Star Safari: What Kinds of Stars Make Up the Constellations?

Double star with nebulaMost things on safari can kill you, and a star safari would be no exception. In fact it would be even worse; while Simba could potentially wipe out your tour party if he got too close, some of the objects in the sky could annihilate the entire solar system from a standing start. But just as a savannah trek could take you closer to untouched nature, a wander through the stars can blow your mind as well as your entire life away. Up until a certain age, you can believe that the magical lands of cartoons and films really exist, and seeing endless oceans of stars and galaxies can almost convince you of this again as an adult. Almost.

So be sure to pack a sense of wonder with your morbid fascination as we look at the kinds of stars you can find in the constellations.

Since there are multiple types, it’s easier to look at them in context, and like any potential mass murderer, a star is best understood if you begin with its troubled and turbulent childhood. Let’s begin by taking a look at its life cycle.

(N.B., if you want the express trip, just read the parts in bold).

 How are Stars Formed?

The Pillars of Creation by Hubble telescope

The “Pillars of Creation” in the Eagle Nebula, by NASA, Jeff Hesten and Paul Scowen.

Even the birthplace of a star can be spectacular, as these Hubble Telescope images show.

Unicorn nebula

The “Unicorn”, part of the Triffid Nebula, by NASA and Jeff Hester

These are sections of nebulae (singular = nebula), misty oceans of hydrogen and other “dust” material in space. When small pockets of them become especially dense, gravity corrals them into clumps and shrinks them down even further. This makes the inside temperature shoot up until the clump becomes a protostar.

Protostar Herbig Haro 46/47

A protostar known as Herbig-Haro 46/47. Courtesy of NASA and the Spitzer Space Telescope.

Career Move

A protostar can go in one of two directions. If it pulls its weight and is more than 0.1 times the mass of our Sun,  its temperature will rise to more than 10 million degrees Celsius (about 18 million degrees Fahrenheit), triggering nuclear reactions at its core, and it will become self-sustaining with a respectable job and a large retirement party that either ends with a bang or by sucking the fun out of everything. More on this later.

If it’s a slacker and is less than 0.1 times the mass of our sun it won’t become hot enough to trigger said nuclear reactions and will simply shine half-heartedly for a long while before fizzling out. On the plus side, such laziness extends its lifetime by several billion years. A star of this type is known as a brown dwarf, but in keeping with the confusing names, it’s actually purple. Eventually it would die and become a black dwarf, but this is even more theoretical, as our universe may not be old enough for us to see any black dwarfs as yet. I’ll get to red dwarfs in a little while, in case you’re wondering.

But what about the respectable ones that are big enough to go out there and make something of themselves?

Star Pupil

Before a star can settle down and become an adult, it goes through an awkward adolescent stage known as T. Tauri.

T Tauri star

It doesn’t photograph well either. This is part of the constellation Taurus – hence the “Tauri” in its name – and was the prototype for this class of star sequence. Atlas Image courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF.

Human teenagers have hormones, but for stars it’s gravity that can make them shrink in on themselves and fluctuate wildly. At the same time they (the stars) send out strong streams of particles known as stellar winds, and eventually they shoo away the dust that was originally surrounding them, much like shrugging off their childhood. Incidentally, if you ever feel or felt that your teenage years were dragging, spare a thought for our Sun – its adolescent T. Tauri stage may have lasted 30 million years.

Throwing off the uncertainty and awkwardness of youth can make anyone glow with happiness, but what about stars?

A Shining Example

Sun and solar flares NASA Goddard Space Flight Centre

Image by NASA Goddard Space Flight Centre

The power of our own star is so strong that it lights and heats the entire planet, can roast your skin in under an hour in some areas and cause permanent blindness if you look directly at it. All this, from up to 95 million miles (153 million km) away. If you think about this too long it can floor you with amazement, so you can see why some cultures worshipped the Sun as a divine being.

Mind you, if it had the ability to speak, it would sound like a squirrel with the wrong kind of nuts trapped in a tree, which would dispel the reverence somewhat. This is because the Sun, and all known stars, shine by fusing hydrogen into helium via the nuclear reactions at their core, and once these reactions have begun, the star becomes stable for a long period.

This is known as the Main Sequence stage, and for stars of 0.1 to 8 times the mass of our Sun this can last around 10 billion years. This would put our shining star in the throes of a mid-life crisis, but worry not, as it isn’t ready to crumble for another few billion years yet. Ample time to build that spaceship/time machine/portal to another dimension.

These nuclear reactions are what stop the star from collapsing in on itself. Gravity is still stubbornly trying to squash it in its invisible fist, but the pressure from the hot gas and radiation pushes outward, providing what’s known as hydrostatic support. Brown dwarfs and protostars also have hydrostatic support, but instead of nuclear reactions this is caused by the internal heat when they contract. Why do they contract? Gravity. So this unstoppable force has essentially shot itself in the foot by creating its own worst enemy.

When a star is fed up of its working life and runs out of hydrogen fuel it can announce its displeasure in one of three ways.

What Happens When a Star Dies?

red giant eats earth

Image by Fsgregs

If a star is 0.1 to 8 times the mass of our own and its hydrogen supply runs low, the helium at its core starts to react and creates carbon. The structure becomes unstable and the outer layers expand outwards, cooling down as they go. This turns the star into a red giant. When this happens to our own Sun, we’ll be screwed if we’re still on Earth, because although the outer layers are “cool” for a star, they would still be hot enough to roast every living thing and more and would expand out past Mars. If it’s any comfort, it would take a few million years for this to happen, so it wouldn’t be a case of looking out of the window one day and having your face melted off.

Once the layers have been completely blasted away, they create what is called a planetary nebula.

Image of planetary nebula

An infra-red image of a the Helix nebula, by NASA’s Spitzer Space Telescope

I’ll give you two guesses as to what’s wrong with that term. If you guessed “it’s neither a nebula nor anything to do with a planet”, awesome job. But, again, sometimes the oldest and most familiar terms stick even if they’re flat out wrong, so this would be the name given to the final fart of our exhausted star. What would be left behind?

The star’s original core would be left sitting in space as an incredibly dim and dense object known as a white dwarf.

White dwarf star

Image by Sephirohq

The most famous example of a white dwarf is Sirius B, the companion to the brightest star in the sky and part of the constellation the Great Dog – Canis Major. Sirius B is barely larger than Earth, but it’s so dense it has a mass as large as the Sun. A white dwarf can have a surface temperature as high as 100,000ºC (about 180,000°F), but eventually it would fade away into a cold black dwarf. Again, in theory.

A star more than 8 times the mass of our Sun would work hard, play hard, and instead of sleeping when it’s dead, erupt in a catastrophic explosion that would obliterate anything within a certain radius. If any of its friends survived, they would call it a supernova.

Supernova

And probably something unprintable.

The reason this kind of star goes out with more of a bang is due to its mass. When it runs out of hydrogen fuel, its core temperatures sky rocket and create new reactions, which in turn create heavier and heavier elements, such as iron. Eventually the core will be made mostly of iron, and since this refuses to burn in the same way, the star collapses and the subsequent explosion hurls most of its material every which way and beyond. This also leaves behind a small, condensed core, this time made up of neutrons, and therefore known as a neutron star.

If we can detect precisely timed radio pulses from a neutron star, it’s classed as a pulsar. A tree falling in the woods with no one around would probably make a noise anyway, so it’s possible all neutron stars send out pulses, but for the moment astronomers only go on whether we can pick them up or not, because it depends on which direction they’re facing. (The radio pulses, not the astronomers.)

Pulsar in Chandra

The pulsar is the bright spark on the right. Image by NASA

If a star is an even bigger monstrosity, its supernova blast will essentially turn it inside out and create a black hole.

Black hole

Artist’s impression of a black hole, by NASA

Gravity will finally have the revenge it’s been seeking all these years and cause the remains of the star to collapse in on itself and pull inwards anything – including light – which passes beyond a certain radius of the ruins. This friendly-sounding place is known as the event horizon, but at the time of writing, Professor Stephen Hawking and other quotable scientific notables are discussing if this is exactly right, and whether there is an “apparent horizon” instead, meaning that light and other objects would be temporarily pulled in but then released in garbled form. Although it’s equally adept at pulling in material and spewing out nonsense, Hollywood wasn’t aware of this term prior to 1997, which is just as well since “Apparent Horizon” sounds less like a terrifying sci-fi thriller and more a sarcastic parody.

In happier news, despite taking out planets, stars and anything else in their path, supernovae can help create new nebulae, which in turn give birth to new stars, and the cycle begins all over again. Here’s a fun fact; you, everything you are using and looking at right now was made from the material hurled into space by a supernova explosion. So we’d be equally screwed without them.

With these cataclysmic events in mind, it’s sensible for astronomers to keep an eye on exactly which types of stars there are in our neighbourhood and the known universe. To do this, they need to be classified, and there are a couple of ways of doing this.

How Are Stars Categorised?

Luminosity 

Alpha Capricorni, by Friendlystar

Alpha Capricorni, in the constellation Capricorn, by Friendlystar

I mentioned apparent magnitude in my previous post, i.e. how bright an object appears with the naked eye. The reason this is sometimes used as a measurement is because the brightness of a star can be relative. For example, some idiot with their fog lights on can be blinding at close range but dimmer than a torch two miles away. By the same token, a star can be thousands of times brighter than the Sun, but due to the distance between us and it, it would be a simple pinprick of light. Which is the case for, I don’t know, pretty much every other star in the sky.

Luminosity is therefore a measurement, usually in watts, of how much energy a star emits per second across all wavelengths, as this determines how bright it truly is regardless of how far away you are. The Sun’s luminosity in watts is 3.846 × 1026, but to make things simpler astronomers often use this as one unit of luminosity and say a star has x solar luminosities.

This is needed to understand one of the most famous tables in astronomy, the Hertzsprung-Russell diagram.

Spectral Types

The Hertzsprung-Russell or HR diagram is named after the Dutch and American astronomers who plotted star types against their luminosity. They did this independently, but their work was used to produce the chart below. As I mentioned, it’s often simpler to speak of luminosity in terms of our Sun rather than an endless stream of numbers, so on the y axis we have solar luminosity, with the Sun at 1.

HR diagram by ESO

Hertzsprung-Russell Diagram. Image by the European Southern Observatory

The x axis shows decreasing temperature, and the letters O, B, A etc. These refer to the stars’ spectral types, i.e. the colours and elements present in their spectrum. There are many colours contained in a beam of starlight, and a spectroscope can break them up as well as show which elements are contained in its atmosphere. For instance, the image below shows the spectrum of our Sun.

Fraunhofer lines in the solar spectrum

Image by NASA.

In simple terms, the dark lines, known as Fraunhofer lines, indicate the elements. For example, those across the yellow band show that there’s sodium lurking about in there somewhere.

Harvard University came up with the definitive way of classifying spectral types and the system used today; despite their best efforts, simply labelling star types as A,B,C and so on was too complicated, so the obvious choice was to go with W,O,B,A,F,G,K,M,C,S, L,T and Y. I hope the following table provides some illumination, but feel free to refer to it when needed rather than slogging through the entire thing from start to finish:

Type Spectrum Surface Temperature Example
W Blue with lines of nitrogen (classed as WN) or lines of carbon (WC, ha ha). up to 80,000°C /140,000°F Gamma Velorum in the constellation Vela
O Blue 40-35,000ºC/72-63,000ºF Alnitak (Zeta Orionis) in the constellation Orion
B Bluish-white, with prominent lines due to helium 25-12,000ºC/45-21,000ºF Spica in the constellation Virgo
A White, with prominent hydrogen lines 10-8000ºC/18-14,000ºF Sirius, in the constellation the Great Dog (Canis Major)
F White or slightly yellow, with very prominent calcium lines 7500-6000ºC/13-10,000ºF Polaris, in the constellation the Little Bear (Ursa Minor)
G Yellowish with weaker hydrogen lines and multiple metallic lines Giants: 5500-4200ºC/10-7500ºFDwarfs: 6-5000ºC/10-9000ºF The Sun
K Orange with strong metallic lines Giants: 4-3000 ºC/7-5000ºFDwarfs: 5-4000ºC/9-7000ºF Aldebaran, in the constellation Taurus
M Orange-red, multiple bands due to molecules Giants: 3400ºC/6000ºFDwarfs: 3000ºC/5000ºF Proxima Centauri, in the constellation Centaurus
C Reddish with strong carbon lines, formerly known as types R and N 2-5000ºC/3-9000 ºF T Lyrae, in the constellation Lyra
S Red, prominent bands of titanium oxide and zirconium oxide 2600ºC/4000ºF Chi Cygni, in the constellation Cygnus
L,T and Y Very cool red dwarfs (‘brown dwarfs’), T are methane dwarfs >2000ºC/>3000ºF GD 165B, in the constellation Bootes.

The keen-eyed among you will have spotted that neither of these tables shows the evolution of a star, because white dwarfs appear behind the red giants even though they are more advanced. We also see some red dwarfs (huzzah), which are small, cool stars on the bottom right of the diagram. (If you don’t know the reason for that cheer, are you sure you should be looking at an astronomy blog?) Red dwarfs are the most common type of star, and career-wise represent university drop-outs working in a fast food chain their entire, longer than normal lives.

As you can see, the hottest stars are blue or white, and our Sun is classed as a G-type yellow dwarf. Colour and size also have nothing to do with a star’s age, as you can have white or blue supergiants and red dwarfs which are perfectly happy in the prime of life, compared to white dwarfs that are in retirement and red giants about to have a meltdown.

You’ll also notice that  “bright” is too vague a description for a star and so requires a specific measurement of energy,  whereas “yellowish/slightly reddish, I guess” is perfectly fine.

Some stars are lucky enough to have companions to share their woes, material and explosions with.

Double Stars

Binary stars Sirius A and B

Binary stars Sirius A and B in the constellation the Great Dog (Canis Major). Artist’s impression by NASA.

Star Wars fans, rejoice – binary star systems are common in our galaxy, so there could be a real Tatooine out there somewhere.  A binary star system is where two stars move around a common centre of mass, so imagine two children holding both hands, facing each other and spinning around, or alternatively, like a pair of bells on a dumbbell. Any planet in this system would in effect have two suns, but depending on how far apart they are, this wouldn’t necessarily be obvious – one of them could be a mere dot on the horizon, or even invisible during the day.

One example of a binary star system can be found in the Great Bear constellationUrsa Major – in the northern hemisphere. Found in the tail of the bear, Mizar is in fact made up of two stars that are close together, aptly named Mizar A and Mizar B. The nearby star Alcor is playing gooseberry – it’s counted as a member of their group too, even though it’s further away.

Binary stars Mizar A B and Alcor

Mizars A and B and Alcor, by Cjkhul

However, as is often the case in space, looks can be deceiving, as not all double stars are binary systems. For instance, Alpha Capricorni from the Luminosity section is made up of two stars, but these two are close together only because of the line of sight and are in fact hundreds of light years apart.

Groups of two or more binary stars are also possible. Theta Orionis, in the constellation Orion, is made up of a group of stars known as the Trapezium, four of which are binaries.

Theta Orionis, the Trapezium, Orion

The Trapezium, in the constellation Orion

Nowadays, binary systems are thought to be formed by stars developing in the same region of space and forever linked by gravity, sometimes exchanging material if one evolves faster than the other (bigger stars age more quickly).

Exchanging (or to be more blunt, stealing) material from its fellow star is one of the reasons some of them don’t shine consistently.

Variable Stars

Variable star in the Chameleon constellation

Variable star Di Cha in the constellation Chameleon, , by Judy Schmidt.

Not all stars appear to shine steadily; if a star becomes dimmer for a time, it could be what’s known as an eclipsing binary, when its binary companion passes in front of it and blocks out some of its light. Algol, in the constellation Perseus, for one. Although Algol A is 100 times as luminous as our Sun, Algol B is a subgiant, so when it passes in front of A its bulk blocks some of its light.

This is also a good example of material swiping, or in astronomy, “mass transfer”Algol B was originally the more massive star, but as it entered its giant phase its outer layers swelled out and Algol A captured them.

A star can also be variable if it’s pulsating, and is known as – wait for it – a pulsating star. A particular type of pulsating star actually proves useful as a distance indicator in space and is known as a Cepheid variable. Delta Cephei, a star in the Cepheus constellation, was the prototype for this idea, as it’s a yellow supergiant that swells and shrinks with a regular pattern. Linking the length of this pattern with its luminosity allows astronomers to gauge distances, and this was instrumental in working out that there are other galaxies hanging about in deep space.

There are multiple types of variable star with just as many reasons for going dim once in a while, but some are more adventurous and clamour for attention whenever they make a change.

Exploding stars

Nova explosion in the Perseus constellation

A nova in the Perseus constellation. Image by the Caelum Observatory.

Eruptive and cataclysmic variable stars, such as Epsilon Cassiopeaie in the northern Cassiopeia constellation or Eta Carinae in the southern constellation Carina, can hurl out material and burn considerably brighter for short periods before dying down again. In fact, during the mid 19th Century, Eta Carinae almost trumped Sirius as beacon of the night, and is on course for a supernova at some point. But these aren’t the only sudden outbursts that can occur among the constellations.

Sometimes a star can blaze out of nowhere and then fade back into obscurity. This is known as a nova, but despite the name it doesn’t mean “new star”. It’s actually due to an outburst from the white dwarf part of a binary system. Even though it’s the dinkier of the two, a white dwarf has a hefty gravitational pull and siphons material away from its neighbour. This builds up into an accretion disk, and the more it builds up, the higher its temperature becomes until it erupts in multiple catastrophic nuclear explosions, destroying the material or flinging it outwards before returning to normal.

Accretion disk from a white dwarf in a binary system

A white dwarf pulling material away from its blue giant binary partner. Artist’s impression by NASA.

Think of it as a bunny-boiling spouse who periodically steals, hoards and then destroys their partner’s possessions in a white-hot rage before calming down again.

Other Deep Sky Objects

As we have seen, stars can be a volatile and deadly bunch, both in birth, during their career and in the throes of death and the afterlife, at the same time helpfully chucking life-facilitating materials about. But they’re not the only monsters lurking in the oceans of deep space, and some of these are cleverly disguised as constellation stars. Much like the glasses in They Live, telescopes and binoculars are sometimes the only way to detect these objects and distinguish them from single pinpoints of light or mysterious shadowy patches.

Star Clusters

M45 Pleiades Star cluster in the constellation Taurus

The Pleiades star cluster in Taurus. Image by Bob Star.

There are two kinds of star clusters – open and globular – and these as well as nebulae were helpfully categorised in 1781 by French astronomer Charles Messier. Funnily enough, he must have counted this as donkey work at the time because he was only trying to sort them out so he could distinguish them from his real passion – comets – but the Messier Catalogue is now what he is most famous for. It assigns each cluster or nebula with an “M” plus a number; for example the Pleiades star cluster in the constellation Taurus is M45.  Likewise there is also the New General Catalogue and Sir Patrick Moore’s Caldwell Catalogue, so this should solve the mystery if you ever saw an interstellar object labelled “Mx”, “NGCx” or “Cx” – these refer to the numbers of these objects in these collections.

Pleiades is an open or “loose” cluster, which is rather like a family of sextuplets, or more likely centuplets given the hundreds of stars they can sometimes contain. All of them were born in the same interstellar cloud and are of the same age, and eventually they will be pulled away by other non-cluster stars, as if they met their significant other and moved out of the family home. Having said this, they develop at different rates and have no definite structure to their group, so this comparison only goes so far.

Conversely, globular clusters are extremely old and can be so densely packed with stars it’s hard to separate them. This is the opposite of a stellar creche, as there are scarcely any nebulae within globular clusters and so no new stars are formed. Think of it as a retirement home; the main stars within are red giants or supergiants. The largest globular cluster is Omega Centauri, part of the constellation Centaurus in the southern sky.

Omega Centauri globular cluster

An infra-red image of Omega Centauri. Huzzah for images that already include their credits!

Incidentally, globular clusters are less common – they make up just 28 of the 110 Messier Catalogue objects – and there are more of them in the southern than northern skies, possibly due to our “lop-sided” view of the galaxy.

Objects don’t have to be sparkling and luminous to grab our attention though.

Dark nebulae

Horsehead Nebula near Orion

One of the horses of the apocalypse? Nope, just the Horsehead Nebula, a dark nebula within the area of the constellation Orion

Despite first impressions, a dark nebula isn’t the evil, soul-sucking villain to the beautiful life-giving nebulae I mentioned at the beginning of this post. It’s still a nebula composed of hydrogen and space dust, but, if you’ll forgive the obvious, it only appears dark because it’s not illuminated enough, and is only visible because its “dust” is thick enough to blot out light from objects behind it. Dobbin here is one of the most famous examples of dark nebulae, but there is also the affectionately named “Coal Sack” within the Southern Cross constellation – Crux.

Both kinds of nebulae can form stunning patterns, but the constellations can also include another, even more exhilarating type of object.

Something called a galaxy.

Galaxies

Image of a spiral galaxy in Eridanus.

A spiral galaxy and its small companion in the constellation Eridanus. Image by the European Southern Observatory.

It’s only in the last 100 years that we’ve discovered that our galaxy, the Milky Way, isn’t the centre of the universe or the only game in town. Thanks in part to Edwin Hubble, Charles Messier and the “standard candle” Cepheid variable stars, we now know that there are oodles of other galaxies out there, also congregating in groups or clusters and home to yet more stars, planets, nebulae and lethal monstrosities such as black holes and supernovae. Given that the lone star in our solar system can inspire knee-trembling awe if you think about it for too long, galaxies must be the wandering gods of the cosmos and therefore named accordingly, right?

Nope. Say hello to the Tadpole, in Draco:

Tadpole galaxy

Image by NASA

and, brilliantly, the Sombrero near Virgo:

Sombrero galaxy near Virgo

Image by NASA

As you can see, galaxies can come in different shapes and sizes, but whether they’re flattened, swirling or seemingly splattered on the cosmic background, they all have a nucleus – thought to be a black hole, supermassive or not – and are found in clusters rather like stars. Our cluster is imaginatively known as the Local Group. Incidentally, the amusingly named examples above are not part of it; only Andromeda and the Triangulum galaxy are included in our exclusive club.

Galaxy clusters can’t wait to get away from their neighbours, but some are faster than others, meaning that Andromeda will at some point collide with our slow coach the Milky Way. Again, no sleepless nights needed, because to quote Sir Patrick Moore:

by then the Earth will no longer exist anyway.

and, if we play our cards right, by then we could either be zipping in between these enormous galaxies or jumping back and forth through time to enjoy them and our own Earth all over again.

Conclusion

Helix Nebula in Aquarius

The Helix Nebula, a planetary nebula (so not a nebula at all really) in the constellation Aquarius. Image by the ESO.

Hopefully by this point the constellations will seem more than just vague specks of light in the night sky. Each individual star is a character in its own right, albeit a ticking time bomb, a master of disguise, part of a stellar creche or even a kind of zombie lighthouse, so imagine what you can find by looking at a whole constellation pattern, let alone their origin and the legends surrounding them. Some of the points in a constellation can contain millions of constellations in and of themselves, assuming any aliens out there also enjoy a spot of dot-to-dot with the skies, and at a moment’s notice, some of them could erupt into blinding haloes and disappear forever.

This isn’t likely to happen soon, but nonetheless it’s time to embark on the main journey of this blog: a constellation cruise.

Thanks for reading!

Sources

A map of the stars

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2. CLARK, S. (2010) Voyager: 101 Wonders Between Earth and the Edge of the Cosmos. London: Callisto and Atlantic Books.

3. CRAY, D. (2012) Tatooine Now: NASA Finds Planet Orbiting Binary Star System. Time Magazine. Available at: http://science.time.com/2012/08/29/tatooine-now-nasa-finds-planet-orbiting-binary-star-system/ [Accessed 04/03/2014]

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5. MOORE, P. (2007) Atlas of the UniverseSixth Edition. London: Philip’s.

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7. SAUL, H. (2014), Stephen Hawking: ‘There Are No Black Holes’. The Independent Online. Available at: http://www.independent.co.uk/news/science/stephen-hawking-there-are-no-black-holes-9085016.html [Accessed 01/03/2014]

8. SOPER, D.E. (n.d.), Luminosity and How Far Away Things Are. The Electronic Universe. Available at: http://zebu.uoregon.edu/~soper/Light/luminosity.html [Accessed 01/03/2014]

9. SPARROW, G. (2013) Constellations: A Field Guide to the Night Sky. London: Quercus.

10. SPARROW, G. and STOTT, C. (2013) Planisphere and Starfinder: The Complete Beginner’s Guide to Exploring the Night Sky. London: Dorling Kindersley.

11. Encyclopaedia Britannica (n.d.), Nature Worship. Britannica.com. Available at: http://www.britannica.com/EBchecked/topic/406588/nature-worship/38286/The-sun-as-the-centre-of-a-state-religion [Accessed 20/02/2014]

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13. NASA (2010), Stars. NASA’s Imagine the Universe. Available at: http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html [Accessed 28/02/2014]

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