An Astronomer Gazes at Heavens
Professor Bryan Gaensler
Astronomer and Federation Fellow at The University of Sydney.
The most important photograph ever taken was a picture of nothing.
The Hubble Space Telescope, which for the last 16 years has orbited 600 km above our heads, is the most powerful observatory ever built. In late 1995, Hubble took a break from looking at the usual assortment of stars, planets and nebulae, and instead devoted ten continuous days to staring at a tiny, non-descript part of the sky in the constellation of Ursa Major. Hubble’s field of view is minuscule – each exposure covers an area on the sky about 1/30th the diameter of the full moon in size. And this particular little patch of the sky had been carefully chosen to be as uninteresting as possible, containing just a few faint stars.
So what did Hubble see at the end of this mammoth 10-day exposure? A handful of faint stars, of course. But filling the spaces between these stars was a riot of smudges and whorls, all of different colours and sizes. If you look closely enough at the image, the dark gaps between these faint smudges are filled with yet fainter swirls and streaks. And between these can be seen even fainter, barely detectable blurs of distant light. In fact, this supposedly blank part of the sky hardly has any dark sky at all in it – this little window onto the heavens suggests that the Universe is filled with light.
What are these smudges? Appropriately, Edwin Hubble himself provided the answer more than eighty years ago, when he showed that each of these faint patches was an entire galaxy, each containing hundreds of billions of stars.
Billions of stars! We can try, perhaps fruitlessly, to put this in perspective. Our own Sun, an ordinary star, is 1.4 million kilometers across, and weighs 2000 trillion trillion tonnes. The Sun is a member of the Milky Way Galaxy, which can be seen on a dark night as a silvery band running through the sky. The Milky Way contains about 400 billion other stars of various temperatures and sizes, and spans a million trillion kilometers from side to side. The Milky Way is itself quite unremarkable – it is dwarfed by its neighbour, the Andromeda Galaxy, which sits 20 million trillion km away, and contains another trillion stars. And the Milky Way and Andromeda are both part of a much larger grouping called the Virgo Supercluster, which consists of thousands of galaxies spread across two billion trillion km of interstellar space.
And so we can return to Hubble’s “Deep Field”, the 10-day exposure of what looked like nothing, now realising that every smudge and speck in this image is an entire galaxy, filled with billions of stars and trillions of planets. Perhaps every galaxy harbours a myriad of sentient civilizations, each looking up at the sky with their own telescopes, and marvelling at the spectacle. And all this in a patch of the sky smaller than your fingernail.
If the Hubble Deep Field were nothing more than a work of art illustrating a tiny part of the night sky, it would still be an incredible image, because it reveals the unimaginable scale of the Universe in which we live.
But to the scientist, it shows so much more.
The light that these galaxies emit, and which is eventually received by the Hubble telescope, travels at a specific finite velocity, the cosmic speed limit of 1079 million km per hour. It thus takes time for this light to reach us. And this means not just a slight delay, like a conversation on a bad phone line, but a wait of billions of years. We have no way of knowing what these galaxies look like now – but only how they appeared when the light that we receive now began its journey.
The sky is a time machine, and the galaxies in the Hubble Deep Field are what our Milky Way might have looked like when it was much younger. Using the smudges and colours in the Hubble Deep Field, astronomers can understand how galaxies, the building blocks of the Universe, are formed and evolve. There are few questions more profound than how we got here – through the images taken by Hubble, we have gone a long way toward answering this question.
Even hardened scientists can’t help but feel dwarfed and humbled by contemplating such issues. The Universe is bigger than we can imagine, older than we can contemplate, and more complex than we can conceive. All our dreams and fears, our times of triumph and of sorrow, our sense of history and our hopes for the future, are utterly irrelevant and insignificant when contrasted to the colossal dance that galaxies play out as they drift and spin through billions of years. Our world could end tomorrow and the stars would not notice.
In some sense this realisation is devastating – regardless of what humanity might ever achieve, the Hubble Deep Field shows that we don’t really matter that much. But to me the Deep Field is also a source of comfort, and of exhilaration.
I feel comfort, because no matter what mistakes we make or calamities we suffer, the night sky will still be there. If we lose a loved one or find ourselves in financial difficulties, the stars will still shine. If we are lonely or sad or afraid, galaxies will still fill the sky. As much as life is filled with fleeting opportunities and uncertainty, as we are at times overwhelmed by the feeling that the years are slipping by and that there is not enough time to do the things we want to do, the Universe patiently, irresistibly, evolves, and nothing we can do can change it. There is one great certainty in life, even if we have to look deep into the night sky with an Earth-orbiting telescope to find it.
And I feel exhilaration, because our ability to contemplate and to calculate what’s out there surely signifies the triumph of pure thought. Our knowledge of geology comes from digging up rocks and looking at their composition and structure. Our understanding of biology is derived from dissecting organs and tissue, and by making cells divide in a Petri dish. And our view of chemistry comes from mixing chemicals in a test tube, or from boiling, freezing or purifying various compounds.
But astronomy is very different. We will never travel to these stars. We will never scoop out a sample of a star to weigh; we can never put a galaxy on a bench and poke it or prod it; we can never create a black hole or a supernova or a star cluster in the lab and see what happens if we heat it up or dissolve it in water.
Instead, all we can do is watch. We collect incredibly weak glints of light from unimaginably far away, and carefully measure their brightness and colour, but those are the only clues we will ever get as to how the Universe works.
Surely this is an impossible task – a gargantuan crossword without any clues, a game of poker played blindfolded. We have no right to even ask questions about the cosmos, let alone try and answer them. And yet the defining achievement of the human mind must be that despite the insurmountable barriers of being grounded on a tiny pebble in an irrelevant corner of an unremarkable galaxy, we have figured all this out. We know what makes stars shine; we know how they form, and when they will die. We know how long ago the Universe began, and we know how it will likely end. A century ago, anyone claiming to understand these things would have been laughed at for their simultaneous lack of humility and good sense. But today all this knowledge is in every astronomy textbook. Using just feeble light collected by telescopes, combined with our ideas and ingenuity, we have made huge strides toward understanding the most complex tableau in existence. The Hubble Deep Field, as much as a great work of art or a multi-layered symphony, symbolises humanity’s achievements.
None of this is to suggest that we should ever feel comfortably smug in our studies of the heavens. As far as we’ve come in our quest for knowledge, there is so much that we still don’t understand. The Hubble Deep Field looks filled with light, but we now have incontrovertible evidence that 85% of the material in the Universe is “dark matter” – a mysterious, exotic, diffuse substance that must be all around us, but which frustratingly remains unseen and unexplained. Even more disturbingly, the Universe is also now known to be filled with “dark energy”, a seemingly inexplicable force, at odds with everything we might expect, that acts against gravity and which is pushing the Universe apart. With these substantial problems as a backdrop, dozens of other phenomena remain as yet unexplained. We see stars in our own Galaxy forming from the debris left behind by the previous generation of dying stars, and presumably those stars were formed from the ashes of a yet earlier generation. But how did the very first stars form? How do galaxies assemble? How are planets created? And amongst all the trillions of stars out there, are we humans alone in being able to think, and to wonder? These are all questions that astronomers are now trying to answer. Given the remarkable but steady progression in our understanding, it is not hubris to expect that these answers will come sooner rather than later.
Can all this beauty, complexity and mystery just be an accident? Or is there a guiding hand that has carefully designed the cosmos? This is of course a question that has been asked for thousands of years, and no-one has yet come up with a definitive argument to settle the issue. What is indisputable is that the vast array of complicated objects that we see in the night sky all results from incredibly simple laws of physics and mathematics. Our current understanding is that all the matter around us is built up from just two types of subatomic particle. And all possible interactions between this matter, be it two people shaking hands, or two distant galaxies orbiting one another, are described by just three forces. With just two types of particles, and three types of force that can act between them, one can let the Universe run its course, and stars, galaxies, rainforests, rainbows, dinosaurs, kangaroos, haiku and hip-hop all naturally emerge, after a wait of a few billion years. As many have noted, if the mass of these particles had been just slightly different, or if the strength of one of these forces had been just marginally stronger or weaker, then stars would not form, DNA could not replicate, and we would not be here to ask our questions of the night sky. Each of the thousands of galaxies in the Hubble Deep Field, along with each snowflake in a snowdrift, and each cloud in the sky, is testament to an astonishing underlying elegance, which we are still yet to fully uncover or understand.
Gazing at Heavens, the story comes full circle. What we see first seems to be galaxies laid out on the flat sky. Peer closer, and the picture breaks up into tangled three-dimensional strands of light, showing deeper complexity. Stare longer, and we see still more. Born of star dust billions of years old, having existed as a species for just 0.001% of the age of the Universe, and animated by a mere 1500-gram bundle of fibres and electrical impulses sitting inside our skulls, we have the capacity both to photograph the full complexity of the night sky, and to transform these scientific measurements into an aesthetic expression of grandeur, wonder and beauty. We learn so much about the cosmos and about ourselves by pointing our telescopes at nothing.
This article is ©2006 Bryan Gaensler, and may not be reproduced or distributed without his explicit written permission.