Undead stars: Pulsars
Manjari Bagchi, The Institute of Mathematical Sciences, Chennai

Our Sun is a star. It shines due to energetic processes taking place in its central core. The fusion processes when four hydrogen atoms are converted to helium atoms in the solar core release energy, and a part of this energy is seen as sunlight.

All stars shine in a similar way: because of nuclear fusion processes in their core. Those stars that fuse hydrogen atoms to form helium atoms in their cores are called main sequence stars. About 90 percent of the stars in the universe, including the sun, are main sequence stars. In such stars, gravity tries to crush the star but the outward radiation pressure due to the hydrogen burning in the core prevents the star from collapsing.

Our sun is about 4.5 billion years old. It is middle-aged; it will shine for another 4.5 billion years. The more massive a star is, the shorter its lifespan. After the hydrogen fuel at the core has been used up, the star evolves into a supergiant, red giant, or directly to a white dwarf.  Our Sun will become a red giant in a few billion years, as will main-sequence stars which have a mass range of 0.3 to 8 times solar mass. Why does it become a red giant? The fuel is exhausted. There is no outward radiation pressure, so the core contracts due to gravity. This heats up the core and causes the outer layers to expand greatly, forming a red giant.

Soon, the fuel is completely exhausted. The star collapses due to gravity. The red giant throws off its outer layers, leaving behind a planetary nebula, which ultimately becomes a white dwarf.

What happens if the main sequence star was heavier than 8 times our sun?  They usually end their lives in a massive explosion called supernova, leaving the core called neutron star behind.  But if they are more than 20 times the solar mass, they end up as black holes.

What is a neutron star?
These neutron stars are extremely compact, with a radius of just 10 km (you could walk across it in a couple of hours!). They have incredible densities of 1014 g/cm3 (the Earth has a density of around 5 g/cm3. So a teaspoon of neutron star material would weigh around a billion tonnes. Gravity has compressed the core of such main-sequence stars to nuclear densities, so that when they blow off their outer layers in the supernova explosion, a very dense object is left behind. But this is not all. Many neutron stars also have very high magnetic fields, 109 to 1015 Gauss. In contrast, earth's magnetic field is half Gauss.

Pulsars and their magnetic fields
There are special neutron stars called  pulsars which are of great interest to scientists.  A pulsar is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. As seen from the schematic view, the sphere in the middle represents the neutron star, the curves indicate the magnetic field lines, the protruding cones represent the emission beams and the green line represents the axis on which the star rotates.

This radiation can be observed only when a beam of emission is pointing toward Earth (similar to the way a lighthouse can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission. As the neutron star rotates, the pulsar beam can be seen on Earth whenever it points in that direction. From this it is possible to tell the rotational time period of such stars.  Various neutron stars have been observed that emit pulses with time period from milliseconds to seconds for an individual pulsar. Can you imagine a star with that size and density rotating once every millisecond? Truly nature is strange and exotic.

How to see these stars?
The most important fact about pulsars is that the emission is in the radio region of the electromagnetic spectrum. So the atmosphere does not block this signal and we are able to find out about distant objects in our universe. The telescopes to be used therefore must be able to detect radio waves rather than visible light. So far 3319 pulsars have been found, Signals from the first discovered pulsar were initially observed by Jocelyn Bell while analyzing data recorded on August 6, 1967 from a newly commissioned radio telescope that she helped build. They were initially dismissed as radio interference but soon scientists realised that they were getting signals from outside the Milky Way galaxy.

The picture shows some of the 30 steerable parabolic raio telescopes that make up GMRT or the Giant Metre Radio Telescope, located at Khodad near Pune in Maharashtra. It has located and tracked many pulsars, galaxies, and supernovae. In August 2018, the most distant galaxy ever known, located at a distance of 12 billion light years, was discovered by GMRT. Many experiments are coming up to study the nature and properties of these exotic stars.

BOX: Earth's atmosphere and its transparency to electromagnetic radiation
An incredible property of Earth's atmosphere helps us to observe the stars. This is the fact that radio waves from the stars can reach the earth because Earth's atmosphere is transparent to these waves. From the figure, it can be seen that the Earth's atmosphere is also transparent to "visible" light (for humans). If visible light (violet to red colour) could not penetrate the atmosphere, then there would be hardly anything visible on Earth to humans since the Sun emits light mostly in this region. (About half of sunligght is emitted in the visible region, and half in the infra-red, with a small fraction in ultra-violet).
However, the atmosphere is also transparent to radio waves which is how you can hear the radio from far-off countries. What if these stars were emitting radio waves? Could you "see" them with telescopes? The answer is yes, and such stars are called pulsars, which are one of the areas being studied at IMSc.
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