Astronomers spend most of their time looking back in time. They peer at light that left an infant universe behind to reach Earth, after travelling for billions of years. This electromagnetic radiation, as well as gravitational waves – ripples of energy in the spacetime continuum released when matter accelerates through space – are ancient energies that sometimes lead astronomers to astonishing discoveries.
To detect gravitational waves, researchers use instruments called interferometers. These are purpose-built to be so sensitive as to sense distortions in the fabric of spacetime to the extent of much less than the diameter of a proton.
Scientists used three such instruments – including the twin LIGOs (Laser Interferometer Gravitational-Wave Observatories) in Washington and Louisiana – to recently ‘spot’ a violent repast in the depths of space: a black hole gobbling up what could be a neutron star, which is the tremendously dense core of a collapsed star. Since the cosmic drama took place 900 million light-years away from Earth, the gravitational waves it produced reached here only now, travelling at the speed of light.
Scientists used the twin LIGOs to make the first direct detection of gravitational waves in September 2015. The waves had been from the merger of two black holes 1.3 billion lightyears away from Earth. Since then, scientists have documented more than a dozen events of black holes merging with other black holes.
However, the latest report is potentially unprecedented. “Whether the second object is a neutron star or a black hole can be established only at the end of a careful analysis, which is currently ongoing,” Parameswaran Ajith, an astrophysicist at the International Centre for Theoretical Sciences, Bengaluru, and a member of the LIGO Scientific Collaboration, told The Hindu.
When a star runs out of material to fuse in its core, it undergoes a series of transformations before its ultimate demise. Depending on the star’s mass, these steps are an extremely hot red giant, an incredibly dense white dwarf, an even denser neutron star and/or finally a black hole.
The story of the neutron star that ended up on the black hole’s menu is no different. Once upon a time, it used to shine just like any other stellar body: by converting its hydrogen into helium and, ultimately, other heavier elements. However, one day, it had become saturated with the heavier elements and couldn’t fuse them further, so it blew up as a supernova.
Whatever was left of the star after the explosion collapsed under its own gravitational weight to become a neutron star. This neutron star then strayed too close to a black hole. Curiously, the same star could have had a different fate and become a black hole itself if it had had some more mass! Any neutron star whose mass exceeds twice that of our Sun is destined to collapse into a black hole.
When you throw a ball with force straight up in the air, it travels faster and higher before gravity pulls it back down. Throw it with enough initial velocity and it could overcome Earth’s gravitational attraction to escape into space. On Earth, this ‘escape velocity’ is 11.2 km/s, whereas it is hundreds of thousands of km/s in the case of stars.
As a neutron star is crushed into ever smaller volumes, its gravitational pull increases, and so does the escape velocity on its surface. So as a star collapses in on itself, at some point, even light – which travels at ~300,000 km/s – doesn’t move fast enough to escape the body’s gravitational pull. A star in this state is called a black hole (although it neither looks nor behaves anything like a star).
Even as astronomers discover more neutron stars, pulsars (rapidly spinning neutron stars) and several likely black holes, gravitational-wave detectors like LIGO and Virgo help detect things like a black hole feeding on a neutron star. The two LIGO interferometers – each an L-shaped building with each side 4 km long – use lasers and mirrors to identify and record variations as small as ten thousandth the width of a proton.
Having confirmed the gravitational signature of such a cataclysmic event, the next task for astrophysicists is to track the electromagnetic trails that could lead them to the actual event. Black holes don’t release any electromagnetic energy, so when two of them merge, gravitational waves are the only way to detect the merger. But when a black hole interacts with a neutron star, the latter could release electromagnetic energy – typically radio waves, X-rays and/or light – that scientists can detect using conventional telescopes.
Albert Einstein’s theory of general relativity predicts that light passing near a black hole’s boundary, called the event horizon, can escape capture and just be bent. This offers astronomers a unique window to study the dying light of a neutron star as it disappeared into a black hole.
As for gravitational waves, with advanced detectors like the Kamioka Gravitational-wave Detector in Japan and the LIGO project in India expected to become operational in the next few years, who knows what new secrets they might reveal about these mysterious gravitational beasts…
Prakash Chandra is a science writer.