Home/ Opinion / Columns/  And yet it moves, and how!

Over in a certain corner of the universe, there’s a happy noise. Well all right, it happened many years ago. Also, I’m not sure it was happy, and I can’t confirm it was even heard at all. But if it was heard, it must have been very loud. When a large star starts running out of material to burn, it can suddenly collapse and explode. This explosion is called a supernova. So this is about a supernova named G292.0+1.8.

Here on Earth, we’ve often observed supernovas. Perhaps there’s suddenly a bright star where we detected none before. Perhaps a star that’s forever been just another nondescript point of light suddenly outshines every other nearby. One such appeared in February 1987 and is known as SN 1987A. Of course, the explosion itself didn’t actually happen in 1987. This collapsed star was in the Large Magellanic Cloud, a sister galaxy to our Milky Way—and the LMC is 168,000 light years away. So the star actually exploded that many years ago, a while before you were born. It’s just that the light from that explosion in about 166,000 BCE reached us on Earth in early 1987. That’s how far away the star was.

Supernova explosions can produce, perhaps counter-intuitively, other, smaller stars called pulsars. These objects rotate at remarkable speeds, which means their brightness varies rapidly but regularly. (Why they rotate, another time.) Their brightness “pulses", and thus the name. This is what happened with supernova G292.0+1.8. Along with all the other debris the explosion expelled, it also sent a pulsar flying through space.

A digression here. Let’s say you’re standing at your window and looking at a short stretch of the nearby road, stopwatch conveniently at hand. Give the scene some numbers: the road is 100m from you, and the stretch is 20m long. As you watch, a car appears at the right end of the stretch of road and you hit the button on your stopwatch. Exactly four seconds later, the car disappears to the left. How fast was the car travelling? That’s easy: Four seconds for 20m, that’s 5m/s, which translates to 18kmph.

One more thing about this thought experiment: if you didn’t know, how might you calculate that the stretch is 20m long? Here’s one way. Point a finger at the right end, then swivel it till it points at the left end. Measure the angle through which the finger swivels. Given that and the 100m distance to the road, some elementary trigonometry lets you calculate the length of the visible stretch of road. In this case, the angle would be just over 11 degrees.

The point of this digression is to put in your mind the question that astronomers naturally ask about the pulsar that G292.0+1.8 kicked out: how fast is it moving?

Though there’s a more basic question: How do we even know the pulsar is moving? In the case of the car, you actually see it move. Not so easy with objects out in the cosmos. Think of any star at all—the ones in the beloved constellation Orion, known in India as Shikari or Kalpurush, for example. As far as humans can see with our naked eyes, none of them has moved for millennia. Yet we know they are constantly moving, because we have instruments that can detect or deduce movement in different ways. But the stars are so distant that without instruments, we’ll never notice their movement.

So with the pulsar in G292.0+1.8. If you could see it in the sky—which you can’t—you might stare at it for hours, weeks and years on end, and you would not detect it moving. So we have telescopes such as Nasa’s Chandra X-Ray Observatory, which was launched into orbit around the earth in 1999. It is designed specifically to observe, among other objects, exploded stars. Chandra produced an image of G292.0+1.8 in 2006, and then again in 2016. Comparing both, astronomers found that a particular dot of light had, just like the car above, moved.

How far had it moved? Well, if you did the same finger pointing exercise spelled out above, your finger would have traced out an angle of...wait for it...0.00006 degrees. An inadvertent twitch of your finger would be far larger. You certainly have no way to measure such an angle. Yet Chandra had detected a movement that minuscule.

Though “minuscule" is hardly the word for this pulsar’s actual movement. G292.0+1.8 is about 20,000 light years from us. Exactly like we can use 11 degrees and 100m to work out the 20m length of that stretch of road, we can use 0.00006 degrees and 20,000 light years to work out how far the pulsar travelled in 10 years. Some fairly easy calculation tells us: about 190 billion km.

Take a moment to comprehend that number. It’s gargantuan. It’s well over a thousand times the distance between the sun and our planet. Gargantuan or not, we can now calculate how fast the pulsar is moving. It took ten years to cover those 190 billion km. That is a speed of...wait for it...over two million kmph. Compare with the car that trundled past at 18kmph.

Scientists calculate that had we humans been able to observe the explosion of G292.0+1.8, that would have been 2,000 years ago. (Which means the explosion actually happened 22,000 years ago, since G292.0+1.8 is 20,000 light years from us.) That is, this was an explosion so monumental, so cataclysmic, that the star—the star!—it expelled into space is, 2000 years later, still hurtling along at two million kmph.

Spend a while thinking about that. About tiny angles, vast distances and unimaginable speeds. A measure of the universe, in that pulsar.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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Updated: 08 Jul 2022, 01:31 AM IST
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