ONE

Our Changing View of the Night Sky

IT IS NO surprise that our ancestors used to think the Earth was at the centre of everything. If you think about it, why would they, or you, have reason to believe any different? Look at the sky in daytime and the Sun seems to move silently around the Earth, rising in the east and setting in the west, taking a day to complete the trip. Look at the night sky and the stars and Moon all seem to travel in a path around us. Even if you make the assumption that everything in the sky is stuck by cosmic glue to the inside of some great celestial sphere then the conclusion is simple: the Earth must be at the centre of it, right? Wrong!

If you look carefully at the sky, particularly at night, there are subtle clues that all is not quite as it seems and it is the careful decoding of these that has developed today’s three-dimensional view of the Universe. In this chapter we are going to take a look at the incredible story of our changing view of the cosmos and at some of the insightful observations of philosophers and astronomers through the centuries.

While it is fair to say not a lot of thought went into the Earth’s place in the cosmos during the early days, there was still an awareness of the motions of the objects in the sky. We know that prehistoric hunters used the sky to mark the passage of time because ancient animal bones have been found with marks thought to represent the changing phases of the Moon. And who can forget the mysterious Stonehenge, which is thought to have been completed as far back as 1600 BC and is believed by many to be an ancient astronomical observatory? It is clear that our ancestors were aware of the sky, but they seemed to be passive observers for many centuries.

It is surprising that we can go all the way back to around 450 BC, to the time of the Greek philosopher Anaxagoras, and find that scientific reasoning was already slowly starting to sow the seeds for our current view of the Universe. Anaxagoras suggested that the Sun was a flaming ball of metal ‘even larger than Peloponnesus’, the large peninsula of southern Greece, and that we saw the Moon because it simply reflected light from the Sun. But the real turning point came around 200 BC, with one of the most insightful observations of all time, not only the realization that the Earth was round but more incredibly the measurement of its circumference.

The Earth is as we know spherical in shape, or more accurately an oblate spheroid (a sphere very slightly squashed at the poles), and we have been brought up with that knowledge, reinforced in modern times by stunning pictures from space. The evidence, though, is there in the sky for anyone to see and amazingly the information is also there for anyone with a basic knowledge of geometry to work it out for themselves.

Eratosthenes was another Greek philosopher who seems to have been annoyingly good at many things from mathematics to poetry and astronomy to athletics! He had heard that at the summer solstice in a town called Syene (now Aswan) in Egypt, the Sun shone directly down to the bottom of a deep well at noon. This meant the Sun was directly overhead, but at the same time in Alexandria the Sun was casting shadows, so could not have been overhead. You may have noticed yourself on your holidays how the Sun climbs higher or lower in the sky depending on where you travel to on the Earth. Eratosthenes realized this but took it a step further.

Knowing that Syene and Alexandria were in a direct north–south line, he set about using this information to work out the circumference of the Earth. The story goes that he actually got someone to walk between the two towns to measure the distance and found they were separated by 5000 stadia. Now, unfortunately, the exact measure of one stadion is unknown but a best guess from historical texts is that 5000 stadia equate to about 925km.

Eratosthenes realized that because he knew the distance between the two locations and the apparent shift in position of the Sun in the sky between them both, he could estimate the Earth’s circumference. Plugging the numbers into some reasonably simple formulae he came up with a figure of approximately 250,000 stadia, or 46,250km, which is remarkably close to today’s figure of 40,075km. This difference in the distances is largely due to experimental error and the inaccuracy of the measurements used but, even so, the result is pretty impressive without a calculator and for around 200 BC. However, though his calculation was accepted by at least some of his peers at the time, it took many years, even centuries, before this view of the shape and size of the Earth became widely accepted.

The next great leap in understanding our place in the Universe came from the movement of objects within it. For many centuries it was known that there was a special group of ‘stars’ which seemed to move or wander around the sky; in fact, their modern name, planets, comes from the Greek word planetes, via the Latin planeta, which means ‘wanderer’. In its earliest form, the model of the Solar System known as the geocentric model had the Earth at the centre with the Sun, Moon, planets and stars fixed upon great crystalline spheres all revolving around the Earth. Beyond the last sphere of the stars was the realm of the gods. While everyday experience suggests this might be accurate, careful study of the sky reveals that the planets seem to move at different speeds and sometimes even backwards. The geocentric model did not quite account for this movement so an alternative explanation was needed.

The backward, or retrograde, motion was later explained in a modification to the geocentric model by Claudius Ptolemy in the second century AD, and, although complex, it did seem to make sense of this odd planetary motion. Ptolemy suggested that each planet was not orbiting the Earth but instead orbited a point (the epicycle), which in turn travelled along a path (the deferent) around the Earth. This meant the planets’ orbits described a rather strange corkscrew pattern through space. In time, the model was modified even more, becoming increasingly complex to account for the observed planetary movements.

The model changed around the sixteenth century when the Polish astronomer Nicolaus Copernicus made the rather bold yet unpopular suggestion that the Earth was not at the centre of everything. Copernicus was also a Catholic cleric and making statements to dethrone the Earth from its central position was not popular with the church. His idea was not new though, as it had been first suggested as far back as the third century BC by Aristarchus, but it was not until careful and precise observations of the planets by Copernicus that evidence started to mount. He realized that it was possible to do away with the complex patterns of epicycles and deferents if the Sun rather than the Earth was at the centre of the Solar System, and if the Earth orbited the Sun like all the other planets.

The key reasoning behind Copernicus’ theory was that the strange movement of the planets was just an apparent and not an actual one. In the same way that a fast car overtakes a slower car, the slower one ‘seems’ to move backwards although in reality it is still going forwards. Unfortunately the new Sun-centred view of the Solar System still required a number of epicycles and deferents but nowhere near as many as were needed in the Earth-centred model.

One of the main issues holding back scientific reasoning and the development of new ideas in these early years was the supremacy of religion. It was considered that all things in the sky from the Moon to the distant stars were divinely created so were nothing short of perfect. The only shape deemed to be perfect was the circle so all things in the sky should be circular and, if they moved, they should move in circular orbits. The reliance on keeping everything circular proved to be a sticking point for some time, and had philosophers and scientists overcome this they might have reached the right conclusion much earlier instead of getting distracted by epicycles and deferents.

The turning point was finally reached by Johannes Kepler, who for a good part of his life was assistant to the renowned Danish astronomer and nobleman Tycho Brahe. In 1572 Brahe observed a ‘new star’ in the constellation of Cassiopeia and noticed its gradually fading light. He also discovered that, due to a lack of parallax (which is an apparent shift in a star’s position in relation to the Earth’s movement in space), it must be at a very great distance from the Earth and therefore be on the outer sphere of the stars and well beyond the planets. This discovery did not sit well with the church as the heavens were supposed to be perfect and unchanging.

Brahe was a prolific observer and left it to Kepler to analyse his countless observations of the planet Mars. Unfortunately for Kepler, Brahe was incredibly protective over his observations so he only let Kepler look at them when absolutely necessary and certainly did not let him copy the records so he could spend more time over them. It was the death of Brahe in 1601 that proved to be a catalyst for Kepler and his work. He was named as Brahe’s successor as Imperial Mathematician to the Holy Roman Emperor Rudolph II. In his new role, Kepler was charged with providing astrological data to the Emperor in an era when there was no differentiation between astrology and astronomy: it was merely a study of the sky. Kepler straddled what we might now consider two modern disciplines in both providing guidance to the Emperor and studying the physical properties of the sky. In his quest to understand the nature of orbits, he hit on an idea which was a precursor to the property we know as gravity. He suggested that the Sun, which like Copernicus he accepted to be at the centre of the system, had a force that was making the planets move, and even suggested it would be weaker at greater distances. This would make the planets further from the Sun move more slowly.

The discovery for which Kepler is best known came from his continued study of Brahe’s observations, which he now had complete access to. These accurate observations of the position of the red planet did not fit with the prediction of the Copernican model, and to account for the discrepancy an ever-increasing level of complexity with more epicycles and deferents had to be employed. He even tried changing the nature of the supposed spherical orbits for an ovoid, or egg-shaped, system but this too proved to be far too complex. After almost fifty attempts at tweaking the theory to match observation he abandoned the use of the ovoid and tried elliptical orbits along a plane. Finally he had hit on a solution that worked and in 1609 was able to articulate this in his three laws of planetary motion:

1. Planets move around the Sun in ellipses, with the Sun at one focus. Essentially this simply says that the distance between a planet and the Sun varies.
2. The line connecting the Sun to a planet sweeps out equal areas in equal times. When the planet is closer to the Sun it moves faster than when it is further away.
3. The square of the orbital period of a planet is proportional to the cube of its mean distance from the Sun. Sounds scary but all it means is that you can work out how far away a planet is from the Sun if you know how long it takes to complete an orbit.

Not only did Kepler’s laws of planetary motion beautifully explain and predict planetary orbits in the Solar System, but when the moons around the outer planets were discovered, the laws held firm to explain their motion around the parent body. Along with Newton’s law of gravity the science is still being used today to send spacecraft to the planets and to understand the movements of planets around other stars.

In 1608, the year before Kepler published his laws of planetary motion, another pivotal discovery had been made. Quite how the Dutch spectacle-maker Hans Lippershey came across the idea is unknown, but to him is attributed the invention of a device ‘for seeing things far away as if they were nearby’, which we now call the telescope. Just a year later the Italian astronomer and philosopher Galileo Galilei heard about the device and going on a rather sketchy explanation managed to re-create one for himself with 3x magnification. He later made some enhancements to the design and managed to achieve a higher magnification of 30x.

Galileo is most famous for being the first person to turn one of these newly invented telescopes to the sky and in doing so making some incredible discoveries. He first used his telescope to look at the planet Jupiter in January 1610 and found what he described as ‘three fixed stars invisible by their smallness’ that lie along a line with the planet. He noted that they moved position from night to night and on one occasion one of them even disappeared. He concluded that these strange objects must actually be in orbit around the giant planet and discovered a fourth just a few days later. He had found four of Jupiter’s moons: Io, Europa, Ganymede and Callisto. This discovery had far-reaching implications though, as it was the first piece of evidence that bodies orbited around something other than the Earth, it supported the idea that the Earth was not really that special in our Solar System after all.

Later the same year Galileo studied the planet Venus and noticed that he could track a full series of phases just like the phases of the Moon. He realized that this too was evidence of the Copernican model of the Solar System with the Sun at the centre since this simply could not happen in the Earth-centred system. Galileo’s discoveries were not only ground-breaking but instrumental in turning the tide from religious belief governing astronomy to science and observation.

Up until now, we’ve seen how it was discovered that the Earth was a sphere, that the Sun was at the heart of our planetary system and that all planets moved in elliptical orbits. The invention of the telescope and subsequent observations set strong foundations for our modern view of the Solar System but still the distances to the stars and the nature of deep space proved to be a mystery.

The next leap forward came almost 230 years later – after the German mathematician and astronomer Friedrich Bessel became the first to measure the parallax of a star. He studied the star 61 Cygni in the constellation of Cygnus and found it seemed to shift by 0.314 of an arc-second (1/5400th the size of the full moon), which was caused by the Earth’s changing position in its yearly journey around the Sun. When this tiny measurement is plugged into a special formula it reveals that the star is at a distance for this shift of 10.4 light years. By the end of the 1800s around sixty stars had been identified as displaying a measurable shift in their position and for the first time in history we had a real understanding of interstellar distance, at least in the nearby Universe. But for the rest of the stars, it remained a mystery.

Telescopic studies also revealed that the strange hazy band of light from the Milky Way arching across the sky was actually millions of distant glittering stars. Quite why there was a concentration of stars in a band across the sky was a subject for much discussion among astronomers. The answer eluded them for nearly two centuries after Galileo’s initial observations but the development of even larger telescopes proved to be crucial in finding the explanation.

William Herschel was a musician who pursued astronomy in his spare time, much like today’s amateur astronomers. His ambitions, though, were big, matched only by his passion for the night sky, so he built his own large telescopes and turned them on the sky to discover new worlds like the planet Uranus. He also spent his time counting stars, a time-consuming and tedious activity, but on counting their numbers in different parts of the sky, he believed he could find the area of highest concentration, which would be the direction of the galactic centre. His search found no such area so he concluded the Sun and its system of planets must sit at the centre of a vast disc-like structure.

It took another hundred years before an astronomer working at the Mount Wilson Observatory in the United States came up with a more accurate and reliable method for estimating the size and scale of the Milky Way. Harlow Shapley was using a huge 1.5m reflecting telescope to study a type of object called a globular cluster. As their name suggests, these are spherically shaped clusters of stars but Shapley realized that a high proportion of the clusters were host to a special type of star.

These special stars are now known as Cepheid Variables and they have a very particular property. Firstly, as ‘variable’ implies, the amount of light they give off varies, but more interestingly there is a correlation between how long they take to change their brightness and how bright they are at maximum output. Knowing this, if I were to measure how long the star takes to fade from brightest to dimmest, I would be able to infer how much light it is actually giving off. From this, I could measure how bright it seems in the sky and, because light fades over distance, I could work out how far away it is. Shapley knew this and calculated the distances to hundreds of Cepheid Variable stars and therefore the globular clusters themselves.

He found the clusters seemed to be roughly distributed in a spherical halo but this halo was not centred on our Sun; instead it seemed to be centred on a point around 32,600 light years away, in the direction of the constellation of Sagittarius. He also deduced that the galaxy was about 100,000 light years in diameter. A light year is a unit of distance used in astronomy and is equal to the distance light can travel in one year at a speed of around 300,000km per second, a pretty big number by anyone’s reckoning, so Shapley’s estimate meant that light reaching the Earth from the galactic centre took around 32,600 years to reach us. He was not far off with his estimate, which was an incredible feat: the currently accepted figure is between 26,000 and 28,000 light years.

The final step in understanding our place in the Universe was made by an American astronomer, Edwin Hubble (after whom the Hubble Space Telescope is named). He, like Shapley, had been employed by the Mount Wilson Observatory in 1919 as a junior astronomer. In his time there, he set about studying the strange spiral nebulae – interstellar dust clouds – which appeared fuzzy in the sky and were thought to be star-forming regions in the Milky Way.

Using the recently completed 2.5m reflecting telescope, Hubble took a succession of pictures of the spiral nebulae, including the so-called Andromeda Nebula. By comparing pictures taken over successive nights he was able to spot changes and on 4 October 1923 he identified another of the Cepheid Variable stars, but this time it was in the Andromeda Nebula. Using the same method as Shapley, he was able to determine the distance to the star and hence to the nebula, and to the surprise of his colleagues, who all thought it would be within our galaxy, he found it to be 900,000 light years away and far beyond the Milky Way. By discovering and measuring the distance to other galaxies, Hubble had increased the size of the known Universe significantly.

Following on from his discovery, it was realized that the type of Cepheid Variable Hubble was observing was a previously undiscovered brighter sort, and comparing it to fainter ones in our own galaxy meant the estimated distance to Andromeda doubled overnight to almost 2 million light years. We now know from even more accurate methods that the distance to the newly renamed Andromeda Galaxy is 2.3 million light years.

Hubble did not stop there though. He busied himself to measure the speed and direction of the motion of the galaxies, continuing the work of his fellow American astronomer Vesto Slipher just ten years earlier. He did this by splitting the incoming light from the galaxy into its component parts using a spectroscope, much like water drops split incoming sunlight into a rainbow. By measuring certain properties of the spectrum, Hubble deduced that some galaxies, such as Andromeda, were moving towards the Milky Way while others were moving away. This was true of the nearby galaxies, but at larger distances it seemed everything was moving away from us at quite incredible speeds. This either meant that the Milky Way was the centre of the Universe or that Hubble had discovered a general expansion of the Universe. A great way to visualize this is to imagine you live on the surface of a balloon and as it is blown up all points on the surface seem to rush away from you. There is no centre of expansion; it is just that every part of the balloon is moving away from every other part.

It seems then that our galaxy, the Milky Way, is one of millions if not billions of galaxies, all of which are like islands in space, made up of stars, clusters, nebulae and even other planets. Hubble’s discovery marks the end of our journey for this chapter and notwithstanding a few more recent discoveries broadly explains how we got to understand the general layout and structure of the Universe we see today.

It is interesting to look back at the way this view has evolved since our ancestors crawled out of the caves. It is easy to understand why early civilizations thought that we sat at the centre of the Universe, since everything in the night sky wheeled overhead. Yet with every discovery came a slight blow to the human ego as we slowly but surely were dislodged from the centre of everything. First the Earth was found not to be the centre of the Solar System, then the Solar System was found to be in a fairly remote part of the Milky Way Galaxy, and finally the Milky Way itself was found not to be at the centre of the Universe but instead just one of countless other galaxies all racing away from each other.

In many ways, the journey of understanding the Universe has also been a journey in understanding our place in it. As science progresses and new discoveries are made this view will undoubtedly change, but I am pretty sure the general overview presented in this chapter will stand the test of time. The last few thousand years have been a testament to human ingenuity and courage as great leaps were made often against much opposition, but as you continue in your journey around the night sky, remember you are not just looking out into space but also looking back through the history of mankind.

January: Northern Hemisphere Sky

Looking at the sky it is easy to see why our ancestors believed all stars were fixed onto a giant crystalline sphere surrounding the Earth. The distinctive stars in the constellation Orion are a great example of what might have contributed to this illusion and it is here that we start the January sky guide.

Remember to read the section starting here, explaining how to use these guides and how to find the celestial equator from your location as all guides start there. Once you are looking roughly in the right direction, just to the west of your gaze should be an almost horizontal line of three bright white stars. These are the stars in Orion’s belt and they lie fractionally below the celestial equator and provide a great starting point. At first glance they look like they are all at the same distance from us, but in reality the stars of Orion’s belt each lie at quite different distances. The eastern star, called Alnitak, is 817 light years away, which is a comparable distance to the western star, called Mintaka, at 916 light years. The central star, Alnilam, is a monstrous 1342 light years away, over 50 per cent as far again as Alnitak. The three stars can easily be seen to be shining with a white colour. Move your gaze north and fractionally to the east to see a star which is distinctly red in colour. This is Betelgeuse (which means ‘armpit of the giant’ in Arabic) and it is a red giant star much larger than our Sun. Simply by looking at the colour of the star we can tell that it is cooler than the stars in the belt of Orion and indeed our own Sun.

Following a line from Betelgeuse due north-east takes you to the constellation of Gemini with its two most prominent stars, Castor and Pollux, marking its north-east border. Castor is the white-coloured star to the north with Pollux to its south looking a little more yellow. Roughly halfway between Pollux and Betelgeuse is Gamma Geminorum, which is the third-brightest star in the constellation. Taking a line from Gamma Geminorum to the north-west takes you to Elnath in Auriga. Scan the area of sky halfway between the two with binoculars and you will notice not only lots of stars from the Milky Way but also an area where the stars seem particularly dense. This is M35, a not especially catchy name, but many galaxies, clusters and nebulae do not have real names but catalogue numbers instead, M35’s coming from the Messier catalogue devised by the astronomer Charles Messier in the eighteenth century. Messier was actually a comet hunter and kept stumbling upon objects which were fuzzy and initially looked like distant comets, but they did not move so clearly were not. He catalogued them all so as not to waste time on them again and in so doing produced one of the most popular deep-sky catalogues in use today. Other catalogues exist, such as the New General Catalogue (NGC) and the Index Catalogue (IC), and many objects will have entries in more than one; for example, M31 is also known as NGC224. M35 is a stunning open cluster of stars with around a hundred stars visible through binoculars or small telescopes. Open clusters like this tend to be found inside our galaxy, unlike the globular variety used by Shapley to determine the shape of the Milky Way, which are found in a halo around the outside.

Almost directly due north from Elnath is a bright yellow star called Capella. In appearance it is much like our Sun, although its colour is the only similarity. Capella is the brightest star in the constellation of Auriga and to the naked eye appears to be a single star. In reality it is two binary star systems, four stars in all, just over 42 light years away. Within the boundaries of Auriga, which looks much like a misshapen square, are three other open clusters all found generally about 5 degrees to the north-east of Elnath. They are named M36, M37 and M38 and sweeping the area with binoculars will reveal them as tiny collections of glittering stars.

To the east of Auriga is the faint constellation of Lynx with Ursa Major and the famous collection of stars known as the Plough just beyond. Just under 10 degrees to the north of Castor in Gemini but still in the constellation of Lynx is the globular cluster known as NGC2419. It was discovered in 1788 by William Herschel and at magnitude 9.1 requires a telescope with an aperture of about 10cm to be seen. At a distance of 300,000 light years it is one of the most remote globular clusters of our galaxy.

Starting from Gemini again and moving south is another example of a star which, like Capella, is in fact a binary system. It is found by looking to the east of Betelgeuse, at a bright star which looks yellow-white in colour. This is Procyon in the constellation of Canis Minor and it lies 11.4 light years away. The larger of the two stars, Procyon A, is twice the diameter of the Sun and the smaller, Procyon B, is a white dwarf star one tenth of the Sun’s diameter.

Starting from the belt of Orion again, look over your right shoulder and you will see the Great Square of Pegasus setting in the west. It is part of the constellation of Pegasus, representing the winged horse, and at this time of year it is up on its corner looking more like a diamond than a square. The northernmost star, called Alpheratz, is the one we are particularly interested in as we can use it to ‘star-hop’ to a real treasure and one that will take you into deep space. From Alpheratz, which is a star also shared with the constellation of Andromeda, continue moving north, slightly to the east and away from the horizon into the constellation of Andromeda. The line bends slightly past the first star (Delta Andromedae) and stops at the second, which is the second-brightest in the constellation. Now you have to be very careful, as you need to find two quite faint stars, which you can discover by turning westwards as though you were heading towards the north-west horizon. You will find one star which is fainter than the one you have just left and another even fainter one that is only just visible to the naked eye in a good dark sky. To the west of this star is a faint fuzzy patch. Binoculars will show it to be a slightly bigger fuzzy patch, and not surprisingly a telescope will reveal it to be a much bigger fuzzy patch. What you will also see though, if you have a large telescope and the skies are dark, are dark lanes against the smudge of light. It may not look incredibly spectacular but, as photographs show, it is a vast swirling mass of stars called the Andromeda Galaxy, the nearest major galaxy to our own at a staggering distance of 2.3 million light years. In other words, because of the time it takes light from the Andromeda Galaxy to get to Earth, we are seeing it as it was over 2 million years ago.

January: Southern Hemisphere Sky

Glance along the celestial equator in the January southern sky and you will see the stars in the constellation of Orion are prominent. The three belt stars, Alnitak on the eastern side, Alnilam in the middle and Mintaka on the western side, are prominent white stars. We start this guide from the central star, Alnilam, and imagine a line dropping to the south. After a very short distance you can just make out a faint fuzzy patch, the Orion Nebula. Do not expect to see it as it looks in photographs though, because human eyes are not as sensitive as a camera lens when it comes to detecting colours if objects are faint. Instead of seeing the stunning reds, greens and blues you will see it only as a wispy grey-green object, but it is still definitely worth a look and is probably the best example of star formation in the sky today. If you have a telescope, see if you can spot any of the stars buried deep inside the nebula which are the hot, young stars called the Trapezium cluster. Infra-red telescopes are used to peer deep inside the dust cloud to reveal its hidden secrets. At a distance of 1344 light years we are looking at the nebula as it was 1344 years ago.

To the south-west of the nebula is the bright star Rigel, still in the constellation of Orion. To the naked eye this bright blue supergiant looks like a single star, but telescopic observation reveals a companion star 500 times fainter. Further studies of the light of Rigel’s companion star reveal that it is a very special sort of binary star called a spectroscopic binary. This means Rigel’s companion is visible only when the spectrum of its light is studied and close examination reveals superimposed lines that are the tell-tale sign of particular gases. The movement of the two stars is shown by the movement of the lines against the background spectrum, first towards the blue end and then towards the red as the companion moves first towards and then away from us.

Another bright star is easily spotted close to Rigel, further over to the east, and it is called Sirius in the constellation of Canis Major. Sirius is one of the brightest stars in the sky, rivalled only by the Sun, Moon and planet Venus. It is another binary star system, consisting of two white stars, the brighter one Sirius A and the older and fainter Sirius B. The companion star, Sirius B, is faint at magnitude 8.5 and due to the proximity and brightness of Sirius A is incredibly difficult to observe. Heading a little further south another bright white star is visible, Canopus in the constellation of Carina. This constellation is famous for a nebula that surrounds a star further over in the east, Eta Carinae, which is a binary star with a Wolf-Rayet star, which is one that is losing mass rapidly as a result of stellar winds, in orbit around a larger companion. Just to the east of Canopus is another bright and unmistakable star of the southern hemisphere, Gamma Velorum, or Suhail al Muhlif, meaning ‘Glorious Star of the Oath’ in Arabic. The name Gamma Velorum suggests it is the third-brightest star (gamma is the third letter in the Greek alphabet) in the constellation of Vela, although in reality it is its brightest star. It is a complex multiple star system, the brightest member of which is a spectroscopic binary.

A little further south lies the South Celestial Pole but it is not as easy to locate as the northern celestial equivalent. To find it, look for a group of stars called Crux, or the Southern Cross, which lies to the south-east of Canopus. From there it is easy to follow the stars forming the longer axis of the cross to the South Celestial Pole, which unfortunately is not marked by a bright star, unlike its northern counterpart.

The Southern Cross actually lies along the same line of sight as the plane of our galaxy, the Milky Way, which can be seen arching across the sky from the south-east to the north-west. The band of light we can see making up the Milky Way arises from the combined light of an estimated 400 billion stars.

Looking at the Milky Way with the naked eye, particularly from a dark location, reveals many dark lanes, which are intergalactic dust clouds, but you can also see brighter regions. One particular bright patch of fuzzy light is found to the south of the bright star Canopus, but it seems a little disconnected from the main band of light. This is the Large Magellanic Cloud. This is not an extension of the Milky Way but, instead, is one of the two main satellite galaxies to our own, at a distance of around 160,000 light years. It straddles two constellations, the long and slender Dorado to the north and the much fainter constellation of Mensa to the south.

In 1987 a star in the Large Magellanic Cloud, just to the edge of the Tarantula Nebula, exploded as a supernova, one of the most violent events in the Universe. Before the event was observed visually, a burst of neutrinos (particles with no electrical charge and difficult to detect) was recorded, which it is thought was emitted at the precise moment of core collapse. Imagine a line from the Large Magellanic Cloud to the north all the way to Rigel, and directly between the two is a globular cluster which lies in the faint constellation called Columba. The cluster has the catalogue designation NGC1851 and at 7th magnitude is just about visible with binoculars from a dark site. It lies 39,000 light years from our Solar System and around 54,500 light years from the centre of the Milky Way.

Forming a great triangle in the sky with Rigel near the celestial equator and Canopus to the south is the brightest star in the constellation of Eridanus, called Alpha Eridani. Also known as Achernar, it is a massive star containing about eight times as much material as our Sun and it is about seven times as big. It marks the southernmost tip of the constellation, which is depicted as a great river stretching from the most northernly point of Hydra, weaving through the sky to Cursa, the northernmost star, which lies close to Rigel in Orion.