NASA
HUBBLE’S MOST IMPORTANT IMAGE
For almost two decades the Hubble Space Telescope has captured the faintest lights and enabled us to rebuild these spectacular images, providing a window onto places billions of light years away and events that happened billions of years ago. These are places forever beyond our reach. However, there is one Hubble image that has done more than any other to reveal the scale, depth and beauty of our universe. Known as the Hubble Ultra Deep Field, this shot was taken over a period of eleven days between 24 September 2003 and 16 January 2004. During this period Hubble focused two of its cameras – the Advanced Camera for Surveys (ACS) and Near Infrared Camera and Multi-object Spectrometer (NICMOS) – on a tiny piece of sky in the southern constellation, Fornax. This area of sky is so tiny that Hubble would have needed fifty such images to photograph the surface of the Moon.
From the surface of Earth this tiny piece of sky is almost completely black; there are virtually no visible stars within it, which is why it was chosen. By using its million-second shutter speed, though, Hubble was able to capture images of unimaginably faint, distant objects in the darkness. The dimmest objects in the image were formed by a single photon of light hitting Hubble’s camera sensors every minute. Almost every one of these points of light is a galaxy; each an island of hundreds of billions of stars, with over 10,000 galaxies visible. If you extend that over the entire sky, it means there are over 100 billion galaxies in the observable Universe, each containing hundreds of billions of suns.
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As we stare at Hubble’s masterpiece we are looking back in time; deep time, time beyond human comprehension…the Hubble Ultra Deep Field transports us back through the history of the Universe.
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However, there is something more remarkable about this image than mere scale, due to the slovenly nature of the speed of light compared to the distances between the galaxies. The thousands of galaxies captured by Hubble are all at different distances from Earth, making this image 3D in a very real sense. But the third dimension is not spatial, it is temporal. As we stare at Hubble’s masterpiece we are looking back in time; deep time, time beyond human comprehension. Just as an ice core leads us back through layer after layer of Earth’s history, so the Hubble Ultra Deep Field transports us back through the history of the Universe.
The photograph contains images of galaxies of various ages, sizes, shapes and colours; some are relatively close to us, some incredibly far away. The nearest galaxies, which appear larger, brighter and have more well-defined spiral and elliptical shapes, are only a billion light years away. Since they would have formed soon after the Big Bang, they are around twelve billion years old. However, it is the small, red, irregular galaxies that are the main attraction here.
There are about 100 of these galaxies in the image, and they are among the most distant objects we have ever seen. Some of these faint red blobs are well over twelve billion light years away, which means that when their light reaches us it has been travelling for almost the entire 13.75-billion-year history of the Universe. The most distant galaxy in the Deep Field, identified in October 2010, is over thirteen billion light years away – so we see it as it was 600,000 years after the beginning of the Universe itself.
It is hard to grasp these vast expanses of space and time. So, consider that the image of this ancient galaxy was created by a handful of photons of light; when they began their journey, released from hot, primordial stars, there was no Earth, no Sun, and only an embryonic and chaotic mass of young stars and dust that would one day evolve into the Milky Way. When these little particles of light had completed almost two-thirds of their journey to Hubble’s cameras, a swirling cloud of interstellar dust collapsed to form our solar system. They were almost here when the first complex life on Earth arose and within a cosmic heartbeat of their final destination when the species that built the Hubble first appeared.
The story hidden within the Hubble Ultra Deep Field image is ancient and detailed, but how can we infer so much from a photograph? The answer lies in our interpretation of the colours of those distant, irregular galaxies
ALL THE COLOURS OF THE RAINBOW
The breathtaking Victoria Falls are one of the most famous and beautiful natural wonders on our planet. Fuelled by the mighty Zambezi River, the falls lie on the border between Zambia and Zimbabwe in southern Africa. The falls were named by David Livingstone in 1855, the first European to see them. He later wrote: ‘No one can imagine the beauty of the view from anything witnessed in England. It had never been seen before by European eyes; but scenes so lovely must have been gazed upon by angels in their flight.’ That’s about right from where I stood. There are few better places on Earth from which you can experience the visceral power of flowing water, but there is an ethereal feature of the falls that is just as enchanting and far more instructive for our purposes, because it holds the key to interpreting the Hubble Deep Field Image.
Hovering in the skies above the falls are magnificent rainbows, a permanent feature in the Zambian skies when the Sun shines through the mist. Rainbows are natural phenomena that have enchanted humans for thousands of years; to see one is to marvel at a simple but beautiful property of light and, as is often the case in nature, they are made more beautiful when you understand the science behind them.
Scientists have attempted to understand rainbows since the time of Aristotle, trying to explain how white light is apparently transformed into colour. Our old friend Ibn al-Haytham was one of the first to attempt to explain the physical basis of a rainbow in the tenth century. He described them as being produced by the ‘light from the Sun as it is reflected by a cloud before reaching the eye’. This isn’t too far from the truth. The basis of our modern understanding was delivered by Isaac Newton, who observed that white light is split into its component colours when passed through a glass prism. He correctly surmised that white light is made up of light of all colours, mixed together. The physics behind the production of a rainbow is essentially the same as that of the prism. Light from the Sun is a mixture of all colours, and water droplets in the sky act like tiny prisms, splitting up the sunlight again. But why the characteristic arc of the rainbow?
The first scientific explanation, which pre-dated Newton by several decades, was given by René Descartes in 1637. Water droplets in the air are essentially little spheres of water, so Descartes considered what happens to a single ray of light from the Sun as it enters a single water droplet. As the diagram opposite illustrates, the light ray from the Sun (S) enters the face of the droplet and is bent slightly. This is known as refraction; light gets deflected when it crosses a boundary between two different substances (point A), then when the light ray gets to the back surface of the raindrop, it is reflected back into the raindrop (point B), finally emerging out of the front again, where it gets bent a little more (point C). The light ray then travels from the raindrop to your eye (E).
The key point is that there is a maximum angle (D) through which light that enters the raindrop gets bounced back. Descartes calculated this angle for red light and found it to be forty-two degrees. For blue light, the angle is forty degrees. Colours between blue and red in the spectrum have maximum angles of reflection of between forty-two and forty degrees. No light gets bounced back with angles greater than this, and it turns out that most of the light gets reflected back at this special, maximum angle. So, here is the explanation for the rainbow. When you look up at a rainbow, imagine drawing a line between the Sun, which must be behind you, through your head and onto the ground in front of you. At an angle of forty-two degrees to this line, you’ll see the so-called rainbow, or Descartes’ ray of red light. At an angle of forty degrees to this line, you’ll see the Descartes’ ray of blue light, and all the colours of the rainbow in between. There is some light reflected back to your eye through shallower angles, which is why the sky is brighter below the arc than above it. You don’t see the colours below the arc because all the rays merge to for
m white light. On the picture on the previous page, you can see the sky brightening inside the rainbow over the Victoria Falls, and the relative darkness of the sky outside it.
So raindrops separate the white sunlight into a rainbow because each of the consituent colours gets reflected back to your eye at a slightly different maximum angle. But why the arc? In fact, rainbows are circular. Think of the imaginary line again between the Sun, your head and the ground. There isn’t just one place at which the angle between this line and the sky is forty-two degrees, there is a whole circle of points surrounding the line. The reason you can’t see a complete circle is that the horizon cuts it off, so you only see the arc. This is also why you tend to see rainbows in the early morning or late afternoon. As the Sun climbs in the sky, the line between the Sun and your head steepens and the rainbow, which is centred on this line, drops closer and closer to the horizon until at some point it will vanish below the horizon.
All the way back to Aristotle, scientists have been trying to understand rainbows and how white light is transformed to colour through this medium. The Victoria Falls are perhaps one of the most spectacular places on Earth to see rainbows; here, these features hover in the sky above the cascading waters whenever the Sun shines through the mist.
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THE ELECTROMAGNETIC SPECTRUM
The electromagnetic spectrum is composed of a range of wavelengths from radio waves at the very longest end to gamma rays at the shortest. Our eyes are sensitive to a limited range in the middle which we know as visible light.
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In fact, rainbows are circular. The reason you can’t see a complete circle is that the horizon cuts it off, so you only see the arc. This is also why you tend to see rainbows in the early morning or late afternoon.
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WHAT MAKES A RAINBOW AN ARC?
Decartes’ theory was based on what happens to a single ray of light from the Sun as it enters a water droplet; he discovered that each colour that makes up this light is refracted, or bent, at slightly different angles to each other.
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These colours hidden in white light are not only revealed in rainbows; wherever sunlight strikes an object the different colours are bounced around or absorbed in different ways. The sky is blue because the blue components of sunlight are more likely to be scattered off air molecules than the other colours. As the Sun drops towards the horizon, and the sunlight has to pass through more of the atmosphere, the chance of scattering rays of yellow and red light increases, turning the evening skies redder. Leaves and grass are green because they absorb blue and red light from the Sun, which they use in photosynthesis, but reflect back the green light.
But what is the difference between the colours that makes them behave so differently? The answer goes back to our understanding of light as an electromagnetic wave. Waves have a wavelength – which is the distance between two peaks (or troughs) of the wave. Blue light has a shorter wavelength than green light, which has a shorter wavelength than red light. Our eye has evolved to discern about ten million different colours, which is to say that it can differentiate between ten million subtle variations in the wavelength of electromagnetic waves. This simple idea is all you need to read the story of the Hubble Ultra Deep Field image
HUBBLE EXPANSION
So, how do we know that the irregular, messy galaxies in the Hubble image are billions of light years away? The picture below shows some of the most distant galaxies we have observed. The most obvious thing about them is that they are all red. Why is this so? To answer this question correctly, we need our friend Edwin Hubble, the astronomer, again.
During the 1920s, Edwin Hubble was using what was then the world’s most powerful telescope at the Mount Wilson Observatory in Pasadena, California, to observe stars called Cepheid variables. These Cepheid variables are stars whose brightness varies regularly over a period of days or months, and they are astonishingly useful to astronomers because the period of their brightening and dimming is directly related to their intrinsic brightness. In other words, it is a simple matter to work out exactly how bright a Cepheid variable star actually is just by watching it brighten and dim for a few months. If you know how bright something really is, then measure how bright it looks to you, you can work out how far away it is. Edwin Hubble’s research project was simply to search for Cepheid variables in the sky and measure their distance from Earth. During his observations, he discovered two remarkable things: firstly, he quickly determined that the Cepheid variables he found in the so-called spiral nebulae (which at the time were thought to be clouds of glowing gas within the Milky Way) were in fact well outside our galaxy. For the first time, Hubble showed that there are other galaxies in the Universe, millions of light years away.
This image shows some of the most distant galaxies that we have observed – and all appear in a bright, sharp, red colour.
Hubble’s second observation was of even greater scientific importance. While he and others were also busy measuring the spectrum of the light from the stars in the spiral nebulae, which thanks to Hubble were now understood to be other galaxies beyond the Milky Way, they quickly observed that many of the galaxies appeared to be emitting light that was redder than it should be. Hubble quantified the amount of reddening in each galaxy as a number called redshift. Remember that red light has a longer wavelength than blue light, so seeing light redshifted simply means the wavelength is longer than expected. Hubble made his second great discovery by plotting a graph of the redshift of the light from the distant galaxies against their distance, which he had calculated from his observations of the Cepheid variables.
To his great surprise, Hubble noticed that his graph was approximately a straight line. This is because the further away a galaxy is, the greater its redshift – i.e. the more its light is stretched, and there is a very simple relationship between the distance and the redshift. Why is this? Well, the interpretation of Hubble’s result is quite remarkable. The more distant the galaxy, the further the light has travelled across the Universe to reach us. Also, the further it has travelled, the more it has been stretched. This relationship between distance travelled and amount of stretching occurs when something very simple but surprising is happening to the Universe. It is expanding! In other words, over the hundreds of millions of years during which the light has been travelling, space itself has been stretching at a relatively constant rate, and this has stretched the wavelength of the light in direct proportion to the distance it has had to travel. This is why the most distant galaxies have the largest redshift – their light has travelled through our expanding universe for longer and has therefore become more stretched. Hubble’s discovery of this so-called ‘cosmological redshift’ was one of the great intellectual moments in twentieth-century science, because he discovered that we live in an expanding universe.
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HUBBLE’S LAW: This diagram illustrates Hubble’s Law; the redshift of the light from distant galaxies is plotted against their actual distance, resulting in a straight line on the graph.
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Stephan’s Quintet is a cluster of five galaxies in the constellation Pegasus, two of which, in the centre, appear to be intertwined. Studying the individual redshifts reveals that one of the galaxies is an interloper: the larger, bluer one at upper left is in fact a foreground galaxy seven times closer to us than the others. So redshifts allow us to create a three-dimensional model of the Universe.
NASA
REDSHIFT
Although first discovered in the early twentieth century, redshifts were really put into their cosmological context through the work of Edwin Hubble. He discovered that there is a very simple relationship between the distance and the redshift of a galaxy – the further away a galaxy is, the greater its redshift. This is because the further light has had to travel, the more the travelling light is stretched, and this occurs when the Universe is expanding.
Nathalie Lees © Harp
erCollins
There is a vast amount of information contained within Hubble’s simple graph. Redshift can be expressed as the amount of stretching you would see if something were flying away from you at a particular speed. The ratio of the redshift expressed in this way to the distance to the galaxy – which is the gradient of the line on Hubble’s graph – is called the Hubble constant. Its value as measured today is 68 kilometres (42 miles) per second, per megaparsec. A megaparsec is a measure of distance commonly used by astronomers – 1 megaparsec is 3.3 million light years. So, another way to think of Hubble’s law is that a galaxy that is 3.3 million light years away will be receding from us at a velocity of about 70 kilometres (45 miles) per second. That’s pretty slow! A galaxy that is 6.6 million light years away will be receding at about 140 kilometres (90 miles) per second, and so on. And further, if you simply invert the Hubble constant, then you get a number with the units of time. For a Hubble constant of 70 kilometres (45 miles) per second per megaparsec, this corresponds to 14.3 billion years, which can be interpreted as the age of the Universe! (For the more mathematically inclined, you can calculate this number easily by converting megaparsecs to kilometres.) As an aside, the attentive reader might have noticed that our current best measurement for the age of the Universe is slightly lower than this, at 13.75 billion years; this is because precision measurements over the last few decades have shown us that the expansion of the Universe is not in strict accord with Hubble’s simple law. The best data we have today tells us that the Universe is accelerating in its expansion due to the presence of something called dark energy.
Wonders of the Universe Page 6
