Astronomy is one of the first sciences – the ancient Greeks were already studying the heavens in the days of Democritus, more than 2500 years ago. But it is also in a sense the final science, because as we learn more about the universe it is abundantly clear that man will never really explore it.

Our galaxy, the Milky Way, is a hundred million light years across, so that even if we could travel at the speed of light we could never hope to explore more than a fraction of it. And there are altogether some two trillion galaxies in the universe.

In the excitement of renewed travel to the Moon and to Mars it is too easy to imagine that humanity will continue to explore ever outwards but it just ain’t so. Doubtless there will be robot probes sent to the outer planets of the solar system but it’s impossible to imagine such probes reaching stars or planets only a thousand light years away – quite close in galactic terms. After all, even with travel at the speed of light that would mean waiting a thousand years for the robot to reach its destination and another thousand years to get any information back from such a probe. It won’t happen.

What that means is that humans will explore Mars and perhaps a few moons beyond that; robot probes will go considerably further; but for the other 99%+ of the universe we will have to rely on what we can learn through telescopes. And although the age of the telescope began with Galileo, in a more profound sense it – and the whole science of astronomy – only began in 1923, when Edwin Hubble discovered that our galaxy was not alone.

Until then, astronomers worked on the basis of one solar system around one star, the Sun, in one galaxy: that was then the whole universe and it existed, unmoving, in a steady state.

Hubble showed that all this was wrong. There were any number of stars and galaxies and the whole universe was expanding at more than 100km a second. And that began an extraordinary race for knowledge, one that has accelerated since we have begun to shoot telescopes into space: we have learnt more about the universe in the last thirty years than in all recorded time before that.

Greatest Scientist

Hubble, perhaps the greatest scientist of his century, was a studious young man and a wonderful athlete. He excelled at baseball, American football, basketball, and on the running track. He studied maths, astronomy, and philosophy at the University of Chicago and won a Rhodes Scholarship to Oxford. However, his father, on his deathbed, made Edwin promise that he would study law as a solid career.

This he did, though his time at Oxford was more remarkable for his staggering athletic performances – he would sometimes win as many as seven events at track and field in a single day. Edwin got his law degree, returned to America, fought in World War I, tried to practise law, utterly hated it and instead became a schoolteacher.

Gradually, he decided to return to his first love, astronomy, and spent the rest of his life at the Mount Wilson Observatory in California, apart from World War II, when he worked on ballistics and became an army major. Only after Hubble’s death did the Nobel Committee recognise astronomy as a science worthy of a prize: otherwise, Hubble would doubtless have been one of the most famous Nobel laureates.

Einstein, who had worked out that the universe must be either expanding or contracting, had invented a constant that had the effect of showing a steady-state universe. But in 1931 Einstein went to Mount Wilson to meet Hubble. He recognised that Hubble was right: the universe was expanding, and he publicly changed his mind. He said that inventing the cosmological constant has been “the greatest blunder of my life”.

In 2013 the Gaia telescope was launched into space by the European Space Agency and was positioned at a Lagrange point 1.5 million kilometres from Earth. Its mission was to construct a 3D map of the Milky Way with the most accurate system possible: every star was measured 70 times. Its mission was to map a billion stars over its presumed five-year lifespan.

In fact, it mapped nearly two billion stars before it ran out of fuel and had to be switched off on 27 March 2025. It was then sent into a “retirement orbit” around the Sun, leaving the precious Lagrange point free for other uses.

Revolutionised

Gaia has already revolutionised our knowledge of our galaxy. And every large telescope is a time machine, analysing light (and thus information) that has travelled for thousands or even millions of years. Accordingly, it has been able to reveal a good deal of the history – and the future – of the Milky Way.

But it takes much time and work to analyse thousands of pages of computer printouts, understand their full import, and turn all that into decent prose. So, as yet, less than a third of its findings have been released. A further data release will occur later this year and a final release will happen in 2030. The world’s astronomers leap upon each release, analysing it for the huge amounts of new knowledge it brings.

Our galaxy, the Milky Way, revolves around Sagittarius A, the supermassive black hole at its centre, which has more than four million times the mass of our Sun. Over 80% of all the matter in our galaxy is dark matter, with its concentration thickest as one moves outward. Our solar system is about two-thirds of the way out from the centre in one of the galaxy’s spiral arms. The Milky Way is one of the three biggest galaxies (Andromeda is the biggest) in what is known as the Local Group of galaxies, including over a trillion stars.

The Local Group in turn is part of the Virgo Supercluster, and Virgo is part of the Laniakea Supercluster, including over 100 000 galaxies and stretching over 520 million light years. Laniakea in turn is part of the Pisces-Cetus Supercluster Complex – the largest structure known to us, containing over one quintillion stars and stretching over a billion light years in length.

Galaxies are not static collections of stars and planets. They are all in motion and they sometimes collide or, more frequently, deal one another a glancing blow. Already Gaia has been able to point to some stars in the Milky Way that were clearly not originally part of our galaxy but that have been captured by it as it has collided with or at least brushed against some smaller galaxy.

When this occurs the conflicting and massive gravitational pull of the larger galaxy always triumphs in the end, though there may be many local instances where the concentration of stars in a smaller galaxy may pull large numbers of stars and planets away from their previous positions in the bigger galaxy.

In the comic-book imagination, a collision of galaxies means stars and planets crashing into one another. In fact, this is vanishingly rare. Space is so enormous and empty that such collisions almost never happen.

Nonetheless, the effect is very great as stars, planets, dark matter, and huge clouds of gas and dust are whirled around, compressed, or expanded. As a result. huge amounts of energy are released into a galaxy, and this triggers star-formation.

Old As the Universe

Gaia has been able to study this. It shows that the Milky Way, which is 13 billion years old, almost as old as the universe, is orbited by at least 59 smaller satellite galaxies, the largest of which is the Sagittarius Dwarf galaxy, which is about 10 000 light years across. It is slowly being devoured by the Milky Way each time that it comes a bit too close. This has happened on three occasions – between five and six billion years ago, two billion years ago, and one billion years ago.

On each occasion this triggered a period of intense star formation. Clouds of gas and dust get compressed by gravitation and as that occurs, they heat up. Ultimately, as the gas and dust become denser, they develop their own strong gravity, creating further compression and greater heat. Sometimes, as this process occurs, the fledgling star swallows older, dead or dying stars.

In the end the new star ignites and bursts into life, with the left-over matter from this process gradually becoming planets and moons in orbit around the new star.

And, it seems, this is how our Sun and its solar system was formed. When the Sagittarius Dwarf first impacted the Milky Way between five and six billion years ago, a process of star formation was triggered and 4.6 billion years ago our Sun came into being, with its planets just a little way behind – the Earth is now 4.54 billion years old.

At least, this is what Gaia suggests, though we have no absolute proof. It may be that further evidence will be revealed as Gaia discharges more data. It is incredibly exciting that at last we may have discovered our own creation story, that by sending a telescope up into space we may have at last understood where, so to speak, we came in.

Nothing Remarkable

Not that there was anything remarkable about our Sun or solar system. The Sun was, thankfully, not blue hot nor even really white hot. But because the third planet out, Earth, lay in the “Goldilocks” region – warm enough to have liquid water but not too close to our Sun to be too hot or to have its own atmosphere burned off – life was possible on Earth or, more precisely, in its oceans. At first those life forms stayed in the depths of the ocean, drawing energy from thermal vents – and thus ultimately from the Earth’s molten core. But as time went by those life forms evolved and developed and began to explore the ocean nearer the surface.

As that happened, something miraculous occurred. Somehow those life forms developed photosynthesis, enabling them to draw energy from sunlight and convert that into a sugary compound, thus storing energy. So the life-forms ceased to draw energy from the depths below and instead drew it from the Sun.

As a by-product of that process, more and more oxygen was discharged and this gradually transformed the Earth’s atmosphere, making it more congenial for vegetation and animal life. And with that the whole adventure of evolution and ultimately the birth of consciousness had begun.

We have no way of knowing if the miracle of photosynthesis has occurred on other planets. All we know for certain is that it happened on Earth. And Earth may be unique. Perhaps in time we will discover whether that is so or not. However, with so many stars and doubtless many times that number of planets, the odds are very strong that other forms of life must exist elsewhere.

Consciousness

But consciousness is something else. Humans evolved only late in the day and owe their existence to the flukish fact that a comet had destroyed all the dinosaurs, which had roamed the Earth for many millions of years. For if the dinosaurs had survived, humans would have had little chance against so many apex predators.

If the evolution of humans was inevitable, then it would surely have happened early on, but in fact it happened late and flukishly. All of which suggests that the evolution of creatures with consciousness is not at all common. So, while the universe almost certainly harbours life, it may not contain other creatures with consciousness. We may indeed be alone.

And that is the strangeness about our voyage of discovery into space. As we go further and become ever more reliant for our knowledge, first on robots and then merely on telescopes, we are learning more about ourselves, our planet, and our Sun. It is an exciting journey, but it may not end with the sci-fi imagining of encounters with other, highly developed aliens. Indeed, the odds are rather against it.

But we will gradually come to understand where we came from and whether we are alone. That, too, would be an exciting as well as a frightening conclusion.