The farther backwards you can look, the farther forwards you are likely to see. – Winston Churchill
Sometimes, we point our most powerful telescopes at the sky, peering as deeply as we possibly can, hoping to shed some light on what the Universe was like oh-so-long ago, as close to the big bang as we can. The Hubble Space Telescope can get us distant galaxies as they were just a few billion years after the big bang.
But Hubble still has never seen one of the elusive, Holy Grails of astronomy: a metal-free star.
You see, immediately after the big bang, the Universe was filled with protons and neutrons, which finally fuse together (when the Universe cools enough) to create hydrogen, helium, and lithium nuclei. A few hundred thousand years later, the Universe cools enough to turn these nuclei into stable, neutral atoms. But that’s it: beyond those three elements, there’s nothing heavier. The Universe can’t make them, not until the first stars form.
And someday, that’s what we’d love to find: a metal-free star. These first stars, without any traces of heavier elements, are responsible for exploding and enriching (or polluting, depending on your perspective) the surrounding space with elements much heavier than lithium. Well, we just determined that a Gamma-Ray Burst earlier this year shattered the distance record:
At a redshift of eight, it’s the most distant object ever discovered. This was light emitted around 13 billion years ago, when the Universe was less than one billion years old. And yet, looking at the spectrum of this one, it’s still full of heavy elements!
When you form stars, anywhere, you make many, many little, low-mass stars, like red dwarfs. But you make a few very high-mass stars, called O-type or B-type stars. These stars are huge. Compared to a G-type star like our Sun, there’s simply no contest.
Huge! Well, there’s a big problem with being huge. What is it? Let’s ask Bladerunner:
The light that burns twice as bright burns for half as long – and you have burned so very, very brightly, Roy. Look at you: you’re the Prodigal Son; you’re quite a prize! –Dr. Tyrell
Bladerunner got the scaling wrong: the star that burns with twice as much mass lives only one-eighth as long! So if a star like our Sun lives for 10 billion years, a star 10 times as massive lives for only 10 million years, and one 100 times as massive lives for just 10,000 years!
So that’s why this gamma-ray burst we’ve found, despite being only an estimated 630 million years into the birth of the Universe, is still chock-full of these heavier elements.
In fact, instead of a redshift of eight, we’d have to get all the way out to a redshift of around forty before we expect to start seeing a metal-free star. And the continued observation of this Gamma-Ray Burst confirms that, despite occurring 95% of the Universe’s lifetime ago, the Universe was very, very similar then to the way it is now. The same stars, the same stuff, the same explosions as the ones we see now. There’s never been a more distant, more comforting observation than this, that tells us pretty much exactly what we expected.
Gamma-Ray Bursts come from stars that die in very certain ways, and this one, from 13 billion years ago, is just like the ones that happened recently — and close — to us. By finding very few differences, one amazing piece of the picture comes into view: the Universe looked a lot like it does now a very short time after the big bang. That we can see things when the Universe was only 5% of its current age is like me looking back on my life and remembering everything that happened when I was 18 months old. Only, I wasn’t able to do all the things I can do now back then. But the Universe can, and did, and now we’ve seen the first pieces of evidence for that! So thank you to that massive star that died all those billions of years ago. It’s shown us that the Universe grows up very, very quickly!