Pulsars: Cosmic labs for extreme science

What would happen if we squeezed the entire sun to the size of a lemon? That's the kind of strange object that nature presents in the form of a neutron star! So what are these intriguing cosmic objects and how are they formed? Read on to find out..

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The tale of supernova

Our sun is an average star, a glowing fireball that emits light. Like a hot air balloon, it burns hydrogen fuel at the centre by nuclear processes, inflating it to a hot radiating sphere. When the fuel at the core runs out, it deflates like a balloon and collapses onto itself. But the story is more interesting for more massive stars. These stars go on to burn one type of fuel, forming and igniting another heavier one in a chain reaction, resulting in an onion like layered structure in its interior. Hydrogen burns to produce helium, which in turn produces oxygen, and similarly carbon, silicon until iron, the most bound element is formed in the core. As iron cannot burn to produce energy, this process stops, resulting in the star running out of fuel to burn. The outer layers collapse towards the centre, until they reach densities as high as in the centre of hard nuclei. This leads to a bounce, eventually ejecting the layers into interstellar space in a cataclysmic event called a supernova. The nuclear core left as a remnant is called a neutron star, as it primarily contains neutrons, the neutral subatomic particles that constitute atoms. Thus neutron stars are neutral objects with incredibly dense matter, what you get on squeezing the mass of about that of the sun within a diameter of a few tens of kilometers.

Laboratories in space

So what can we say about the properties of such an exotic object? Honestly, nothing. Our knowledge of current physics is limited to nuclear experiments that can be performed in laboratories. Such matter can only be found at the crust (outer layers) of a neutron star. But as we go deeper towards the interior, matter would be more and more dense, and we know essentially nothing about this type of matter. The closest possibility is to compare with the hot dense matter produced in heavy-ion collision experiments at the large accelerator facilities. But the temperature of the particles at the interior of neutron stars is low compared to their energies, hence the properties of such cold and dense matter could be distinctly different.

An era of multi-messenger astronomy

The only possibility of knowing more about these mysterious objects is to resort to theoretical techniques. By constructing theoretical models, scientists try to paint a picture by looking at astronomical clues, much like police sketches in investigations. Using different types of space-based and ground-based telescopes, astronomers can observe a wide range of astrophysical phenomena involving neutron stars across different wavelengths, from visible to infrared, ultraviolet, X-ray, radio or gamma. Just like medical examinations in X-ray or radio frequencies are performed to probe the interior of the human body, neutron star interiors can be probed using different astronomical tools. Recently, a new era of astronomy has been evoked with the first detection of gravitational waves from neutron stars.

As neutron stars warp the fabric of space-time around them, non-radial oscillations may create ripples in the space-time generating gravitational waves. These waves, when detected using the new generation of interferometers, reveal unforeseen facts about their interior. In conjunction with observations at other wavelengths, one may then be able to hint at interesting information of the behaviour of high density matter.

Multi-messenger astronomy

GW detectors

In addition to the ultrahigh densities and strong surface gravity, neutron stars also display a myriad of extreme properties, such as rapid rotation, fast transverse speed, very low interior temperature, ultra-strong magnetic fields and much more. By virtue of their strong magnetic field, neutron stars emit a beam of radiation of charged particles along their poles, which due to their rotation, sweep across the earth like a lighthouse beam. Such neutron stars, or pulsars, were the first to be detected fifty years ago, thanks to a young PhD student by the name of Jocelyn Bell at Cambridge. Although first suspected to be signals of extraterrestrial origin, due to the highly periodic nature of the signals, the origin of the radiation of pulsars was later realized.

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With the planned and upcoming astronomy missions and gravitational wave detectors, we are expecting our pool of information on neutron stars to only get richer. Extracting the behaviour of matter subject to extreme conditions allows us to expand our horizons of the knowledge of physics beyond our imagination. In order to venture beyond the known realms of extreme machines that we have on earth, nature provides us with intriguing phenomena that we are only ever more curious to understand.