Meet the oldest object ever seen
4 min read 05 Aug 2022, 12:27 AM ISTGLASS-z13 is also the oldest galaxy humans have ever observed; it is 13.4 bn years old
PremiumOne of the great scientific endeavours of our times has just started delivering. Maybe you’ve noticed? I refer to the James Webb Space Telescope (JWST), which was fired into space last December. Settled into its position about a million miles away, it took its time to unfurl mirrors and get its equipment functioning.
By now, you’ve undoubtedly seen the spectacular first few images from the JWST. There’s the Southern Ring Nebula, like some glowing beetle. There’s the Carina Nebula, which looks like a bank of roiling orange clouds with stars gleaming above in a deep royal blue field. There’s Stephan’s Quintet, five galaxies like ethereal nymphs doing an elaborate dance; that one photograph is actually a mosaic of about 1,000 separate images. And this week, there’s a dramatic view of Jupiter like you’ve never seen Jupiter, in gleaming black-and-white like some ancient warrior’s shield.
Just beautiful, each photograph. And yet they are not what’s really remarkable about the JWST’s performance. What is remarkable is how far into the cosmos it has been able to peer. Let me give you a flavour of that and the implications.
Take the Southern Ring Nebula. It is about 2,000 light-years (ly) from us on Earth. How far is that, really? What does that distance actually mean?
Start by remembering that a “light-year" is a measure of distance, not time. It is the distance that light travels in a year. Now light moves along at about 300,000 km every second. That’s so fast as to seem pretty much instantaneous, which is why you see the lamp on your wall turn on as soon as you press the switch. But of course, it is not really “as soon as", not actually instantaneous. Think about this by imagining that when you press the switch, the fixture on the wall gently lobs a tennis ball at you. The ball takes a moment - maybe a second - to reach you. In the same way, the light from the lamp takes a slice of time - a tiny fraction of a second - to reach you.
Over much larger distances than the few metres between you and your lamp, the time light takes to travel becomes perceptible rather than apparently instantaneous. For example, light needs about eight minutes to get from the Sun to the Earth. That’s a distance of about 150 million km, but we could also say that the Sun is eight light-minutes from us. Jupiter, for its part, is 36 light-minutes away.
One light-minute, the distance light travels in a minute, is 300,000 km x 60 = 18 million km. We can do a similar calculation for a light-year:
1 ly = 300,000 km x 60 x 60 x 24 x 365 = 9,460,800,000,000 km.
Call it 9.5 trillion km. The Southern Nebula is 2000 ly, or 19,000,000,000,000,000 (19 quadrillion) km away. All those zeros tell you why, for astronomical distances, the light-year is an easier unit to work with than the kilometre. Still, whichever unit you use, the Southern Nebula is almost unimaginably far off.
Yet the truth of our awe-inspiring universe is, stacked up against other objects out there, the distance to the Southern Nebula is almost unimaginably minuscule. The Carina Nebula is 7,600 ly distant, or over twice as far as the Southern Nebula. If you’re not impressed by “over twice", Stephan’s Quintet is—wait for it—290 million ly away. That’s 150,000 times as far as the Southern Nebula.
Take your time to grasp that. I find it hard because even that vast distance is tiny compared to some still farther objects the JWST has shown us. In mid-July, a team led by Harvard astronomer Rohan Naidu announced that they had spotted a galaxy, GLASS-z13, whose light took about 13.4 billion ly to reach JWST. That’s nearly 50 times as far as Stephan’s Quintet. That’s also the most distant galaxy humans have ever observed.
Now grasp this: GLASS-z13 is also the oldest galaxy humans have ever observed. How do we know this?
Remember, the Sun is eight light minutes away. So when we look at it in the sky, we see it where it was eight minutes ago. When it “rises", it’s actually eight minutes farther along the path it will trace in the sky through the day. Extend that reasoning to the Southern Nebula. Because light from there has taken 2,000 years to reach us, we are actually seeing it as it was 2,000 years ago.
In a very real sense, when you look out at objects in the sky, you are looking back in time. You are looking at an eight-minute-old Sun, a 2000-year-old Southern Nebula, at a 7,600-year-old Carina Nebula. It’s a good bet the Sun hasn’t exploded or vanished in those eight minutes, but what about those Nebulae? For all we know, they may not even exist today. And these are all relatively nearby objects. The five galaxies in Stephan’s Quintet are 290 million years old. GLASS-z13 is 13.4 billion years old.
Compare that last number to the age of our universe itself. Since it was born in the vast cataclysm we call the Big Bang, it’s been about 13.8 billion years. So the very old GLASS-z13 offers us a glimpse of what a very young universe - aged a mere 400 million years - was like.
And this is also why astronomers typically refer to objects such as GLASS-z13 by age—“the oldest galaxy ever observed"—rather than distance. In fact, given that everything in the universe is also moving, GLASS-z13 is today certainly not where it was 13.4 billion years ago. Astronomers estimate it is now about 33 billion ly from us.
How do they know that? The “z13" is a clue. That story is in another column. For now, let’s just savour JWST’s early findings and savour the wonder and excitement in them.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
