Twinkle twinkle gargantuan star

Astronomers detected more than 230 supernova incidents in 2013, and nearly 60 in 2015


Supernova 1987A, as the supergiant explosion was called, was the first one in nearly 400 years to be visible to the naked eye. Photo: NASA/Reuters
Supernova 1987A, as the supergiant explosion was called, was the first one in nearly 400 years to be visible to the naked eye. Photo: NASA/Reuters

Forget Donald Trump and what I’m calling that Rs500/1,000 thing. Let’s focus for a while on stars. You know, those little blinking dots in the night sky. Specifically, I want to tell you about something that happened with a star nearly 30 years ago. Well, actually it happened about 168,000 years ago, but we earthlings first found out about it on 24 February 1987. This happened: A star exploded. In fact, a truly massive star, one astronomers call a supergiant, exploded.

Such an explosion is known as a supernova, and they are not that unusual: a typical supergiant that has reached the end of its life will, typically, explode. Given that there are countless billions of stars out there, and many of them are supergiants, and at any given time, many of those arrive at the end of their lives—well, these starry explosions must be happening pretty regularly. And in fact, astronomers detected more than 230 in 2013, and nearly 60 in 2015 (the International Astronomical Union’s Central Bureau for Astronomical Telegrams, or CBAT, maintains an ongoing list here: www.cbat.eps.harvard.edu/lists/Supernovae.html).

True, we don’t see most of these cosmic fireworks. That’s because most stars are so unimaginably far away that we cannot see them with telescopes, let alone with our naked eyes.

That is, even if you’re far from a city on a clear night and look up at a sky filled with stars, you’re seeing only a tiny fraction of all the stars in the universe. So even if stars are exploding all the time, the chance that one that’s visible to us will explode is correspondingly tiny (one estimate is that a given human will see—or like me, get news of—a supernova only once in their lifetime).

Which is why that February 1987 sighting made headlines around the world: it was actually visible to millions on Earth (though only in the southern hemisphere). That means this was a relatively close star. As it turns out, it was just 168,000 light years away.

In any sense you and I would comprehend, 168,000 light years is a vast, vast distance. But on the scale of the universe, that particular supergiant exploded practically in our pockets. Still, light from there does take that many years to reach us. So if we saw it in 1987, understand that this supergiant really exploded about when our primate ancestors first evolved into modern human beings.

Puts it in some perspective.

Supernova 1987A, as it was called, was the first one in nearly 400 years to be visible to the naked eye (astronomers have speculated that the Star of Bethlehem, which guided three wise men to where Jesus was born, was a supernova too). So it was the first one to be widely observed and studied using modern equipment.

Nearly 30 years later, we can still observe the residues of this cataclysmic event. In 2013, for example, a team of astronomers used a radio telescope in Chile to produce a spectacular image of 1987A, an orange core surrounded by a halo of shining blue and green spots, like a necklace. That halo is actually material from the star that the supernova flung into space, and it is still travelling outward at more than 7,000km per second.

Which number, by itself, should give you an idea of the titanic power a supernova packs (our most powerful rockets reach 11km per second). But don’t worry! Even at that speed, it will be more than 7 million years before 1987A’s flying detritus reaches us.

Astronomers classify supernovae by certain chemicals they contain. So 1987A was a Type II. Type Ia supernovae turn out to have a particularly useful characteristic: they all release about the same amount of energy when they explode, and thus are almost equally luminous. This means they are good “standard candles”—astronomer-speak for milestones in space. That is, how bright a Type Ia appears to us is a direct indication of how far it is—and thus other nearby objects are—from us.

In the late 1990s, astronomers used a database of Type Ia supernovae to reason that the expansion of the universe is accelerating, which was something of a surprise.

But with a much larger database of Ia supernovae available now, Professor Subir Sarkar of Oxford University and two colleagues have just challenged this idea of acceleration. “We find, rather surprisingly,” they wrote in Nature (October), “that the data are still quite consistent with a constant rate of expansion (of the universe).”

The arguments that drive astronomy and its practitioners.

Last year, some of them detected the most luminous supernova we’ve ever found, ASASSN-15lh. Since it is about 3.8 billion light years away, it’s far too faint to see. But the explosion was about 570 billion times brighter than our Sun. Try imagining that much light. And had it been in our Milky Way galaxy, one astronomer explained to the New Scientist that “it would shine brighter than the full moon, there would be no night and it would be easily seen during the day.”

And I’ll leave you with this to chew on. Astronomers think most of the atoms inside us humans were actually created by stars. How did they get here? When the stars exploded, the supernovae propelled the atoms into the void, and they spread across the universe.

Some became you. And me. And Katrina Kaif. And Donald Trump.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers explores the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at dilip@livemint.com. Read Dilip’s Mint columns at www.livemint.com/dilipdsouza

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