Hubble Galaxy Classification
The Realm of the Galaxies
WE LIVE ON a planet orbiting around a fairly average star which itself orbits around the centre of something we call the Milky Way. Nearly every single object that can be seen in the night sky is also a member of this vast galactic family, which we have named after its ghostly, milky appearance. It is like a giant island in the never-ending emptiness of space and is home to most objects that light up the night-time sky. From the 4000 or so stars that can be seen by the naked eye, to the planets, star clusters and gas clouds, they are the objects that, bound together by the force of gravity, make up our galaxy. Yet there are a couple of objects in the sky, mere smudges to look at, which hint that the Milky Way is not alone.
The reality is that the Universe is peppered with billions of galaxies of different shapes and sizes and with varying numbers of stars in each; indeed, it is often said that there are more stars in the Universe than there are grains of sand on the Earth. Perhaps one of the most exciting aspects of astronomy is that not only are the furthest corners of the cosmos being probed by studying the galaxies, but also the depths of time. The nearest major galaxy to our own, the Andromeda Galaxy, is visible to the naked eye in the constellation of Andromeda and lies a whopping 2.3 million light years away. That is nothing, though, compared to the most distant object so far discovered, a galaxy with the catchy name UDFj-39546284 a mind-boggling 13.2 billion light years away – the light forming the image we see today left that galaxy some 8 billion years before our Solar System began to form.
Before looking at some of the other galaxies scattered throughout the Universe, it is interesting to examine first how our view of the Milky Way has changed over the years. When humankind first looked at the sky the view was unimpeded by artificial lighting and the spectacle would have been truly stunning, achievable today only from some of the most remote places on the planet. Our ancestors would have seen the band of stars defining the Milky Way with amazing clarity, yet it was not until the invention of the telescope and the curiosity of Galileo in probing the band of light that things started to change. He found that, under magnification, the light separated out into thousands of individual stars.
Not much changed until another astronomer, William Herschel, who was working at his own observatory in England, turned one of his large telescopes on the Milky Way to try and measure the distance to as many stars as possible. Making the rather rash, and incorrect, assumption that all stars give off the same amount of light, he estimated their distance based on their apparent brightness in the sky, fainter ones therefore being further away than brighter ones. We now know that stars vary considerably in the amount of light they give off so his estimates would have been quite wrong even though he was simply working on a comparison of distances with each other rather than actual distances from us. He drew the conclusion that we were located inside a giant disc of stars, with the Milky Way representing the plane of the disc, a pretty accurate view.
Other than Herschel’s disc-shaped view of our galaxy, very little was known about its actual size and shape until 1914, when another astronomer, Harlow Shapley, started to study clusters of stars with the 1.5m reflecting telescope at Mount Wilson Observatory in California. He found that some of those under observation seemed to contain a type of variable star whose actual light output was directly linked to how long it took the star to change from minimum to maximum brightness. By observing these very special Cepheid Variable stars in the distant clusters, he could time how long it took for them to change in brightness and therefore deduce how much light they really gave off. Comparing this to how bright they seemed in the sky would allow him to calculate their distance and, hence, the distance to the cluster.
When Shapley plotted the positions of some of the clusters, a remarkable picture emerged. Their distribution seemed to be centred on a point a staggering 60,000 light years away, and the galaxy itself appeared to be about 300,000 light years in diameter. We now know that the diameter is in fact about a third of Shapley’s figure, at around 100,000 light years, and the galactic centre is around 30,000 light years away in the direction of Sagittarius.
Since Shapley’s studies, a lot more has been learnt about the shape of the Milky Way, primarily by using radio telescopes to study the locations of clouds of hydrogen gas out of which stars form. By mapping their distance and position a picture emerges of a galaxy shaped like a flattened disc with spiral arms emanating from a bar which runs across a central bulging nucleus. The name for this type of galaxy is not surprisingly a barred spiral galaxy. A good analogy for the appearance of the Milky Way from the side is two fried eggs stuck back to back with the white representing the plane along which the spiral arms exist and the yolk representing the central bulge. More detailed studies of the motion of gas clouds at the centre of the galaxy show that there is a central object of very large mass. By measuring the speed of the orbiting gas clouds it is possible to determine the mass of the object and it is now believed that a large supermassive black hole lurks there.
The Milky Way is not alone though, as there are an estimated 170 billion galaxies in the observable Universe. The first recorded observation of another galaxy was as far back as the tenth century, by a Persian astronomer, Abd al-Rahman al-Sufi, who had noted a ‘small cloud’, which we now call the Andromeda Galaxy. Without the magnifying power of a telescope its true nature was not known – in fact, for many years it was referred to as a ‘nebula’ (Latin for ‘cloud’) due to its fuzzy appearance. In the centuries that followed there were many theories as to the origin of these fuzzy blobs, some of which were not all that wrong, including the English astronomer and mathematician Thomas Wright’s, who in 1750 suggested the tiny faint smudges were separate ‘Milky Ways’!
The real breakthrough came with the invention of the spectroscope, used in 1912 by the American astronomer Vesto Slipher to study the nebulae, which appeared to be spiral in shape. He was trying to learn what they were made of but instead discovered that they seemed to have a high red shift, which is seen as an apparent shifting of certain features in the spectrum towards the red end. This showed that the objects were heading away from us at a great speed, faster than the speed needed to escape the gravitational pull of the Milky Way, so they cannot have been gravitationally bound together. The conclusion was simple: they were a long way outside our galaxy. A figure was finally put on the distance to the brightest of the spiral nebulae, the Andromeda Galaxy, by the American Astronomer Edwin Hubble, who was using a giant 2.5m telescope. By trying to resolve individual stars in our galactic neighbourhood, he found Cepheid Variable stars, which were used to help determine distance. This led to the currently accepted figure of 2.5 million light years.
When Edwin Hubble started to study galaxies he realized that they fell into three broad categories: spirals, ellipticals and irregulars. Each category is then subdivided further, with spiral galaxies defined by the tightness of the spiral arms and whether they have a central bar, and elliptical galaxies by their shape from spherical to ellipsoid (shaped like an egg). The irregular galaxy category is a curious one and is more of a catch-all group for galaxies that do not fall into the other categories. Which one a galaxy belongs to is determined purely by its physical appearance and not by any other features, such as star formation rate, for example.
Hubble Galaxy Classification
The shape of a galaxy seems in no way related to a stage in its evolution; generally, once a galaxy forms, it broadly retains its shape unless it happens to collide with another galaxy. The formation process that determines the shape of a galaxy is still very much under debate, but we do know that all galaxies, whether elliptical or spiral, emerged over millions of years from tiny fluctuations in the distribution of matter after the Big Bang. It is thought that the tiny fluctuations grew slowly and eventually small proto-galaxies started to appear. Over time they clumped together, creating the galaxies we see today, and the gas slowly formed into the first generation of stars.
The theory that galaxies formed out of the variation in matter distribution after the Big Bang works well, but it does not describe why we see different types of galaxies. The spiral galaxies, for example, differ quite markedly from the large elliptical galaxies, in that they are really quite thin relative to their diameter and rotate quickly, and generally we see a significant amount of star formation going on in them. The elliptical galaxies, on the other hand, are much larger and do not rotate, and there is barely any star formation going on. To understand why we see different galaxies we need to look at their formation and interaction in a little more detail.
Shortly after the Big Bang, the tiny fluctuations grew, leading to massive haloes of so-called dark matter, which is material that neither emits nor absorbs electromagnetic radiation. The only way it can be detected is through its gravitational interaction with other matter and light. The proto-galaxies formed out of the dark matter haloes and, over time, the smaller galaxies merged to form larger galaxies with the dark matter haloes remaining around the outside. We can see this distribution of dark matter haloes even around today’s galaxies. The gas content of the young massive galaxies quickly contracts under the force of gravity and it is this initial motion which starts the galaxy spinning. This rotating motion tends to force the material outwards, producing a thin, disc-shaped galaxy. For some unknown reason, the contraction seems to cease, perhaps because of the rotational movement of the galaxy or maybe due to the gravitational pull from the dark matter, but for now the cause remains a mystery.
The classic spiral galaxy seems to form the greater proportion of galaxies and in appearance these resemble a flat rotating disc that is home to the spiral arms which emanate from a central nucleus. Surrounding the galaxy, along with the invisible dark matter halo, is a nearly spherical halo of stars which is made up almost entirely of dense globular star clusters. One of the long-standing mysteries of spiral galaxies is just how they retain the spiral structure. It is not possible for the stars to be arranged in a spiral structure because over time the arms would wind up tighter and tighter until they eventually dissipated. Studies of the motions of stars in the spiral arms actually show the more distant stars rotating faster than expected, which leads to two possible explanations.
The older of the two theories suggests a phenomenon known as density waves, which rotate around the centre of spiral galaxies. They can be thought of like a traffic jam on a motorway: individual cars move through it but the traffic jam stays where it is and does not move. In the same way, stars move through the wave and so observationally you would see a different set of stars in the arms at different times as the wave continues on its orbit. When the wave meets gas, it compresses it, leading to a burst of star formation and the appearance of young, hot and luminous stars that make the arms stand out from the surrounding galaxy. The other, rather less popular, idea suggests that the explosion of stars and solar wind being emitted from all stars generates shockwaves which continue on around the galaxy. As the shockwaves interact with gas, they compress it, leading to a burst of star formation in the same way as in the density wave model. M33 is a great example of a spiral galaxy in the northern skies during October and is just visible to the naked eye under exceptionally good conditions between the Andromeda Galaxy and the stars of Aries.
A slight variation of the spiral galaxy is the barred spiral, which retains the spiral arms feature but, instead of protruding from the nucleus, they sweep out from a bar which itself extends through the nucleus. Interestingly, surveys have shown that there are more barred spiral galaxies than pure spirals. The widespread presence of the barred spirals suggests that the existence of the bar may well be an evolutionary stage in the life of a spiral galaxy, and it is thought that these may be the younger relatives of the fully grown spirals. Orbital resonance is the mechanism that may be the driving force behind the strange feature where stars and gas in orbit around the nucleus exert a gravitational force on each other. Eventually their orbits become gravitationally synchronized, leading to the formation of the bar. This process may also encourage the birth of new stars in and around the core of the galaxy until the bar reaches such a mass that it becomes unstable and the galaxy evolves into a spiral. There are a number of examples of barred spiral galaxies around the sky but NGC55 is one of the brighter ones, although appearing edge-on to us. Like most galaxies it is not visible to the naked eye, but a beginner’s telescope will hint at some detail. It is possible that the Small Magellanic Cloud in Tucana was once a barred spiral galaxy that has been distorted by the immense gravity of the Milky Way.
The other main class of galaxy is the elliptical, whose appearance varies considerably from almost spherical galaxies at one end through to nearly cigar-shaped at the other. The stars in the elliptical galaxies travel in random motion, unlike those in spiral galaxies, which orbit a central nucleus. This is seen clearly to be the case in M32, one of the companion galaxies of the Andromeda Galaxy.
Typical elliptical galaxies are made up of old, low-mass stars and have almost negligible quantities of gas, which means star formation is rare. The absence of star formation suggests that elliptical galaxies are old, but surprisingly they seem to be few and far between in the early Universe. We can tell this because it takes time for light to travel the great distances between the galaxies, so looking at distant ones means we are looking back in time. This leads us to the conclusion that the giant ellipticals are actually the result of collisions between, or mergers of, two or more galaxies of equal mass, rather than having originally been formed in that shape.
Given the incredible distance between the galaxies it is hard to believe that they can ever get close enough to merge. Our own galaxy, the Milky Way, is over 2 million light years away from its nearest major galactic neighbour, the Andromeda Galaxy, yet even they are heading towards each other. There is still some uncertainty as to whether they will actually collide, but if they do, it could happen in as little as 5 billion years from now. This all sounds pretty scary stuff but the reality of galactic collisions and mergers is not quite as terrible as it seems. Without you even realizing it, the Milky Way is in the process of devouring a smaller galaxy called the Sagittarius Dwarf Elliptical Galaxy. Typically, when galaxies collide they have a relative impact speed of around 500km per second, yet very little real damage is done. Because of the distance between the stars, it is probably more accurate to call it a merger rather than a collision; indeed there are very few if any impacts. To put that in context, if the average star were scaled down to the size of a golf ball and placed in the centre of London, then the next nearest star would be at Zurich in Switzerland and the chances of the two of them colliding would be negligible.
The series of events during a merger is very much dependent on how the process unfolds, but we do know collisions occur as there are many fine examples of them in the sky such as the Large and Small Magellanic Clouds in the southern hemisphere sky, which are now thought to have collided with the Milky Way billions of years ago. An example in the northern sky is the Whirlpool Galaxy, found in Canes Venatici. The system is stunning through a telescope, which reveals two galaxies in the process of colliding. It can just be seen through binoculars, although detail in the spiral arms is only revealed in telescopes.
If collisions like this take place between two equal-mass spiral galaxies such as the Milky Way and Andromeda, then before getting too close the stars will orbit around the respective nuclei in a fairly orderly fashion. As the galaxies get closer and start to merge, the gravitational effects throw this all into disarray and the stars are disturbed in their orbits, sending them off on different paths. Interestingly, this is what we see in elliptical galaxies, which supports the theory that ellipticals are the result of galaxy mergers. The two galaxies will for a short time maintain their original motion so they pass through each other, sending the stars into random orbits and disturbing the gas, stirring up a burst of star formation. The shape of the galaxies will change dramatically, with some stars and gas being ejected into space. If the galaxies have insufficient momentum then the force of gravity will pull them back together again and the process continues until the galaxies eventually merge. On rare occasions the galaxies will be massive enough and travelling fast enough for them to continue on their way and not merge into one. Instead, the transient gravitational interaction will simply change their appearance for ever.
At a local level it seems galaxies are sparsely distributed – after all, the 2.3 million light years or 21 trillion kilometres to Andromeda does seem quite a distance. Looking further afield at the wider cosmos, galaxies are clustered together into gravitationally bound groups. There is actually a difference between a galaxy group and a cluster but it is chiefly one of size, with galaxy groups having up to fifty members and measuring around 6 million light years across. Clusters are larger than this but there is no other real differentiation between them and groups.
Our own galaxy is part of a group of galaxies called the Local Group, of which there are around forty known members. The two largest members are the Milky Way and the Andromeda Galaxy, and they are joined by a host of smaller galaxies, including the Small and Large Magellanic Clouds, which lie at a distance of 200,000 light years and 160,000 light years respectively. They can only be seen in the southern hemisphere sky and appear like disconnected patches of the Milky Way; and are small in comparison to many other galaxies, including the Milky Way, measuring just 7000 and 14,000 light years across.
Beyond the Local Group are more clusters of galaxies, such as the Virgo Cluster – which contains up to 2000 galaxies and includes the giant elliptical galaxy M87.
The Virgo Cluster, the Local Group and almost a hundred other galaxies and clusters are collectively known as the Virgo Supercluster. A galaxy supercluster is the largest structure in the Universe, and in the case of our own Virgo Supercluster measures 100 million light years from side to side. Interestingly, superclusters, of which there are about a million in the observable universe, are not gravitationally bound. Their existence is believed to represent the large-scale fluctuations in the Universe following the Big Bang and between them are vast expanses of empty space where few if any galaxies exist.
There is one final type of galaxy to be considered and it has been left to the end of the chapter due to its strange and exotic nature. These are the galaxies with something quite unusual at their core called the active galactic nuclei. These ‘active’ galaxies have a higher than usual output of energy at their core, be it in visible light, X-rays, radio waves or any other portion of the electromagnetic spectrum. It is thought the source of the energy is the accretion of matter around a supermassive black hole at the centre of the galaxy. The gravitational pull of the black holes is so high that they drag matter towards them at high speeds. As the matter gets nearer it forms an accretion disc which spins faster and faster the closer it gets to the black hole’s event horizon (the point where even light does not travel fast enough to escape). As matter is accelerated to higher speeds it heats up and emits the intense quantities of energy observed. Another feature of some active galaxies is jets of radiation (relativistic jets) that extend out over millions of kilometres and, while the exact cause of these is unknown, they do seem to contribute significantly to the overall brightness.
There are a few different types of active galaxy which are divided into two sub-categories: those that emit highly in the radio wavelengths, called ‘radio loud’, and those that do not, called, unsurprisingly, ‘radio quiet’. The key difference between the two is the emission from the jet that is dominant in radio wavelengths in the radio loud group. Within the radio quiet group is the best-known type of active galaxy, the quasar, which is short for ‘quasi-stellar object’, and Seyfert galaxies, which were the first active galaxies to be identified. Within the radio loud group are the blazars, which are the most luminous of all. All of them have supermassive black holes in the centre, surrounded by an accretion disc, and the so-called relativistic jets, but they differ because of their orientation in space relative to our vantage point here on Earth.
The Seyfert galaxies, like M77 in Cetus, which is 47 million light years away, are the closest, and in the case of M77 can be seen visually as a barred spiral galaxy in amateur telescopes. While they are the nearest of all active galaxies, it seems that the relativistic jet is not pointing towards the Earth, so while they do have bright cores they do not appear to be among the most energetic. The quasars are generally seen to be more energetic than the Seyfert galaxies but are much more distant. When they were discovered, they appeared like stars visually but seemed to be in exactly the same part of the sky as a strong radio source. The beam of a quasar is thought to be pointing more towards the Earth than is the case with Seyferts, but still not directly at us. The closest quasar is an object called 3C273 at a distance of around 2.5 billion light years in Virgo, but a good-sized amateur telescope is needed to see it. The blazar is the most energetic of all with significant observed emissions of radiation, particularly in the radio spectrum. They seem to be hosted inside giant elliptical galaxies and it is thought the beams are pointing directly at us so we see the characteristic intense amounts of energy coming from a compact source.
The family of galaxies around the Universe is a diverse one and, while space is to all intents empty, these giant islands light up the darkness. It is fascinating that they are all generally home to the same objects yet their appearance in the night sky varies wildly, making them a great target for amateur observations.
October: Northern Hemisphere Sky
Pisces is very well placed in the northern sky during October and sits just north of the celestial equator. To the west can be seen the Circlet pattern of stars, which actually looks more like a pentagon than a circle. The first bright star to its east is Alpha Piscium, which is the third-brightest star in the whole constellation. The rest of Pisces forms a large V-shape, with Alpha Piscium at its tip, that points to the south-east.
To the west of Pisces is the unmistakable Square of Pegasus. The square itself is just a portion of the constellation, which represents the winged horse, but the four stars marking the corners of the square are a great signpost to the October sky. The south-east corner star, known as Algenib, is a striking blue star with a surface temperature of 21,500 degrees, but of real interest is NGC7814, just a few degrees off to its north-west. NGC7814 is a beautiful edge-on spiral galaxy but a little faint at 10th magnitude, although even a small telescope will reveal its long, thin shape. Telescopes with an aperture of 15cm or more will start to show the dark dust lanes against the faint light of the galaxy.
The north-east corner star of the Square of Pegasus, Alpheratz, actually belongs to the neighbouring constellation of Andromeda, found just to its east. Continue along the line of stars from Alpheratz to the east, past Delta Andromedae, then turn slightly north to find Mirach, a cool red giant star. Now turn further north and locate Mu Andromedae, then a little further on the slightly fainter Nu Andromedae, before shifting your gaze fractionally to the west to find a faint fuzzy blob. This is the famous Andromeda Galaxy and from dark locations it is quite easy to spot with the naked eye. It is the nearest major galaxy to our own and lies around 2.3 million light years away, although, in contrast to the motion of NGC7814, which is heading away, it is actually heading towards us. The Andromeda Galaxy is an easy target for the naked eye under dark skies and with binoculars, but telescopes will reveal dark dust lanes. Small telescopes will even reveal its two companion galaxies, M32 and M110, much like the companions of the Milky Way, the Large and Small Magellanic Clouds that can be seen in the southern hemisphere sky.
Starting back at Mirach again (the bright star to the north-east of Alpheratz), find the orange star to its south-east, which is Hamal in Aries. This one is easy to spot as it is prominent in an otherwise bland area of sky. Between the two is a faint constellation called Triangulum, in the shape of a thin triangle and pointing to the west. A little further off to the west of the triangle itself and almost halfway between Mirach and Hamal is M33. At a distance of almost 3 million light years it is the most distant object that can be seen with the naked eye under exceptionally dark conditions. Binoculars will reveal it as a large fuzzy blob but it takes a telescope with an aperture of at least 15cm to start to reveal a hint of the spiral structure.
The prominent star to the north of Triangulum is the most easterly star of Andromeda, known as Almach. Further east of Almach is another bright star, by the name of Algol, in Perseus, and about a quarter of the way towards it is another fine example of an edge-on spiral galaxy, NGC891. At 9th magnitude it requires a telescope to be seen in any detail, but even a small one will show a thin needle-like smudge of light. Larger telescopes will reveal a dark dust lane along its equator. Studies in infra-red light suggest the galaxy has a bar similar to the Milky Way but with its edge-on orientation it is not directly visible to us.
To the north-west of Almach are a couple of fainter stars separated by no more than a couple of degrees: Upsilon Persei (orange) and Phi Persei (white-blue). To the north of Phi Persei by a fraction of a degree is one of the faintest planetary nebula, M76, otherwise known as the Little Dumbbell Nebula because it resembles a smaller version of the famous Dumbbell Nebula found in Vulpecula.
Further north from Andromeda lies an easy-to-recognize group of stars called Cassiopeia, which looks like a giant celestial ‘W’. Caph, the star at the western end of the ‘W’, lies 54 light years away and is thirty times more luminous than the Sun. The next star to the east in the shape that makes up the ‘W’ is a yellow star by the name of Shedar. It looks to be of a similar brightness to Caph and therefore it would seem safe to assume it is at roughly the same distance but it is actually over four times further away. This illusion is a result of Shedar’s greater size and luminosity – it is 855 times brighter than our Sun. Just to the east of Shedar is the emission nebula known as the Pacman Nebula, which, at magnitude 7.4, is only visible with optical aids. A telescope aperture of at least 10cm should reveal it nicely, but more detail, such as the dark dust lane cutting into the nebula, will only be seen in large instruments. To the eye, the nebula appears grey-green, but in images of the area a beautiful red light is seen, caused by energy from nearby stars exciting the hydrogen-gas atoms in the cloud and making them glow. Those same hydrogen atoms were created at the birth of the Universe.
Moving further along Cassiopeia to the east finds the stars Navi and Ruchbah, and scanning the sky with binoculars to the east of Ruchbah by 1.5 degrees reveals M103, a 7th magnitude open cluster. It is not so easy to spot with a telescope as the larger aperture, and therefore greater light-gathering power, also reveals more background Milky Way stars, making the cluster much less obvious. It lies about 7000 light years away and, like many open clusters, has a small number of stars – forty in this case.
October: Southern Hemisphere Sky
The constellation of Cetus straddles the celestial equator and is the starting point for October’s southern sky guide. Its brightest star, Deneb Kaitos, also known as Beta Ceti, is prominent just 18 degrees south of the equator and is easy to spot due to its yellow-orange colour. Like all stars it generates heat and light by fusing hydrogen into helium and helium into carbon deep in its core. In the case of Beta Ceti, it looks to be a normal Sun-like star nearing the end of its life but it seems to be emitting a high amount of X-ray radiation as if it already has a contracting helium core.
When Beta Ceti finally dies, it will gracefully lose its outer layers to space, producing a beautiful planetary nebula like the one that lies to its north by just 8 degrees. Its name, the Skull Nebula, gives a clue to its ghostly appearance when seen through modest-sized telescopes. An aperture of at least 15cm is needed to see the nebula and a scattering of stars in front gives an illusion of transparency, with dark knots in the cloud giving the impression of empty voids. A filter can be fitted to telescope eyepieces to enhance objects like this, such as an OIII filter that will make the nebula much more prominent.
Visible just to the north of the celestial equator and to the north-west of Beta Ceti is Alpha Ceti, which is more red in colour and a little fainter. Delta Ceti, a blue giant, is the next star found heading back from Alpha towards Beta Ceti, and less than a degree to its south-east is M77, one of the few Seyfert galaxies visible to amateur astronomers. It appears as a 9th-magnitude barred spiral galaxy face-on to us. Heading from Alpha Ceti through Delta Ceti and on the way to Beta Ceti is a famous star called Mira. It is a red giant whose light output varies as the star pulsates over a period of 332 days. In that time, it brightens to a maximum magnitude of 3.5 before slowly fading from view to magnitude 9.5. It has a temperature of around 2000 degrees, making it one of the coolest stars in the sky. The name Mira, which means wonderful, is now used to describe variable stars of this type and there are about 6000 of them across the whole sky.
Starting back at Beta Ceti again, the next bright star to the south and roughly as bright is Alpha Phoenicis, or Ankaa. It is a yellow-orange star about 77 light years away and is joined to its south-west by Beta Phoenicis, which is a little fainter. Between these two stars in Phoenix and Beta Ceti is Alpha Sculptoris, the most westerly of the bright stars in Sculptor and a blue giant star around four times the diameter of the Sun. Sitting almost at the centre of a triangle made up from Alpha Sculptoris and Alpha and Beta Phoenicis to the south is a beautiful example of a spiral galaxy, NGC300.
NGC300 covers about the same area of the sky as the full moon and at a distance of just over 6 million light years is one of the closest galaxies to our Local Group of galaxies. It was once thought to be a member of the Sculptor group of galaxies but the cluster lies some 4 million light years further on. At magnitude 8, it is not visible to the naked eye but telescopes with an aperture of around 10cm will show it as a fuzzy disc, with larger instruments starting to reveal detail in the disc, but the spiral arms only become prominent in telescopes of 25cm or more. To the west of NGC300 by around 10 degrees is NGC55, an example of an edge-on barred spiral galaxy, lying 7 million light years away.
South of Phoenix and slightly to the west is the bright star Achernar, the ninth-brightest star in the sky and the brightest star in the constellation of Eridanus, which depicts a river. It marks the end of the river and is a giant blue star 144 light years away with a temperature in excess of 15,000 degrees. It spins on its axis at a speed of 225km per second, which causes the star to bulge out around its equator so its diameter is less from pole to pole. The remainder of the stars in Eridanus extend to the north past the constellations of Horologium, Fornax and Caelum before almost reaching the celestial equator near Rigel, the brightest star in the constellation of Orion.
To the south of Archernar is a large celestial triangle with its northernmost tip pointing just to the west of it. The triangle is the constellation of Hydrus and is marked by Alpha Hydri at its northernmost tip, Beta Hydri at the eastern tip and Gamma Hydri to the west. The brightest of the stars is Beta Hydri, which lies 24 light years away and is one of the most Sun-like stars in the night sky, though it is probably a little older and a little further down its evolutionary path than our star at an estimated 6 billion years old.
Along the line between Beta and Alpha Hydri is what seems to be a slightly detached portion of the Milky Way. It is actually one of our four closest satellite galaxies, the Small Magellanic Cloud (or SMC), the other three being the Sagittarius Dwarf Elliptical Galaxy, the Canis Major Dwarf Galaxy and the Large Magellanic Cloud. Just off the eastern edge of the SMC is a 4th magnitude star that looks a little hazy to the naked eye. Binoculars simply make the fuzzy star look bigger and there is an increase in brightness towards the core. Using a telescope with at least a 10cm aperture will start to turn the haze into individual stars, revealing the globular cluster 47 Tucanae in all its glory. The cluster extends over an area of sky equivalent to the full moon which in reality is 120 light years across. In that area of space there are around a million stars, in comparison to an equivalent region of space around our own neighbourhood where there are an estimated 15,000 stars.