The Hubble Ultra Deep Field is one of the most spectacular and important pictures taken by the Hubble Space Telescope. This image shows nearly 10,000 galaxies of various ages, sizes, shapes and colours. The nearest galaxies appear larger and brighter, but there are also around one hundred galaxies here that appear as small red objects. These are the most remarkable features in this image; these are among the most distant objects we have ever seen.
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.
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.
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
