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An artist’s impression of a stellar-mass black hole. photo courtesy: NASA/JPL-Caltech/Robert Hurt
An artist’s impression of a stellar-mass black hole. photo courtesy: NASA/JPL-Caltech/Robert Hurt

Black holes: Gravity is the key

Astronomers are now using X-rays to study black holes that stay hidden behind clouds of gas and dust

Black holes remain one of the most mysterious entities science has ever encountered. Questions remain over their origin, nature and presence. But 2018 might be the year when scientists will finally be able to see a black hole with the help of radio telescopes.

Poshak Gandhi is an associate professor and astronomer working at the department of physics and astronomy at the University of Southampton in the UK. His research focuses on multiwavelength time domain and compact object astrophysics. In lay terms, his research is focused on black holes.

In an interview, Gandhi explained how there’s a black hole at the centre of every galaxy and why interstellar travel for humans is not an impossible feat. Edited excerpts:

You were a speaker at the Astrosat View of AGN Central Engines in Pune? What was it all about?

The conference was focused around Astrosat, which was the first astronomy-focused mission launched (in 2015) by ISRO (Indian Space Research Organization). Most missions have been launched to study earth, remote sensing, weather monitoring, communication, etc. But this was one of the first dedicated satellites to look out into space for pure scientific research. Its focus is on X-rays and ultraviolet studies of the universe.

Could you explain how astronomers study active galactic nuclei (AGNs) or supermassive black holes using X-ray?

Objects in the universe can emit all kinds of radiation. They can emit radio waves, ultraviolet, infrared rays and X-rays. Now, X-rays are very high-energy radiation and they come from very hot regions of the universe. And one of the extreme environments that we know about are black holes.

This (pointing to a screenshot of a stellar massive black hole) is not a supermassive black hole but we think the physics is very similar. The black hole sits at the centre and its gravity is strong enough to capture any material that comes close enough to it. There happens to be a star in orbit around this black hole, and it is pulling off material from the star. It comes in the form of a whirlpool. There’s a sink in the middle and all the material is being drawn towards the sink. This is called a disc. The temperature here is so hot and material is moving so fast, (that) there’s a lot of friction. The material is colliding with each other at a very high speed in a magnetized environment. A lot of radiation, energy can be extracted from just these frictional processes. And that’s the light we see from black holes. So when we talk about studying black holes with X-rays, we are not looking at the black hole itself. We are always looking at the material that is in the immediate environment of the black hole. The temperature here can be hundreds of millions of degrees Celsius. That’s why they emit X-rays and that’s why we use satellites like Astrosat to study them.

A growing supermassive black hole surrounded by clouds of gas and dust. Photo courtesy: NASA/JPL-Caltech
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A growing supermassive black hole surrounded by clouds of gas and dust. Photo courtesy: NASA/JPL-Caltech

How would you explain black holes to a lay person?

Black holes were actually predicted to exist before they were found. That doesn’t happen often in science. (Indian American astrophysicist Subrahmanyan) Chandrasekhar worked out that any star which is more massive than approximately 1.4 times the mass of our sun cannot sustain itself towards the end of its life cycle. At that point, it collapses into something called a white dwarf. If you have something bigger, about three times more massive than the sun, then even a white dwarf cannot hold itself. It will collapse into a neutron star and there will come a point when nothing can sustain the stability of such an object. The only option is for the material to keep collapsing under its own gravity.

Once these were predicted to exist, people went looking for them. They found objects which had puzzling properties, like strong X-ray radiation, for example. They appeared to have very high masses. Then the idea slowly started to gain credence that perhaps black holes do really exist in nature. We now know that almost every big galaxy has a supermassive black hole at its centre.

What’s the proof?

There’s no absolute proof. We are always searching for more. As scientists, we are always trying to disprove our models. The biggest, unambiguous proof we have is gravity (he shows us a view of the center of the Milky Way). You can see billions of stars here, but if you zoom in right to the centre, there was a mysterious point found. When the orbits of the stars going around this object or point were mapped, it was found that there was actually nothing there. They seem to be going around empty space. Some of the stars are going extremely fast—we are talking thousands of km per second.

But how can a star whirl around empty space? There must be an object there which is exerting a force of gravity strong enough to hold these stars in their orbit. If you use simple Newton’s Law to estimate the mass of such an object, it turns out to be a few million times the mass of our sun. So there are a few million suns all packed into this very small region of space. This is one of the best pieces of evidence we have. We don’t have a picture of such a black hole yet, but we hopefully will soon.

You’re heading a survey called NuLANDS. Tell us more about it.

The main theme of the survey is to complete a census of supermassive black holes in our local cosmic neighbourhood. We are talking about finding all the supermassive black holes out to approximately 500 million light years away from us. That might seem like a big distance, but it’s actually very little in cosmic terms. Even within our local universe, we still haven’t found all the black holes that are growing and active. The reason is that most supermassive black holes are obscured behind thick clouds of gas and dust.

In order to find them, we need to find a way to penetrate these clouds. One way to do that is to use X-rays. It’s like going for a medical X-ray. One common misconception is that we are emitting these rays. We are not emitting any rays, unlike an X-ray machine. We are acting just as the X-ray film. We are capturing the X-rays coming from these black holes. The X-rays come from these black holes; it penetrates the skin, which in this case is the dust cloud. They pass through and we detect (capture) them through telescopes. NuLANDS is led by us in Southampton, but we are using a telescope called NuSTAR (a Nasa mission), which is one of the best X-ray telescopes in the world at the moment. All X-ray telescopes are in space, because we need to go above the earth’s atmosphere to capture the X-rays. We have proposed up to 90 galaxies or so and the observations should hopefully be completed in two years.

Poshak Gandhi.
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Poshak Gandhi.

The James Webb Space Telescope is scheduled to launch from French Guiana in spring 2019. How big is that for astronomy?

It is going to be completely transformative. I don’t remember the exact numbers but it is going to be a lot more sensitive than all previous infrared telescopes. It is going to be able to detect objects which are highly obscure, for example the AGNs. That’s because if you have a black hole that emits all the light, it is absorbed by these dust clouds. But this light doesn’t disappear. It’s converted into heat. You can detect this heat from the gas and dust because it is re-emitted as infrared, which the James Webb telescope will be able to detect. It will also be able to detect many other galaxies which are far away.

We now know that almost every big galaxy has a big, supermassive black hole at its centre... The biggest, unambiguous proof we have is gravity.- Poshak Gandhi

There are developments in India too—a Laser Interferometer Gravitational-Wave Observatory (Ligo), the third in the world, will be built here by 2025. What do you think about the progress happening here?

This is a great time to be an astronomer in India. There are a lot of projects in development and there is a lot of funding available for scientific research. So, there’s Astrosat, which was launched in 2015. Then there is Ligo. India has also built a couple of other telescopes; there is one in Nainital called the Aries observatory. One new, big telescope site is Devasthal, which now has a big 3.6 m telescope. There is definitely huge potential for growth in India.

We spoke about the film ‘Interstellar’. In reality, we are talking about colonization of planets and space travel. Do you feel all this will be possible?

In terms of interstellar travel, it must happen someday. Eventually, we will have to expand beyond the earth and move; the simple reason being that the earth will become uninhabitable in a billion years from now. It must happen at some point in the future. We still don’t have the technology to attempt interstellar travel with humans at least, but we may soon be able to send nano-satellites to other systems.

I think the first step will be our intelligent machines going and exploring the nearby universe for us. And then we will follow them step by step.


Be a stargazer

While some constellations—like the Big Dipper—are visible to the eye, other cosmic landmarks are not easy to spot. Here are some smartphone apps that will make the sky above you come alive.

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Official Nasa app

The official app from the US space agency Nasa not only gives you detailed information on its latest missions, but also comes with some stunning images from space. You can track the International Space Station and other earth-orbiting satellites with the help of 2D maps and 3D earth models.

Available on iOS and Android

NightSky makes your iOS device a portal to the cosmic world.
NightSky makes your iOS device a portal to the cosmic world.


An Augmented Reality (AR)-powered planetarium, NightSky makes your iOS device a portal to the cosmic world. You can use your iPhone, iPad and even the Apple Watch to spot constellations and satellites. Some premium features include a worldwide tour of the night sky and a virtual tour to the moon. You can also set notifications for when satellites are passing over your location.

Available on iOS

Star Chart

Star Chart promises you a virtual planetarium on your Android device with the help of GPS and a 3D model of the universe. The app shows all 88 constellations and has a “Time Shift" feature that allows you to shift up to 10,000 years forward or back in time to see how the sky was back then. It also comes with voice control that lets you explore the sky with commands such as “Fly me to the Moon" or “Tour Mars".

Available on iOS and Android

Sky Map

This app was developed as the Google Sky Map but is now open-sourced. Sky Map lets you point your smartphone camera towards any star in the sky. Once the app spots it, it gives you the star’s name and other details. You can also view constellations in detail—names, structure and so on. The app mostly uses your device’s GPS and sensors to analyse the sky.

Available on Android

SkyView Free app uses the camera to spot and identify celestial objects in the sky.
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SkyView Free app uses the camera to spot and identify celestial objects in the sky.

SkyView Free

With the SkyView Free app, all you need to do is point your phone towards the sky. It uses the camera to spot and identify celestial objects in the sky—doesn’t matter if it’s day or night. Users can find popular constellations, locate planets in the solar system and view satellite fly-bys with this app which also uses AR.

Available on iOS and Android

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