The Electromagnetic Spectrum
The Invisible Universe
WHAT WAS THE last concert you went to? Whatever the kind of music, it is incredible to hear all the different sounds coming together to produce the songs that you were probably humming for several days after. Listen carefully to a rock band, say, and you can pick out the beat of the drum, the noises from the other individual instruments and the different vocal harmonies. Now, just imagine tuning in to the beat of the drum and nothing else: you would have no idea what the song actually sounds like. You have to tune in to all the elements before you get to hear the complete piece of music.
As with music, if we want to get a full picture of the Universe in all its glory we must tune in not only to visible light but to all the different types of radiation; otherwise we are missing out on an incredible amount of information. Our eyes have evolved to detect visible light, which is just a tiny portion of something called the electromagnetic (EM) spectrum. You are probably familiar with the ‘colours of the rainbow’ – red, orange, yellow, green, blue, indigo and violet – but extending either side is a much larger range of radiation that we are incapable of detecting with our eyes. Beyond the red end we find infra-red and radio waves; and beyond violet are ultra-violet, X-rays and gamma rays. To make that a bit clearer, see the illustration at the bottom of this page.
You can see an example of how different objects give off different types of radiation simply by looking up at the stars. There is an easy-to-recognize constellation called Leo which lies just north of the celestial equator. Leo can be seen from most parts of the world, although it will appear to be upside down from southerly latitudes. The brightest stars in the constellation are Regulus, marking the base of the lion’s head, Denebola, marking its tail, and Algieba, which is the Lion’s mane. Look closely at the stars and it can be seen that their light is not the same: Denebola, shining white, is not too dissimilar to the blueish hue of Regulus, but both are distinct from Algieba, which is orange in colour. These variations are the result of the distinctive temperatures of the stars and show nicely that stars emit radiation at different wavelengths in the visible-light range. Of course, they emit radiation in wavelengths other than visible light too, so to be sure we are capturing all available information we must study objects in all ways possible.
The Electromagnetic Spectrum
The real beauty of studying the Universe in all these various types of radiation is that you can reveal detail and information that otherwise would be lost to you. There is a challenge here though, since not all types of radiation make it to the surface of the Earth. Our atmosphere blocks much of the incoming radiation from space so, in a number of cases, we have to launch a telescope into orbit, beyond the filtering effects of our atmosphere.
Before we get too far into looking at how our view changes by tuning in to different types of radiation, it is worth briefly looking at how they actually vary. All types of radiation in the EM spectrum share the same wave-like properties and, for the purposes of this discussion, it is wavelength and frequency that interest us. Just like a wave in a lake, a wave of radiation has a crest which is followed by a trough before another crest arrives. The distance between two successive wave crests or troughs is called wavelength, and the number of waves that go past in a second determines frequency. More waves per second equates to a higher frequency and fewer waves to lower frequency. Scientists use these terms to describe the radiation’s behaviour.
Considering the EM spectrum in its entirety, it starts with radio waves, which have the longest wavelength at thousands of metres (lower frequency), continuing through visible light to the other extreme, the shorter wavelength gamma rays at 100,000 millionths of a metre (higher frequency). (There is a mathematical relationship between wavelength and frequency but we do not need to worry ourselves about it now.)
The really useful thing about the EM spectrum is that different types of radiation can punch through different materials. If you have been outside on a clear dark night well away from street lights, then you may have seen the Milky Way stretching overhead and perhaps spotted some dark patches among the hundreds of glittering stars. These are huge dark dust clouds which lie between us and the more distant stars. They are so dense that they absorb visible light and block the distant starlight from our view. It turns out, though, that while these clouds block visible light, they do not block other types of radiation, so if you tune in to radio waves, for example, you can look right through them. This principle applies to clouds in our atmosphere too. Radio telescopes, which as their name suggests tune in to radio waves, can still work in the thickest of clouds, whereas optical telescopes are rendered useless.
The interaction of different parts of the EM spectrum with different materials allows astronomers to probe the deepest, darkest reaches of the Universe. This is not just of use to the professional astronomer though. A blight for the amateur astronomer is the ever-increasing level of artificial lighting in residential areas, but we can now use very special filters to cancel out most of this, effectively blocking out parts of the EM spectrum, leaving the visible light from the cosmos to shine through. We are already familiar with visible light and what we can see with it, so for now let us concentrate on the stuff we cannot see. Moving from visible light and heading to the right along the spectrum, we leave red light behind and enter the realm of infra-red radiation.
You already detect infra-red radiation (or heat, to use its common name) without even thinking about it. The warmth from a fire and even the heat from the Sun on a summer’s day are examples of infra-red radiation. It was discovered back in 1800 by Sir William Herschel, a Hanover-born German astronomer who was trying to measure the temperature of each colour of the visible spectrum. He set about passing light from the Sun through a prism, which split the incoming white light into its component colours. He then placed a thermometer in each colour in turn and a further one at either end, beyond the visible light, to act as a control for his experiment. During this exercise, he noticed two things: that the temperature seemed to increase from the violet end to the red end and, more surprisingly, that the thermometer beyond the red end, where there was no ‘light’, seemed to register a higher temperature than in the red! Further experiments by Herschel showed that this new radiation, which he called ‘calorific rays’, acted just like the light he could see.
Following Herschel’s discovery of what we now call infra-red radiation, it was found that water vapour would absorb it, a real nuisance for astronomers wishing to observe the sky in infra-red since 1 per cent of the Earth’s atmosphere is made up of water. This means the greater part of incoming infra-red radiation is absorbed and only the smallest amount reaches us here on the surface. We get around this by placing telescopes high up in the atmosphere, above the majority of the water vapour – on top of high mountains, for example. NASA have taken this a step further with their airborne SOFIA facility, the Stratospheric Observatory for Infra-red Astronomy, which is a converted Boeing 747. Infra-red telescopes have even been launched high above the Earth’s atmosphere, allowing them unimpeded opportunities to observe infra-red. The Infra-red Astronomical Satellite, or IRAS, is the first of its kind and was launched in 1983, becoming the first space observatory to survey the entire sky at infra-red wavelengths.
The measure of temperature, which effectively defines how much infra-red energy is being emitted, is actually a measure of the movement or vibration of atoms. If they are stationary then the temperature will be the lowest it can ever be, and is said to be at absolute zero. This means they will be emitting no infra-red energy, but as their movement increases the temperature goes up and so does the amount of energy being given out. The real benefit of infra-red studies is that they allow us to examine the temperature distribution of objects even if they are blocked from view by thick interstellar dust clouds.
We can study these interstellar clouds, or nebulae to use their correct term, in great detail. A fine example is the Orion Nebula, found in the constellation of the same name that we looked at in the January sky. Just below the stars in the belt of Orion, for viewers in the northern hemisphere, or just above them for those in the southern, is a faint row of three ‘stars’ which depict the hunter’s sword. From a dark location, look a little closer at the centre star and you can see that it is not a pinpoint of light but looks a little fuzzy. Try looking at it through binoculars or a telescope and you will discover an amazing sight: wispy strands of clouds surrounding what appear to be four faint stars in the middle. Infra-red studies show many more stars than the four easily visible ones, and we will look at how they are created out of these vast interstellar clouds in a later chapter.
Moving a little further along the EM spectrum we enter the world of radio waves and, in particular, a specific type of radio wave called microwave radiation. If you, like me, are a totally uninspired chef then you will know the real beauty of this lies in its ability to cook food. Inside those handy little ovens is a device that generates microwave radiation, which is in turn absorbed by the water, fat and sugar molecules in the food. As the molecules start to vibrate thanks to the extra zap of energy, they heat up, cooking your meal. Neat.
For astronomers, though, microwaves represent the most intense type of radio waves, but their first detection coming from the sky was really a matter of luck. In 1964, Arno Penzias and Robert Wilson were working at the Bell Laboratories in New Jersey and were experimenting with a special type of telescope that was designed to pick up microwaves bouncing off balloon satellites (a type of satellite which is spherical in shape and simply reflects communication signals back down to Earth, unlike conventional satellites, which receive signals and then retransmit them).
In order to be able to pick up the faint reflected signals, they had to remove as much interference as possible, which they managed, except for a faint yet persistent signal. It remained pretty constant in whichever direction they pointed the horn-shaped telescope and, suspecting pigeon droppings, they set about giving this a thorough clean. The signal continued and they eventually concluded that it must be coming from beyond the Earth, from deep space! Penzias and Wilson had in fact detected the remnant radiation from the birth of the Universe, the Big Bang, which occurred 13.7 billion years ago.
This is a good opportunity to consider a rather strange concept in astronomy. Light travels at almost 300,000km per second and in our everyday lives this is of no real consequence. In astronomy, though, it is incredibly useful to know this fact. Because of the vast distances across the cosmos it takes time for light to get from one place to another; to the Earth from the Moon it takes just over a second, from the Sun 8.3 minutes, from the nearest star around four years, and from the nearest neighbouring major galaxy 2.3 million years. This means that by looking at distant objects, we are actually looking back in time. The latest studies from the space-based microwave telescopes, known as the Wilkinson Microwave Anisotropy Probe (WMAP), have looked at what we now call Cosmic Background Radiation in unprecedented detail and determined that its source is so far away that it gives us a view of the Universe as it was just 400,000 years after the Big Bang.
If you look at the night sky with your own eyes you will see it as visible light. You will see individual specks of light, most of which are stars, but some of these tiny pinpricks will be planets; others that look a little fuzzy will be clusters of stars or even distant galaxies like the Andromeda Galaxy in the northern sky. In between the stars, it will seem dark and black, except of course if you look towards the inside of our own galaxy, the Milky Way. But if you could look at the sky as microwave radiation then you would not see many, if any, of the stars you are familiar with; instead, you would see the Cosmic Background Radiation covering the entire sky. It would be a sky full of light with a few darker patches here and there. It is believed that the slight variations in the intensity of the radiation ultimately led to the evolution of the vast galaxy clusters that we see in the Universe today.
It was some thirty years before Penzias and Wilson revealed microwave radiation in the sky that a telecommunications engineer called Karl Jansky discovered the more general form of radio waves from astronomical objects. He was investigating interference detected on transatlantic voice transmissions and was using a directional antenna that allowed him to tell where a signal was coming from. He picked up some interference that seemed to be peaking every 23 hours and 56 minutes which, in consultation with an astrophysicist, he concluded must have been astronomical in origin because it took the Earth that long to rotate on its axis. By comparing his results with maps of the night sky, he saw it could only have been one thing: he had been picking up radio signals coming from the centre of our galaxy.
The real beauty of the great proportion of the radiation at the radio end of the spectrum is that it nearly all passes through the atmosphere, so it can be observed from the ground – and in broad daylight. This means that radio astronomy studies of the Universe can continue around the clock almost regardless of weather conditions, except perhaps during some thunderstorms.
The patch of sky where Jansky had first detected astronomical radio waves lies in the direction of Sagittarius and the galactic centre of the Milky Way. To the naked eye this area of sky does not look like anything special, although it does seem pretty much packed with stars if you study it with a telescope. The main reason for this, though, is that most of the light coming from the centre of our galaxy is blocked by interstellar gas clouds but, interestingly, it is these gas clouds that contribute to the radio waves detected by Jansky in the early 1930s.
Today’s professional radio telescopes are a little larger than the directional kind used by Jansky. They are usually shaped like a huge dish and in size tend to dwarf all other types of telescope. One factor that determines the diameter of any telescope is how much detail it can see; in other words, its resolution. The bigger it is, the finer the level of detail it can detect. The wavelength of radiation being studied also has a bearing on this and a longer wavelength needs a bigger telescope to see the same level of detail that a shorter-wavelength telescope can see. This is why radio telescopes are so huge. That said, it is possible for amateur radio astronomy set-ups – and I’ve even seen a pretty rudimentary radio telescope made out of a bin lid and a few basic electronic components – to detect radiation from Jupiter and the Sun.
Another great way to try radio astronomy for yourself is from the warmth and comfort of your car during a meteor shower. The Lyrids are a good example of a meteor shower in the April sky and peak around the 20th/21st of the month. Whether or not a good show is seen depends on the amount of meteoric material the Earth encounters, the orientation of the Earth at peak activity, cloud, light pollution and even the Moon. The trick is to tune your car radio to a commercial FM station that you cannot normally pick up, ideally one that is around 1000km away. For now you will just hear the hiss of noise, but as meteors zip in through the atmosphere radio waves will bounce off their trail, allowing you to hear the distant station for a brief moment. You might also hear pops and whistles as the meteors arrive.
That is it for the longer-wavelength end of the electromagnetic spectrum; moving back to the other end of the visible portion we come to violet light. Beyond violet light is ultra-violet, or UV, radiation and its discovery was, like many scientific finds, a matter of chance. By the start of the nineteenth century infra-red radiation, and its warming effect, was known about, so in 1801 a German physicist called Johann Ritter set about looking for an equivalent radiation that might have a cooling effect. He started looking in the dark regions beyond violet light and, while he did not find any ‘cooling’ light, he did notice that a white chemical called silver chloride turned dark when placed beyond violet light. He had discovered UV radiation, although he first called it ‘chemical rays’ as a result of its observed ability to produce chemical changes.
Many birds and insects can see in UV but humans are blind to it, although we can ‘detect’ it from the way the Sun burns our skin. The ozone layer in our atmosphere blocks around 95 per cent of the incoming solar and astronomical UV radiation, so astronomers wishing to study it must once again send telescopes up into space. The view through telescopes that can see in UV, e.g. the Hubble Space Telescope, is quite different since most stars are in the middle years of their lives and surprisingly are relatively cool, while ultra-violet radiation is the mark of hotter objects like stars at the beginning or end of their lives. Unlike radio waves, which can pierce galactic dust clouds, UV is blocked by them, so it is very difficult, but not impossible, to study the stars of the Milky Way in UV. It can be used effectively to study the chemical make-up of the interstellar clouds and also to probe distant galaxies and learn about their evolution.
Moving beyond UV poses a challenge for astronomers because the radiation has shorter wavelengths so is much more energetic, which means it blasts through most materials if pointed straight at them. This property is used very successfully in medicine, which exploits the ability of X-rays to see inside humans without having to subject them to an operation. This does mean, though, that when researchers use a conventional telescope designed to capture incoming light, the X-ray radiation has so much energy that it will fly straight through the mirror. The solution is to tilt the mirror so that the light strikes it at a shallower angle.
Because X-rays are blocked by the atmosphere, the high-energy telescopes used to study them must be placed into Earth orbit. The first source of X-rays discovered beyond the Sun is known as Scorpius X-1, which has been studied and found to be a type of star known as a neutron star. Nothing amazing about that, but what does make it special is that it is part of a binary star system too. Neutron stars have an incredibly strong gravitational pull and can literally rip material off a companion star. This is the case with Scorpius X-1, and the material builds up into a disc which is accelerated to incredibly high speeds, causing it to start to emit X-rays, which is the radiation we can detect from 9000 light years away (the distance light can travel in 9000 years).
Beyond X-rays is the final piece of the EM spectrum and it represents some of the most energetic and violent events in the Universe: gamma radiation. It had been known about and observed in natural processes here on Earth for many years – for example, during radioactive decay – long before it was discovered in deep space. The existence of this most energetic form of radiation had been predicted during extreme events such as the supernovae that mark the death of a supermassive star, but it was not until the late 1960s that it was actually detected. Satellites from the Vela military satellite group were searching for bursts of gamma rays on the ground from the detonation of nuclear weapons, but they also picked up flashes of gamma rays from deep space. Careful study later showed that these brief bursts of gamma radiation lasted for only a fraction of a second and came from totally unconnected parts of the sky, quite randomly.
As I’ve touched on before, these brief events are known as gamma ray bursters (GRBs) and when a star explodes it sends out a burst of energy so great that in just a few seconds it produces more energy than our Sun over its entire 10-billion-year lifetime! Unfortunately GRBs are incredibly hard to study because they are so short-lived. However, following the initial burst, an afterglow is often present that emits radiation in longer but fading wavelengths. If astronomers can respond swiftly to a GRB event, it is possible for them to detect the afterglow and identify the origin.
GRBs are a fitting way to end this section as the only way to understand them is by studying them over a range of different wavelengths. Without this approach, they would remain a mystery. As with the rock band at the start of the chapter, it is only by opening our senses to the whole EM spectrum that we can ever hope to get a full picture of the cosmos.
April: Northern Hemisphere Sky
In the previous section I referred to a constellation called Leo and it is a great place to start the April guide. To find it, look just to the north of the celestial equator and a little to the west to find a pattern of stars that resembles a backwards question mark. This marks the head of Leo and at the southern end of the grouping is the brightest star in the constellation, Regulus, which is a blue-white giant star 78 light years away. Following the shape of the backward question mark to the north, the next star is Eta Leonis, followed by Gamma Leonis, which is 125 light years away. Comparing Gamma Leonis with Regulus, it is perhaps surprising that Regulus is almost half the distance away. At the opposite end of the constellation to the east is a star called Denebola, which is about the same brightness as Gamma Leonis yet is much closer than even Regulus at well under half the distance at 34 light years.
The distances to the stars in Leo are insignificant when compared to the incredible distances between the galaxies. To the west of Denebola is the white star Theta Leonis and just to the south is a pair of galaxies called M65 and M66. Neither is visible to the naked eye but they can be seen through a good pair of binoculars. Telescopes will show M65 to be a spiral galaxy which is presented to us at an angle and is the brighter of the two; M66 is another spiral galaxy but more face-on. Compared to the stars in Leo, such as Gamma Leonis at 125 light years, the galaxies beat them hands down at a staggering distance of around 35 million light years.
Off to the south-east of Denebola is the constellation of Virgo and scattered over its northern boundary is the Virgo Cluster of galaxies. The cluster is thought to span around 15 million light years of space, which is not much larger than our own Local Cluster of galaxies, although it has significantly more members. Red shift measurements taken by studying the spectrum of some of the Virgo members show how fast they are travelling away from us, and from that it is possible to calculate that the centre of the cluster is about 54 million light years away.
Just to the north-east of Virgo and its cluster of galaxies is the unmistakably bright orange star Arcturus in Boötes, which is a strong contrast to the blue-white colour of Regulus in Leo. Not only is it the brightest star in the constellation but it is also the brightest star in the northern hemisphere of the sky. The name Arcturus means ‘bear watcher’, which has its origins in the fact that it follows Ursa Major, the Great Bear, as it circumnavigates the Pole Star. At a distance of 37 light years it is the nearest giant star to us, giving a great opportunity to study the evolution of these stellar monsters. Moving to the east of Arcturus takes us to Zeta Boötis, which is a binary star system 180 light years away. The stars of this system orbit around a common centre of gravity but along unusually elliptical orbits. Extending the line between Arcturus and Zeta Boötis points to the brightest star in the constellation Serpens, right next to Virgo. This star is called Alpha Serpentis and to its south-west is a tiny fuzzy-looking blob just visible to the naked eye. This is the globular cluster called M5 and it lies at an estimated 24,500 light years away. As with many globulars, it is thought to be home to a huge number of stars, maybe as many as half a million, and it is their combined light which gives it the appearance of a fuzzy blob.
A little to the north-west of M5 is the location of the largest known galaxy, with the most uninspiring name of IC1101. Regardless of its monstrous size – it is over fifty times bigger than our own galaxy – at a distance of 1.07 billion light years it appears only as a faint smudge in the sky so is beyond the range of most amateur telescopes.
Skipping back due west from Arcturus is Diadem, the brightest star in Coma Berenices, and it lies at a distance of 47 light years. Just 1 degree to the north-east of Diadem (remember, you can estimate this distance as it is the same as a finger extended up at arm’s length) is the globular cluster known as M53, and a degree to the south-east of that is NGC5053. Although a little fainter, NGC5053 is still visible through amateur telescopes but, of the two, M53 is by far the easier to find. The distance to M53 is 58,000 light years, which means it is among the most distant of the Milky Way’s globular clusters, while NGC5053 is a little closer at 53,500 light years.
Hopping back due west to Leo and then north is a rather less conspicuous constellation representing the lesser lion, Leo Minor. Even the brightest star in the constellation struggles to shine brighter than 4th magnitude and the other stars, stretching out to the west in a rather haphazard line, are fainter still. To the north of Leo Minor is the constellation of Ursa Major, containing the famous arrangement of the stars which resemble a Plough or Big Dipper.
The stars in the Plough are easily seen and very familiar to northern hemisphere observers, but those in the rest of Ursa Major are less prominent. The head of the bear stretches out to the west, with its front leg to the south, and the rear leg extends south from Phecda, the star at the south-east corner of the pan of the Plough. There are around 200 notable stars in the constellation and their distances vary wildly. Of those easy to see with the naked eye the nearest star, Alula Australis, is found just to the south of Alula Borealis, which is the star at the bottom of Ursa Major’s rear leg and a mere 27.3 light years away. The most distant star easily visible with the naked eye is 83 Ursae Majoris, at a distance of 549 light years and found between and slightly to the north of Alkaid and Mizar, the two end stars in the bear’s tail. This distance, though, is nothing compared to the most distant star in the constellation, T Ursae Majoris, at an impressive 5930 light years and located between and slightly to the north of Alioth and Megrez, the two most westerly stars in the tail.
The two stars at the western end of the pan of the Plough, known as the Pointers, point directly to a 2nd magnitude star called Polaris, the North Star. It lies 430 light years away and marks one end of Ursa Minor, the Lesser Bear, which looks just like a small version of the Plough. Between the north of the tail of Ursa Major and the pan of Ursa Minor is the end of the constellation Draco, which is one of the largest in the northern hemisphere sky. From here, it curves round behind Ursa Minor and finishes to the north of Hercules in the south-east.
April: Southern Hemisphere Sky
Spica in Virgo offers a great starting place for our April guide to the southern hemisphere sky as it is just south of the celestial equator and in a fairly sparse area of sky. Careful study of the light from Spica reveals that it varies over a regular period of about four days. The reason is that it is not a single star but in reality a binary star system with the two stars in orbit around each other at only 18 million kilometres apart, which is nearer than the Sun and Mercury. The close proximity of the stars means they complete an orbit of each other in just over four days, leading to the regular dip in brightness as one of the stars eclipses the other. Just to the south-east of Spica is a constellation of much fainter stars, called Corvus, whose stars are a little brighter than 3rd magnitude, compared to the much stronger Spica at around 1st magnitude.
The stars in Corvus are arranged in a distorted square shape and are all broadly the same brightness as you look at them in the sky, yet their distances vary. Starting at the star nearest Spica then moving around to the south are Algorab at 88 light years, Kraz at 140 light years, Minkar at 303 light years and finally Gienah Corvi, the brightest star in the constellation, at 165 light years. Just to the west of Kraz at the south-west corner of Corvus is Cauda Hydrae, the second-brightest star in Hydra, the snake. Hydra is the largest constellation in the sky and winds across the top of Corvus, Crater and Sextans to the east before stopping at Cancer in the northern hemisphere sky.
Directly to the south of Cauda Hydrae is the prominent constellation Centaurus, with its brightest star, Rigil Kentaurus, or Alpha Centauri as it is more commonly known. Any uncertainty in identifying this star can be resolved due to its proximity to Hadar just a few degrees to the east. Alpha Centauri is a multiple star system of three stars: Alpha Centauri A and B, which orbit each other over an eighty-year period, and Proxima Centauri, which is a red dwarf star. At a distance of 2.2 trillion kilometres from the other two it is uncertain whether Proxima Centauri is gravitationally bound to them or just a visiting star on its way through the system. Either way, what is certain is that it is the closest star to our Solar System at 4.22 light years, which is 40 trillion kilometres. The distance to Proxima Centauri is easily determined by measuring its parallax, which is its apparent shift caused by the movement of the Earth around its orbit.
To the east of Alpha Centauri and Hadar are the stars of the Southern Cross, or Crux, which looks like a cross with its long axis aligned north–south. The most southerly star of the cross, Acrux, is also the brightest in the constellation and lies 320 light years from us. It is a multiple star system where two of its stars can be easily separated in small telescopes, but the brighter of the two is also a spectroscopic binary which means its hidden companion is only visible through studies of the spectrum of the star. The most westerly star of the cross is called Becrux, and lying to its south-west is perhaps one of the finest star clusters in the sky, the Jewel Box. It earned its name from Sir John Herschel, who when describing what he saw through his telescope said it looked like ‘a casket of variously coloured precious stones’. A telescope is not essential to appreciate this object though, as even binoculars will reveal its glittering collection of stars.
A little to the east of Acrux is Lambda Centauri, in Centaurus, which as a constellation straddles the Southern Cross. The star lies 410 light years from us, which means the light we can see today left around the time Galileo first turned a telescope on the sky! To the south-west of Lambda lies a cluster of stars embedded in beautiful nebulosity, visible to the naked eye and at a distance from us of 6000 light years. In photographs the nebula appears red in colour, which is a trademark of an emission nebula. In these interstellar nurseries the energy from the hot young stars causes the atoms to glow, giving off their own light at a very specific wavelength. The nebula, which is often called the Running Chicken Nebula because it supposedly resembles a chicken mid-dash, is only visible from a dark site and often only when employing a technique called averted vision. This observational skill simply means looking slightly to one side of a faint object, which allows the light to fall on a more sensitive part of the eye and make it appear brighter.
Directly to the south of Crux and its brightest star, Acrux (which is a shortened version of Alpha Crucis), is one of the smallest constellations in the sky, Musca. It traces out the shape of another distorted square, with its brightest star, Myla, which is the second-most northerly star in the constellation. In the south-west corner is Delta Muscae, which is distinctly orange in colour, and to its north is NGC4833, a lovely example of a globular cluster. It is just beyond the limit of visibility to the naked eye but an easy target for binoculars even though it is 21,200 light years away. NGC4833 is, like most globulars, composed of old stars and is one of hundreds of clusters in a halo around our galaxy. It was studying the distance and distribution of objects like this that led astronomers to gain insight into the size and shape of our galaxy.
The Southern Pleiades is an open or galactic cluster which can be seen to the north-east of Musca by the naked eye. Unlike the globular clusters such as NGC4833, open clusters are composed of young hot stars and around 50 million years old. The name comes from its appearance, which is similar to the Pleiades star cluster seen in the northern hemisphere sky in the constellation of Taurus.
The real showcase of the southern sky in April is the Milky Way itself, an unmistakable glow of light stretching across the sky from east to west. It is easy to see why understanding the shape and size of our galaxy was a tricky business – after all, we live inside it and have no way of assessing it from afar. The study of globular clusters started to unlock its secrets, but following on from that it was mapping the interstellar dust clouds like the Running Chicken Nebula in Centaurus that gave us the final answers. By their nature, the stellar nurseries are most common in the spiral arms, which meant studying them enabled a map to be built up of the structure and scale of our galaxy, with its centre about 30,000 light years away in the direction of Sagittarius in the western sky.
It is easy to see how our ancestors thought all objects in the night sky were stuck onto giant crystal spheres surrounding the Earth. Observations of real objects over many many years, some of which have been covered here, have slowly changed our view of the Universe and given us a real three-dimensional impression of our place within it.