Why we study astronomy

I’m an astronomer, and I think there are four reasons we do astronomy. The first is simply exploration, to find out what’s out there. The second is to make sense of what we see, to understand it in terms of the laws of physics and to perhaps discover new laws of physics. The third is to understand cosmic evolution: was there a beginning, and if so, then how have things evolved from that beginning to the amazing cosmos we see around us and are a part of? And that leads to a final reason: the mystery of why things exist, and why the universe is the way it is.

All these things are part of astronomy and what I do, and of course, pure thought doesn’t get you very far. But thanks to improved instruments, we’ve made huge progress. The last 50 years of astronomy have been one of the real highlights of science, up there with the standard model of particle physics and the double helix. I’ve been privileged to be part of this huge cosmic exploration.

Expanding horizons

Photo by Triff

Astronomy is really a story of expanding horizons. To the ancients, the Earth was the centre, and the Sun and planets orbited around it. Post-Copernicus, we realised that we are part of a planetary system orbiting the Sun. We’ve since realised that there are many, many other planetary systems orbiting other stars; our Sun is just one of billions of stars in our galaxy; and our galaxy is just one of many billions of galaxies that can be seen through a large telescope.

Our entire universe is a dynamic entity. Everything started off very compressed and compact, and about 13.8 billion years ago it started expanding, and it gradually cooled down. Stars and galaxies emerged, and we are coming to understand how this happened. Indeed, when I was starting my research about 50 years ago, it wasn’t clear that there was a beginning. There was a rival theory called the steady-state theory, according to which our universe had existed from everlasting to everlasting, and although it is expanding, new galaxies form in the gaps as the old ones moved apart.

Problems with the steady-state model

This theory was certainly tenable until 50 years ago. Then, two things happened. First, it became possible to look at objects so far away that we were looking back billions of years into the past. In the steady-state theory, things would have looked the same then as now, but it was clear that the galaxies were different far away and, therefore, far back in time, so the universe and the things in it were evolving.

Even more importantly, evidence was found that the entire universe is full of microwave radiation coming from all directions, which can’t be understood except as the afterglow of the hot beginning of the universe. The universe started up very hot and dense. It expanded and cooled. But this radiation is still around. It fills the universe. It’s got nowhere else to go. This is really unambiguous evidence that the universe had a hot, dense beginning, and later data pinned down the time of that as about 13.8 billion years ago.

Key questions in astronomy today

We are trying to understand how the universe evolved from that beginning to its present stage – how the first galaxies formed, how the first stars formed. Why do galaxies have the properties they do, and why do stars evolve as they do?

One of the most exciting developments in the last 20 years has been the definite realisation that our solar system is not unique. Most of the stars in the sky are other suns, and they have a retinue of planets orbiting around them, just as the Sun has the Earth and the other familiar planets orbiting around it. So this is an exciting new subject. Unfortunately, these planets orbiting other stars are so faint that we can’t really see them with existing instruments. But for the next generation of telescopes, we’ll actually be able to study the light from planets orbiting other stars. And this, of course, leads to the most exciting question, which everyone asks me when they know I’m an astronomer: is there life out there?

We’d like to know if on these planets around other stars there could be life, vegetation, even intelligent life. It’s worth a search. And within 10 years, I think we will know whether some of these planets, like the Earth, orbiting other stars, like the Sun, have vegetation, oxygen-rich atmospheres and other indicators that there’s been biological evolution. At the moment, we only know that life exists in one place, but if we could find it in a few nearby planets, around other stars, that would tell us that the entire universe must be teeming with life, and that’ll be the most fascinating discovery of all time.

Corroborating the Big Bang

I’ve been part of a collective enterprise over the last 50 years trying to make sense of what telescopes – which are ever more powerful and varied – tell us. One area which has made great progress is understanding the early stages of the universe.

Photo by Zora

We can say what the universe was like when it had been expanding just for a few seconds. We can say that because at that time, its temperature would be about 10 billion degrees. That’s a temperature where nuclear reactions occur, turning hydrogen into helium and deuterium and other substances. One of the triumphs of our understanding is that our predictions of the proportions of those kinds of atoms, if they were made in the Big Bang, agree with what’s observed. So we have corroboration of the Big Bang, right back to when the universe was a second old.

But we’d like to go back even further because there are still many mysteries. In particular, we don’t know why the universe is expanding the way it is. We don’t know why it has the mixture of things it has: it’s got some radiation; it’s got some atoms; it’s also got some mysterious stuff we call dark matter. We’d like to understand this. The answer to those questions requires us to understand not just the first second, but the first tiny, tiny fraction of a second.

The challenge of going back to the very beginning

The problem is that if we imagine extrapolating back closer and closer to the original instant, everything gets denser and hotter, and the physics become more extreme. For the first nanosecond, the physics is far more extreme than anything we can simulate in the lab, even in a big machine like the particle accelerator at CERN in Geneva.

We can go back to a nanosecond. But before that time, we have to just extrapolate the physics without being able to battle-test it by experiments. So the first nanosecond is still speculative, and unfortunately, many of the key features of our universe – its content and the way it’s expanding – may have been determined back then. That’s still a challenge.

But it’s a huge triumph that over 50 years, we’ve got to a stage from when we didn’t know if there was a Big Bang at all to being able to talk about what happened, from when the universe was a nanosecond old to when it was 13.8 billion years old. That is what we are trying to do now. And we’re also trying to understand how the entities in it, mainly galaxies, evolve, and what their structure is.

We’re in a galaxy called the Milky Way, which is rather typical of other big galaxies. If you look with a small telescope, you can see the Andromeda Galaxy, which is three million light years away. That’s rather similar to what our Milky Way would look like if we could get outside it and look at it.

Star explosions and Crab Nebula

Stars are really gravitationally confined nuclear reactors. They’re very massive. Gravity holds them together, and they are very hot in the centre. Nuclear fusion reactions take place inside stars. Stars gradually burn their hydrogen to helium, and then they burn that helium further up the periodic table. And stars eventually die.

The Sun has been shining for four and a half billion years, but after another six billion years, it’ll run out of fuel, and it’ll then blow off its outer layers and settle down as a white dwarf star. Heavier stars burn their fuel more rapidly and therefore face an energy crisis earlier on, and then they explode. They leave behind a dense remnant, a neutron star or black hole, and blow off their outer layers. And this is something which you might think is entirely recondite and irrelevant to us, but it’s crucial for our existence.

Photo by Outer Space

To illustrate this, one of the most amazing objects in the sky is something called the Crab Nebula, which is expanding debris from a star that exploded in the year 1054 AD, nearly a thousand years ago. This exploding star was witnessed and recorded by the Chinese court astronomer, who reported to the Chinese emperor that a guest star had appeared, and it became brighter than the moon and faded after a few weeks. At that place in the sky, we see this expanding glowing debris of the remnants of the star.

We are the ashes of dead stars

Now, why is that relevant to us? We wouldn’t be here were it not for things like the Crab Nebula, because before a star explodes, it turns hydrogen into helium, and then it turns helium into carbon, carbon into oxygen, oxygen into iron etc. Stars have an onionskin structure, where the inner parts are processed higher up the periodic table. We can calculate that when a massive star explodes, it flings back into space gas, which has all the chemical elements in it. And that is how the chemical elements in our universe are made, and then new stars condense from those.

When we look at our solar system, it contains all the atoms, like oxygen, carbon and iron, that we are made of. It makes us feel an intimate part of the universe when we realise that every atom in our bodies was forged in an ancient star which lived and died before our solar system formed, maybe five billion years ago, and then our Sun and our planets condensed from gas, contaminated by the debris from earlier generations of stars. We are literally the ashes of long dead stars. Or, if you are less romantic, we are the nuclear waste from the fuel that made stars shine. This theory, which dates back to the 1950s, is really one of the great achievements – up on the level with Darwin – enabling us to understand how we and all the chemical elements were made in stars.

Discover more about

exploring the cosmos

Rees, M. (2018). Our Cosmic Habitat. Princeton University Press.

Rees, M. (2001). Just Six Numbers. Hachette.

Rees, M. (1998). Before the Beginning: Our Universe and Others. Basic Books.

Planning ahead for a decisive century

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