A Down to Earth Guide to the Cosmos

The relevance of the different wavelengths is that they determine how light interacts with materials, and in particular how much it is bent or refracted. A shorter wavelength is refracted more than a longer wavelength, which means that the shorter-wavelength violet light is bent more than the longer-wavelength red light. As a result of this, a beam of light will, when passed through a prism, split into its component colours because they are bent by different amounts. Nature demonstrates this beautifully in the rainbow, where water droplets act like prisms. Of course, astronomers do not rely on water droplets – instead we use instruments called spectroscopes, which attach to telescopes and act on the incoming light from distant stars in the same way as prisms.

In Chapter 5 I touched on the technique where, if a star’s spectrum is looked at in fine detail, it is possible to see not only the individual colours but also a series of dark lines, called absorption lines, superimposed on the colourful spectrum. These lines are produced because a gas, present in the star, will absorb light passing through it and the type of gas present will determine the exact pattern of lines seen. Measuring the position and arrangement of lines, it is possible to work out what gases are present and therefore what a star is made of.

There is much more that can be understood from the spectra of stars and galaxies, but for now it is important to grasp how we can learn about distant stars just by looking at their light. You can do it for yourself, too, if you look at the colour of a star in the sky. A good example is Enif, at the south-western corner of Pegasus, or Altair, in the west of Aquila; by looking at their colour you can tell if they are a hot star or a relatively cool one. Imagine a workshop in which metal has to be heated to high temperatures. As the metal gets hot, its atoms start to give off light, initially shining with a red glow, then turning to yellow as the temperature increases, and on to white and ultimately blue. Simply by looking at the colour of the light given off from the metal, it is possible to tell roughly how hot it is, and in the same way it is possible to estimate the temperature of a star from its colour.

Stars differ in many ways other than colour; even their sizes span a huge range. There are some which are small and comparable in size to the Earth and others, like VY Canis Majoris, in the constellation Canis Major, which is thought to be around 2000 times the size of the Sun. If it were at the Sun’s position it would be large enough to extend to a point just before the orbit of Saturn. Though VY Canis Majoris is probably one of the largest stars ever discovered, it is very inconspicuous in the sky and needs binoculars even to be glimpsed.

Whether a star is visible in the sky or not is determined primarily by two things: the amount of light it is actually producing and its distance from us. These two factors are combined to determine a star’s absolute magnitude, which allows us to compare their real brightness rather than how they appear in the sky. This is done by calculating how bright the star would be at a distance of 10 parsecs (equal to 32.6 light years or 308.5 trillion kilometres, as described in Chapter 5), giving a basis for comparison. At a distance of 10 parsecs, in other words, the Sun’s absolute magnitude is just +4.7, making it quite a faint star as we would see it in the sky, visible from dark sites only, whereas VY Canis Majoris is a mighty –9.4, which would make it brighter than Venus at its most luminous, even casting shadows here on Earth. As explained later in this chapter, a magnitude with a minus value is brighter than one with a positive value. The absolute magnitude scale is great for allowing us to compare the real brightness of objects, but to understand how bright they appear to us in the sky now, regardless of distance, we need another measure, called apparent magnitude.

The origins of the apparent magnitude scale come from Greek astronomers around 200 BC, when they divided the stars visible to the naked eye into six groups, with the brightest being assigned a value of one and the faintest a value of six. Each brightness value (or magnitude, to use the modern term) was estimated to be twice as bright as the next, so a 1st-magnitude star was twice as bright as a 2nd magnitude star. The system was formalized around the middle of the nineteenth century by Norman Pogson, who defined a 1st magnitude star as 100 times brighter than a 6th magnitude star. This means the difference from one magnitude to the next is a multiplication by 2.51.

The system we use today is not limited to just the six magnitudes originally conceived by the Greeks, as the invention of the telescope revealed many more stars fainter than 6th magnitude. With the Hubble Space Telescope a distant galaxy was spotted at 30th magnitude, which places it at about one four-billionth the brightness of objects visible to the naked eye. We also have negative numbers in the modern magnitude scale to allow for objects brighter than 1st magnitude; for example, the Sun comes in at –26 and Venus at around –4.8 at its brightest. On maps of the sky, such as those in this book, the apparent magnitude scale for stars is represented by dots of different sizes, with fainter stars showing as smaller dots.

To complicate matters a little more, some stars vary in the amount of light we receive from them here on Earth, as a result of changeability either in the amount of light given off or in the amount that is blocked from reaching the Earth. These variable stars, as they are known, are common in the night sky but their presence is not particularly obvious; even the amount of light our Sun gives off varies, but only by about 0.1 per cent.

The first variable star was discovered in the seventeenth century and is known as Mira. It is a star about 300 light years away in the constellation of Cetus and is a binary star system made up of two components, a red giant star nearing the end of its life (Mira A) and a white dwarf star (Mira B). Mira A pulsates, leading to an increase and decrease in light output over a period of time, usually about eleven months. At its faintest it shines at around magnitude 10, making it undetectable by the naked eye, but at its maximum it can be seen shining at around 2nd magnitude just south of the celestial equator. The variability of Mira and the mechanism that causes this are not uncommon, and since its discovery several others have been identified and are now classified as Mira Variables. There are many other types of variable stars that actually change in their luminosity and it is a popular area of study for amateur astronomers. Any strange activity detected is then picked up by professional astronomers and studied in more detail.

Another reason for a change in the brightness of stars in the sky is their light being obscured by another orbiting companion star. The star Algol in Perseus is a good example of this; every three days its brightness dips from magnitude 2.1, when it is easily visible to the naked eye, down to 3.4. Its variable nature was first noticed in 1667 but it was almost a century later that the reason for this change was identified. It turned out that Algol was the first discovered example of a binary star system where the two component stars orbit in line of sight with the observer on Earth, so every few days one star eclipses the other, blocking a small amount of light from reaching the Earth and causing the star to dim a little. There are in fact two dips in brightness, a small drop only visible with instrumentation when the fainter of the two stars passes behind the brighter, and the main dip when the fainter star passes in front of the brighter one. There is actually a third star in the system but it is not responsible for any of the brightness changes we can see in the sky. The nature of this system was discovered by spectroscopic studies in which it was noticed that the star in the sky was wobbling back and forth as the smaller of the two tugged as it orbited, much like a hammer thrower spinning around. By studying this motion it is possible to calculate the mass of the two individual stars.

Aside from the spectroscopic binary stars like Algol there are many other stars in the sky that are part of binary or multiple star systems, many of which can be seen in amateur telescopes. Around 50 per cent of all stars in the sky have companion stars when studied but there are a few which are really stunning to look at.

Some of the stars in these binary systems orbit so close to each other that their proximity is quite destructive. If a red giant star orbits around a white dwarf, it is common for the red giant to spill material onto the white dwarf, whose mass increases slowly. Eventually the white dwarf reaches a critical mass and it explodes as a type 1a supernova, in the process ejecting its companion into space.

Violent explosions like this are not uncommon and are mainly associated with the death of a star. All stars die; even our Sun will one day, although this is not likely for another 5 billion years. Earlier in the chapter we looked at the nucleosynthesis process that transforms one element to another through fusion deep in the core of a star. Quite how a star ends its life is determined by its mass and the critical number is around nine times the mass of the Sun. When a star of less than nine solar masses has converted all the hydrogen in its core into helium it is left with a helium-rich core surrounded by a shell of hydrogen. The fusion process in the core now fades and, as a direct result of this, the force of gravity momentarily wins and the core is compressed. The compression of the core not only increases the temperature, from around 15 million degrees up to a staggering 100 million degrees, but the pressure too.

This is a key stage in the evolution of stars like the Sun as the increase in temperature and pressure starts to fuse helium in the core to carbon and the hydrogen shell into helium. As the fusion process picks up again, the outward thermonuclear pressure climbs, causing the star to swell up, increasing in size and energy output. In the case of the Sun, it is likely that it will increase in size so much that it will engulf the orbits of Mercury, Venus and perhaps even the Earth. As the star enters this new red giant phase of its life its increased energy production is spread over a much larger area, which results in a lower surface temperature and a shift in colour to the red part of the spectrum. Among the best-known examples of a red giant is Betelgeuse in the constellation of Orion, which shines with an unmistakable red light.

For most average-sized stars like the Sun this is as far as things go. Larger stars will continue to fuse carbon into heavier elements, but for the rest the repeated core contraction, temperature and pressure increase, followed by an increase in size and a subsequent reduction in temperature, will lead to the star slowly pulsing. The pulsations build up over time until they become so intense that the outer layers are eventually ejected into space and become a planetary nebula with just the core of the star remaining as a white dwarf.

A planetary nebula has nothing to do with planets although this term suggests a connection. They earned their name because their appearance through small telescopes is generally circular and planet-like. The reality of course is that they are the outer layers of a star expelled following the red giant phase. There are some beautiful examples of planetary nebulae within the grasp of modest amateur telescopes, such as the Ring Nebula in Lyra in the northern hemisphere and the Helix Nebula in Aquarius in the southern hemisphere. There are many more examples around the sky, some circular and others more dumbbell-shaped. It is thought the form is sculpted in some way by the magnetic field of the star, its rotation and also by the cloud’s orientation in space.

Stars that are about nine times the mass of the Sun or more suffer a rather more catastrophic end. Due to their larger mass the core compressions are more intense, with an increase in pressure and temperature that allows the fusion process to continue beyond carbon and through the heavier elements – neon, oxygen, silicon and iron – although each cycle lasts for a shorter period of time. Through these repeated cycles the outward pressure from fusion is helped in resisting the force of gravity trying to collapse the core by something called electron degeneracy pressure. Electrons are the tiny things that orbit around the nucleus of an atom and when they are packed closely together they try and move around more, generating pressure which, along with pressure from fusion, pushes against the force of gravity and resists further core compression.

Ultimately, though, this process is doomed to fail and when the core is finally fused into iron, further fusion will not produce any surplus energy so it is left to the pressure from the electrons to try and stop the collapse of the core. The iron core slowly gets bigger as the shell of silicon around it fuses into even more iron, until the mass of the core reaches about 1.4 times the mass of the Sun. At this mass, the pressure from electron degeneracy cannot overcome the crushing force of gravity and the core catastrophically collapses.

For a star that is less than twenty times the mass of the Sun another source of pressure stops a complete and total collapse and it is called neutron degeneracy pressure. A strange term but, effectively, during the collapse, electrons and protons (which along with neutrons make up atoms) crash together to form more neutrons and other particles. The neutrons are compressed so much that they exert an outward pressure and the core becomes one big neutron and further collapse is halted. Neutron stars are typically only a few tens of kilometres across but measure several times the mass of the Sun – in fact, a teaspoon of neutron star material would weigh the same as a few hundred big cathedrals. The collapsing outer layers rebound off the dense core and in an explosion which equates to letting off 100 million billion billion nuclear warheads they are ripped off into space in the blink of an eye. These are the type 2 supernovae, which are among the most violent events in the Universe.

The explosion of a supernova of this type gives off a phenomenal amount of energy – one star going supernova can outshine all the stars in a galaxy put together. Many examples of supernova remnants can be seen in the sky, such as the famous Veil Nebula in Cygnus, which is the result of a star that exploded about 7000 years ago.

If the original star is between twenty and forty times the mass of the Sun, it is set for an even more extreme fate since neutron degeneracy pressure is unable to oppose the collapse. The core of the star is compressed into an object even denser than a neutron star and known as a black hole. For the real giants among the stars, those over forty times the mass of the Sun, the star collapses completely into a black hole without ejecting any of the outer layers into space.

These strange and exotic objects rather defy common sense as the entire core of the star is compressed down to an object, called a singularity, so small that it has no size, but is simply a point in space. Do not try and think about it too hard. The mass of a black hole is so high that even light, travelling at about 300,000 kilometres per second, is unable to escape so the black hole neither emits nor reflects any light, hence its ‘blackness’. The boundary of the black hole is called the event horizon, which hints at the fact that any event beyond this point will never be seen by an observer in the outside Universe. Crazy stuff.

Black holes are not just the stuff of science fiction though, because we can detect their presence by measuring the movement of objects around them. By studying the spectrum of material being sucked into the black hole we can tell how fast it is moving and, from that, calculate the mass of the thing in the middle, the singularity. No black hole, by their very nature, has ever been directly observed, but the first strong candidate for one was found in the constellation of Cygnus, near the star Eta Cygni, and was named Cygnus X-1.

The planetary nebula and supernova stages mark the death of a star but also signal the start of another process lasting billions of years, in which the outer layers of the star will go on to form the next generation of stars. After the first generation of stars emerged following the creation of the Universe, the next generation formed out of material scattered throughout the galaxies from dying stars, including all the heavy elements created during the nuclear fusion process. By studying the chemical make-up of stars using spectroscopes it is possible to tell whether a star is one of the first, second or third generation of stars, which all have increasing amounts of heavy elements present.

In observing the stars we are not only learning about their lives, how they are formed and how they die, but gaining a real insight into how our Universe has evolved. Their study even leads us to understand where we have come from as every atom in our bodies has been synthesized inside the core of a star. So the next time you look up at a clear, dark sky, do not gloss over the stars to hunt down the planets and galaxies: take time to look at the stars – they are your heritage and they will not disappoint you.

September: Northern Hemisphere Sky

Sitting just to the north of the celestial equator in September are the stars that mark the extreme western end of Pisces. Forming what can only be described as an upturned classical pentagon when viewed from the northern hemisphere, the faint stars then extend out to the east, making it one of the largest constellations. The pentagon shape is actually known as the Circlet, with Gamma Piscium the brightest star in the group.

To the north of Pisces is the unmistakable constellation of Pegasus, which is dominated by a large square in the sky. The corners of the square are marked by Algenib in the south-eastern corner, Markab in the south-west, Scheat to the north-west and finally Alpheratz to the north-east. Alpheratz actually means ‘shoulder of the horse’, although the star is officially now a member of neighbouring Andromeda rather than Pegasus. Off the south-eastern corner extends the neck and head of the horse, ending at the red star Enif, the horse’s muzzle. Scan the sky with binoculars just a few degrees to the north-west of Enif to spot the globular cluster M15.

To the north of Pegasus is a rather bland region of sky with a small, faint constellation called Lacerta, with its stars no brighter than 4th magnitude. The stars are arranged in a zig-zag fashion running north–south for about the same distance as one side of the square of Pegasus. There are a couple of open star clusters in Lacerta, one of which, NGC7243, is at the limit of visibility to the naked eye. It can be easily picked out with binoculars just to the west of Alpha Lacertae and 4 Lacertae at its northern edge.

Over to the east of Lacerta is the constellation of Andromeda curving up to the east from Alpheratz, the north-east corner star of Pegasus. At a point roughly halfway between Alpha Lacertae and Alpheratz is a small planetary nebula called the Blue Snowball Nebula. At magnitude 8.3 it will not be visible to the naked eye but telescopes of 15cm or more will reveal a slightly fuzzy disc with a hint of blue colour. Although there is some uncertainty over the distance, it is thought to be just under 6000 light years away and, if this is the case, that makes it just under a light year across.

Now getting higher in the sky is the well-known ‘W’ of Cassiopeia, which appears to the north-east of Lacerta. The stars marking the main shape of Cassiopeia are all more or less the same brightness and its most westerly star, Caph, is a useful pointer to an easy-to-find open-star cluster known as Caroline’s Rose. Located just a few degrees to the south-west of the star, it was named after its discoverer, Caroline Herschel, in 1783. Binoculars or a low-power telescope will give the nicest view of this, while larger telescopes will reveal many more of its stars. Its age has been estimated at 1.6 billion years, which makes it one of the oldest clusters of its type.

Over to the west of Lacerta is the famous constellation of Cygnus, which looks like a great cross in the sky along the line of the Milky Way. Its brightest star, Deneb, is seen at the northernmost point and at the eastern wing tip is Zeta Cygni. Between Zeta Cygni and the slightly brighter star to its north-west is the Veil Nebula, one of the finest examples of a supernova remnant in the sky, although, unfortunately, one that is only visible to the naked eye under exceptionally dark skies.

Even further to the west is the distinctly bright star Vega in the constellation of Lyra. Off to its south-east is a faint parallelogram of stars and halfway between the two stars marking the southern side is the Ring Nebula, one of the best examples of a planetary nebula in the northern sky. The nebula is not visible to the naked eye; binoculars will reveal it as a fuzzy star but point a telescope at it to see it in its full glory, though the central star shown in photographs will only be seen through the largest of amateur telescopes.

Further north of Lacerta is the constellation of Cepheus, famous for being the home of Delta Cephei, the benchmark for Cepheid Variable stars. It looks like a slightly wonky house with its roof pointing to the north-east. The brightest star in Cepheus is Alderamin, found in its south-west corner. Just a few degrees to the south-east of Alderamin is Mu Cephei, otherwise known as Herschel’s Garnet Star. It is one of the most deeply coloured stars in the sky, appearing as its name suggests a deep red. In reality it is a red supergiant star and is one of the largest stars in our galaxy. Just to its south is an open cluster of stars called IC1396, which is enshrouded in faint red nebulosity. It is difficult to see visually but looks incredible in photographs. Within the nebulosity is a dense and dark concentration of interstellar dust that gives the appearance of an elephant’s head, hence its name, the Elephant Trunk Nebula.

Lying almost directly between the stars of Cepheus and Caroline’s Rose is another open cluster that was added to Charles Messier’s catalogue as M52. It contains an estimated 200 stars, which through binoculars look like a faint misty patch of the Milky Way, but turn a small telescope on it and the stars of the cluster pop into view.

M52 lies almost an equal distance from two stars in Cepheus, Iota Cephei to the north and Delta Cephei to the south. Delta Cephei was the second variable star to be discovered that exhibited the period–luminosity relationship that is common among stars of this type. The variability is due to the star physically pulsating and at maximum brightness it will have swollen to forty times the diameter of the Sun before returning to its original size. The whole process lasts no more than 5.37 days and runs as regular as clockwork. Alderamin is the brightest star in Cepheus and is easily found to the north-west of Delta Cephei.

September: Southern Hemisphere Sky

Aquarius, the water bearer, is a large but faint constellation well placed for observation in September and lies just south of the celestial equator. Alpha Aquarii is found almost on the celestial equator and is very slightly fainter than Beta Aquarii to its east. Due north of Beta Aquarii lies M2, a globular cluster that is great for smaller telescopes, although due to its huge distance of 37,500 light years larger apertures are needed to resolve the individual stars.

Over to the west of Alpha Aquarii is a large almost elliptical pattern of stars which represents the lower half of Aquarius’ body and legs. The brightest star on the eastern side of the ellipse is called Skat and to its south lies the brightest star in this part of the sky, Fomalhaut in Piscis Austrinus. Imagine a triangle with these two stars marking the base, then the top of the triangle to the east is the location of the Helix Nebula, which at 650 light years away is the closest of all the known planetary nebulae. Its proximity means it appears large in our sky, just over half the size of the full moon, but regardless of that binoculars and small telescopes will show it only as a fuzzy blob. Larger telescopes will make it easier to see but the use of a technique called ‘averted vision’ will also help. This involves looking slightly to one side of the object and using the more sensitive parts of the retina.

The prominent 1st magnitude star Fomalhaut at 25 light years away is fairly close to us, which means its system can be studied in great detail. It has a surface temperature of around 8500 degrees and gives off fifteen times more light than the Sun. A red dwarf companion star orbits Fomalhaut at a distance of 1 light year, but in 2008 the discovery of planet Fomalhaut b was announced. It orbits at a distance of 115 astronomical units, taking at least 800 years to complete one orbit. This was also the first exoplanet to be directly imaged using the Hubble Space Telescope.

The area of sky to the west of Fomalhaut is devoid of bright stars and is where the constellation of Sculptor is found. It looks like a ‘J’ on its side marked out by mostly 4th magnitude stars. There are a handful of moderately bright galaxies in the western half of the constellation, notably the well-known Sculptor Galaxy in the north-western corner, although at magnitude 7 it needs a moderately sized telescope to appreciate it in its full glory. Phoenix lies to the south of Sculptor, with its brightest star Alpha Phoenicis, which looks distinctly yellow in colour and is the nearest bright star to Sculptor. Due east of Phoenix is Grus, which to the ancient Egyptians represented a flying crane, with Alnair as its brightest star to the south-east and Beta Gruis to its west, and together they represent the crane’s feet. There is not much difference in brightness between the two stars but there is a stark contrast in the colours from the searing 13,500 degrees of the blue giant Alnair to the cooler red giant Beta Gruis at a modest 3400 degrees. Gamma Gruis at the northern end of Grus was once part of the neighbouring constellation, Piscis Austrinus, and marked the fish’s tail, which is still reflected today in its alternative Arabic name, Al Dhanab, meaning ‘the tail’.

Midway between Al Dhanab and Alpha Phoenicis is a faint cluster of four galaxies known as the Grus Quartet. These four spiral galaxies, NGC7552, 7582, 7590 and 7599, lie at a distance of around 60 million light years and are all gravitationally interacting with each other. This can be seen from the bursts of star formation in two of the galaxies and tidal tails of stars and dust clouds being drawn out from the others. The galaxy cluster is quite faint though, and telescopes with apertures of at least 15cm will be needed to pick them up, with a little more detail being visible through larger instruments.

Moving further south from Grus leads to Tucana, a constellation that was introduced in the seventeenth century to celebrate the discovery of the Toucan in South America. It looks a bit like a diamond in the sky with a few extra stars thrown in to distort the shape a little. Alpha Tucanae is an orange giant star 200 light years away and is the brightest star in the constellation. It is found at the eastern point of the diamond, has a temperature of 4300 degrees, yet kicks out as much energy as over 400 of our Suns.

At the opposite end of the constellation is Beta Tucanae, which is a multiple star system made up from six individual stars. To the eye, two stars are visible, the brighter Beta-1 Tucanae and fainter Beta-3 Tucanae, which are apart by about 6 light years. Through a telescope, Beta-1 Tucanae separates out to become two stars, Beta-1 and Beta-2 Tucanae. Taking well over 150,000 years to complete an orbit of each other, the main pair are separated by over 1000 times the Earth–Sun distance. Each of the stars is then accompanied by a companion star, making this a sextuple system.

Over to the south-west of Tucana but still within its boundaries is NGC292, otherwise known as the Small Magellanic Cloud (SMC). It looks like a detached piece of the Milky Way to the naked eye yet at 200,000 light years is well beyond it. In the sky, it appears about six times the size of the full moon and by comparing this against its distance it is possible to calculate that it is around 7,000 light years in diameter. As in many other galaxies, open clusters, globular clusters and nebulae can all be found in the SMC, many of which have entries in the New General Catalogue and can be picked out through large amateur telescopes with apertures of 25cm and above.

There are two globular clusters within the vicinity of the SMC: 47 Tucanae and NGC362. October’s guide looks at 47 Tucanae in detail but NGC362 is sadly often overlooked because of the prominence of the other. It can be found slightly off the north-west edge of the SMC and at 6th magnitude is just detectable as a faint fuzzy star with the naked eye from a dark site. Binoculars will not show much more, a small telescope from 10cm upwards will start to reveal more detail, but a 15cm telescope or larger is needed to be able to start resolving some of the individual stars. It lies at a distance of 27,700 light years and is 130 light years in diameter.