is predicted by Einstein’s general theory of relativity. “The amount of shift also depends on the circumference of the neutron star, and from this we can calculate the radius,” says Strohmayer.

The Nasa scientists looked at light emitted by iron atoms around the neutron star, and made the best-ever measurements so far. They concluded that the size of the neutron star was 30 km at the most. Scientists always had a good idea of how massive a neutron star is. Now they are beginning to have a good idea of how big it is. They are probing deeper and could soon come up with some surprises.