Wonders of the Universe

by Professor Brian Cox

  Beyond the S-stars, the galactic centre is a melting pot of celestial activity, filled with all sorts of different systems that interact and influence each other. The Arches Cluster is the densest known star cluster in the galaxy. Formed from about 150 young, intensely hot stars that dwarf our sun in size, these stars burn brightly and are consequently very short-lived, exhausting their supply of hydrogen in just a couple of million years. The Quintuplet Cluster contains one of the most luminous stars in our galaxy, the Pistol Star, which is thought to be near the end of its life and on the verge of becoming a supernova (see Chapter 2). It is in central clusters like the Arches and the Quintuplet that the greatest density of stars in our galaxy can be found. As we move out from the crowded galactic centre, the number of stars drops with distance, until we reach the sparse cloud of gas in the outer reaches of the Milky Way known as the Galactic Halo.

  This artist’s impression shows the Arches Cluster, the densest known cluster of young stars in the Milky Way Galaxy.

  NASA

  Along with the Arches Cluster, the Quintuplet Cluster is located near the centre of the Milky Way Galaxy.

  NASA

  The bright white dot in the centre of this image is the Pistol Star, one of the brightest stars in our galaxy.

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  The distance between the Sun and the outermost planet of our solar system, Neptune, is around four light hours – that’s one-sixth of a light day. You would have to lay around 220 million solar systems end to end to cross our galaxy.

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  In 2007, scientists using the Very Large Telescope (VLT) at the Paranal Observatory in Chile were able to observe a star in the Galactic Halo that is thought to be the oldest object in the Milky Way. HE 1523-0901 is a star in the last stages of its life; known as a red giant, it is a vast structure far bigger than our sun, but much cooler at its surface. HE 1523-0901 is interesting because astronomers have been able to measure the precise quantities of five radioactive elements – uranium, thorium, europium, osmium and iridium – in the star. Using a technique very similar to carbon dating (a method archaeologists use to measure the age of organic material on Earth), astronomers have been able to get a precise age for this ancient star. Radioactive dating is an extremely precise and reliable technique when there are multiple ‘radioactive clocks’ ticking away at once. This is why the detection of five radioactive elements in the light from HE 1523-0901 was so important. This dying star turns out to be 13.2 billion years old – that’s almost as old as the Universe itself, which began just over 13.7 billion years ago. The radioactive elements in this star would have been created in the death throes of the first generation of stars, which ended their lives in supernova explosions in the first half a billion years of the life of the Universe (see Chapter 2)

  THE SHAPE OF OUR GALAXY

  As well as being vast and very, very old, our galaxy is also beautifully structured. Known as a barred spiral galaxy, it consists of a bar-shaped core surrounded by a disc of gas, dust and stars that creates individual spiral arms twisting out from the centre. Until very recently, it was thought that our galaxy contained only four spiral arms – Perseus, Norma, Scutum–Centaurus and Carina–Sagittarius, with our sun in an off shoot of the latter called the Orion spur – but there is now thought to be an additional arm, called the Outer arm, an extension to the Norma arm.

  Close to the inner rim of the Orion spur is the most familiar star in our galaxy. The Sun was once thought to be an average star, but we now know that it shines brighter than 95 per cent of all other stars in the Milky Way. It’s known as a main sequence star because it gets all its energy and produces all its light through the fusion of hydrogen into helium. Every second, the Sun burns 600 million tonnes of hydrogen in its core, producing 596 million tonnes of helium in the fusion reaction. The missing four million tonnes of mass emerges as energy, which slowly travels to the Sun’s photosphere, where it is released into the galaxy and across the Universe as light

  The Andromeda Galaxy is our nearest galactic neighbour, and our own Milky Way Galaxy is believed to look very much like it.

  Located 5,000 light years away, the Lagoon Nebula is one of a handful of active star-forming regions in our galaxy that are visible from Earth with the naked eye.

  NASA

  A STAR IS BORN

  Our sun is in the middle of its life cycle, but look out into the Milky Way and we can see the whole cycle of stellar life playing out. Roughly once a year a new light appears in our galaxy, as somewhere in the Milky Way a new star is born.

  The Lagoon Nebula is one such star nursery; within this giant interstellar cloud of gas and dust, new stars are created. Discovered by French astronomer Guillaume Le Gentil in 1747, this is one of a handful of active star-forming regions in our galaxy that are visible with the naked eye. This huge cloud is slowly collapsing under its own gravity, but slightly denser regions gradually accrete more and more matter, and over time these clumps grow massive enough to turn into stars.

  The centre of this vast stellar nursery, known as the Hourglass, is illuminated by an intriguing object known as Herschel 36. This star is thought to be a ‘ZAMS’ star (zero ago main sequence) because it has just begun to produce the dominant part of its energy from hydrogen fusion in its core. Recent measurements suggest that Herschel 36 may actually be three large young stars orbiting around each other, with the entire system having a combined mass of over fifty times that of our sun. This makes Herschel 36 a true system of giants. Eventually Herschel 36 and all the stars in the Milky Way will die, and when they do, many will go out in a blaze of glory.

  Eta Carinae is a pair of billowing gas and dust clouds that are the remnants of a stellar explosion from an unstable star system. The system consists of at least two giant stars, and shines with a brightness four million times that of our sun. One of these stars is thought to be a Wolf-Rayet star. These stars are immense, over twenty times the mass of our sun, and are engaged in a constant struggle to hang onto their outer layers, losing vast amounts of mass every second in a powerful solar wind. In 1843, Eta Carinae became one of the brightest stars in the Universe when it exploded. The blast spat matter out at nearly 2.5 million kilometres (1.5 million miles) an hour, and was so bright that it was thought to be a supernova explosion. Eta Carinae survived intact and remains buried deep inside these clouds, but its days are numbered. Because of its immense mass, the Wolf-Rayet star is using up its hydrogen fuel at a ferocious rate. Within a few hundred thousand years, it is expected that the star will explode in a supernova or even a hypernova (the biggest explosion in the known Universe), although its fate may be sealed a lot sooner. In 2004, an explosion thought to be similar to the 1843 Eta Carinae event was seen in a galaxy over seventy million light years from the Milky Way. Just two years later, the star exploded as a supernova. Eta Carinae is very much closer – at a distance of only 7,500 light years – so as a supernova it may shine so brightly that it will be visible from Earth even in daylight.

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  Out in the Milky Way we can see the whole cycle of stellar life playing out. Roughly once a year a new light appears, as somewhere in the Milky Way a new star is born.

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  Eta Carinae is one of the most massive and visible stars in the night sky, but because of its mass it is also the most volatile and most likely to explode in the near future.

  NASA

  Seeing the light from these distant worlds and watching the life cycle of the Universe unfold is a breathtaking reminder that light is the ultimate messenger; carrying information about the wonders of the Universe to us across interstellar and intergalactic distances. But light does much more than just allow us to see these distant worlds; it allows us to journey back through time, providing a direct and real connection with our past. This seemingly impossible state of affairs is made possible not only because of the information carried by the light, but by the properties of light itself

  Eventually all the stars in the Milky Way will die, man
y in spectacular explosions. Herschel 36 was formed from just such a stellar explosion, which occurred within the Eta Carinae system.

  WHAT IS LIGHT?

  If we aspire to understand the world around us, one of the most basic questions we must ask is about the nature of light. It is the primary way in which we observe our own planet, and the only way we will ever be able to explore the Universe beyond our galaxy. For now, even the stars are far beyond our reach, and we rely on their light alone for information about them. By the seventeenth century, many renowned scientists were studying the properties of light in detail, and parallel advances in engineering and science both provided deep insights and catalysed each other. The studies of Kepler, Galileo and Descartes, and some of the later true greats of physics – Huygens, Hooke and Newton – were all fuelled by the desire to build better lenses for microscopes and telescopes to enable them to explore the Universe on every scale, and to make great scientific discoveries and advances in the basic science itself.

  YOUNG’S DOUBLE-SLIT EXPERIMENT

  By the end of the seventeenth century, two competing theories for light had emerged – both of which are correct. On one side was Sir Isaac Newton, who believed that light was composed of particles – or ‘corpuscles’, as he called them in his Hypothesis of Light, published in 1675. On the other were Newton’s great scientific adversary, Robert Hooke, and the Dutch physicist and astronomer, Christiaan Huygens. The particle/wave debate rumbled on until the turn of the nineteenth century, with most physicists siding with Newton. There were some notable exceptions, including the great mathematician Leonhard Euler, who felt that the phenomena of diffraction could only be explained by a wave theory. In 1801, the English doctor Thomas Young appeared to settle the matter once and for all when he reported the results from his famous double-slit experiment, which clearly showed that light diffracted, and therefore must travel in the form of a wave.

  Diffraction is a fascinating and beautiful phenomena that is very difficult to explain without waves. If you shine light onto a screen through a barrier with a very thin slit cut into it, you don’t see a bright light on the screen opposite the slit, but instead you see a complex but regular pattern of light and dark areas.

  The explanation for this is that when you mix lots of waves together they don’t only have to add up. Imagine two waves on top of each other with exactly the same wavelength and wave height (technically known as the amplitude), but aligned precisely so that the peak of one wave lies directly on the trough of the other (in more technical language, we say that the waves are 180 degrees out of phase), and so the waves cancel each other out. If these waves were light waves you would get darkness! This is exactly what is seen in diffraction experiments through small slits. The slits act like lots of little sources of light, all slightly displaced from one another. This means that there will be places beyond the slits where the waves cancel each other out, and places where they will add up, leading to the light and dark areas seen by experimenters like Young. This was taken as clear evidence that light was some kind of wave – but waves of what?

  The results of Young’s double-slit experiment are revealed in this detailed, wide pattern. The experiment demonstrates the inseparability of the wave and particle natures of light and other quantum particles.

  GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY

  The movement of waves across the ocean can be explained by a set of equations; Maxwell discovered a similar form of equation explained waves within magnetic fields.

  MESSENGERS FROM ACROSS THE OCEAN OF SPACE

  As is often the way in science, the correct explanation for the nature of light came from an unlikely source. In the mid-nineteenth century, the study of electricity and magnetism engaged many great scientific minds. At the Royal Institution in London, Michael Faraday was busy doing what scientists do best – playing around with wire and magnets. He discovered that if you push a magnet through a coil of wire, an electric current flows through the wire while the magnet is moving. This is a generator; the thing that sits in all power stations around the world today, providing us with electricity. Faraday wasn’t interested in inventing the foundation of the modern world, he just wanted to learn about electricity and magnetism. He encoded his experimental findings in mathematical form – known today as Faraday’s Law of Electromagnetic Induction. At around the same time, the French physicist and mathematician André-Marie Ampère discovered that two parallel wires carrying electric currents experience a force between them; this force is still used today to define the ampere, or amp – the unit of electric current. A single amp is defined as the current that must flow along two parallel wires of infinite length and negligible diameter to produce an attractive force of 0.0000007 Newtons between them. Next time you change a thirteen-amp fuse in your plug, you are paying a little tribute to the work of Ampère. Today, the mathematical form of this law is called Ampère’s Law.

  By 1860, a great deal was known about electricity and magnetism. Magnets could be used to make electric currents flow, and flowing electric currents could deflect compass needles in the same way that magnets could. There was clearly a link between these two phenomena, but nobody had come up with a unified description. The breakthrough was made by the Scottish physicist James Clerk Maxwell, who, in a series of papers in 1861 and 1862, developed a single theory of electricity and magnetism that was able to explain all of the experimental work of Faraday, Ampère and others. But Maxwell’s crowning glory came in 1864, when he published a paper that is undoubtedly one of the greatest achievements in the history of science. Albert Einstein later described Maxwell’s 1860s papers as ‘the most profound and the most fruitful that physics has experienced since the time of Newton.’ Maxwell discovered that by unifying electrical and magnetic phenomena together into a single mathematical theory, a startling prediction emerges.

  Electricity and magnetism can be unified by introducing two new concepts: electric and magnetic fields. The idea of a field is central to modern physics; a simple example of something that can be represented by a field is the temperature in a room. If you could measure the temperature at each point in the room and note it down, eventually you would have a vast array of numbers that described how the temperature changes from the door to the windows and from the floor to the ceiling. This array of numbers is called the temperature field. In a similar way, you could introduce the concept of a magnetic field by holding a compass at places around a wire carrying an electric current and noting down how much the needle deflects, and in what direction. The numbers and directions are the magnetic field. This might seem rather abstract and not much of a simplification, but Maxwell found that by introducing the electric and magnetic fields and placing them centre stage, he was able to write down a single set of equations that described all the known electrical and magnetic phenomena.

  These picture strips illustrate maps of the Milky Way Galaxy as they appear in different wavelength regions.

  NASA

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  Maxwell’s equations had exactly the same form as the equations that describe how soundwaves move through air or how water waves move through the ocean.

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  THE RELATIONSHIP BETWEEN ELECTRICITY, MAGNETISM AND THE SPEED OF LIGHT IS SUMMARIZED IN THE EQUATION:

  Where c is the speed of light and the quantities 0 and 0 are related to the strengths of electric and magnetic fields. The fact that the velocity of light can be measured experimentally on a bench top with wires and magnets was the key piece of evidence that light is an electromagnetic wave.

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  At this point you may be wondering what all this has to do with the story of light. Well, here is something profound that provides a glimpse into the true power and beauty of modern physics. In writing down his laws of electricity and magnetism using fields, Maxwell noticed that by using a bit of simple mathematics, he could rearrange his equations into a more compact and magically revealing form. His new equations took the form of what ar
e known as wave equations. In other words, they had exactly the same form as the equations that describe how soundwaves move through air or how water waves move through the ocean. But waves of what? The waves Maxwell discovered were waves in the electric and magnetic fields themselves. His equations showed that as an electric field changes, it creates a changing magnetic field. But in turn as the magnetic field changes, it creates a changing electric field, which creates a changing magnetic field, and so on. In other words, once you’ve wiggled a few electric charges around to create a changing electric and magnetic field, you can take the charges away and the fields will continue sloshing around – as one falls, the other will rise. And this will continue to happen forever, as long as you do nothing to them.