Beginners GuideHome
HomeResourcesAdvice/ Rough Guide/

CAS LiveLinks

Links relevant to whatís in the news now . . .

 

Link to printer friendly pageA Beginnerís Guide to the Night Sky

Stuart Atkinson, our founder member and EAS Secretary provides a beginners guide the the night sky. Your introduction starts here . . . .

Starting OutÖ..

Youíve looked up at the night sky before - no big deal, right? After all, whatís up there? A few dozen stars and, possibly, the Moon. Some of the stars look brighter than the others, and on exceptionally clear nights you have just about convinced yourself that one of them looked slightly red, but thatís about all youíve noticed.

Well, there is a lot more to see if you only take the time and look, believe me.

But to see the night sky in all its glory, you need to get back to nature - away from all the artificial light our modern society is so obsessed with. Travel out of town and keep going until the blue glows of living-room TV sets and the ghastly orange glare of streetlights is far behind you, and then you can begin to get to know the sky. If you canít do that, then go to your local park, or take a walk over to your nearest playing fields. Just do it.

But why do you have to get away from the lights?

Well, once you are away from them your eye will "dark adapt" and you will see literally thousands more stars. Your pupils will expand to let more light into your eyes, and chemicals will be activated which also increase their efficiency... Then, once youíre in the darkness, very quickly youíll notice there are many, many more stars visible than there were from your garden, or street. After just a few minutes you will realise that the stars are many different brightnesses. Some are so faint they can only just be seen out of the corner of your eye, While others are so bright they look like finely-cut jewels, or chips of ice shining up there above you. You might think thatís because some are nearer than others, but thatís not the case, and to understand why it isnít, you have to understand what stars actually are...

What are the stars anyway ?

The Sun, our nearest star The Sun our nearest star

People have all sorts of funny ideas about stars.

Some think they are balls of fire hanging above us in the atmosphere. Others think they are millions of pieces of rock out in space reflecting the Sunís light. But the truth is even more incredible.

You see, our Sun - you know, that brilliant ball of orange-yellow light which appears in the sky once every week or so - is a star, the nearest star to the Earth. It is a blazing ball of gas, a naked nuclear fusion reactor out in space, surrounded by a family of nine planets, of which Earth is one.

All the stars you see after the Sun has set are also suns immense globes of incandescent gas - but they are so far away that they just look like tiny pin-pricks of light to us here on Earth. Hereís a brain-aching thoughtÖ if there are aliens out there in space, they see our Sun as just a tiny star, twinkling in their night sky..!

You might think that all the stars are the same distance away from us - after all, they do look rather like dots of luminous paint spotted on the inside of a spherical shell surrounding the Earth. In fact, the stars are all different distances away from us, so when you look up youíre actually looking out into an endless ocean of blackness, where stars shine like tiny islands of light in the ink-black void. Space isnít up above us, itís out there, all around us, stretching off to infinity...

So why are some stars brighter than others?

The brightness of a star in the sky is rather misleading. A "bright" star might actually be a very weak star in terms of luminosity - i.e. how powerful it is - but may look very bright simply because it is close to us. On the other hand, a star that appears very "faint" to us might actually be a monster, a blazing beacon many thousands of times more powerful than our own Sun, but if it is very, very far away its brightness will be reduced accordingly. ( Think of stars as huge lightbulbs and you'll get the idea. If a 60W bulb and a 120W bulb are held near you at the same distance, which will be brighter? The 120W obviously. But if you moved the 120W bulb a mile away, the 60W bulb would appear brighter, just because it was closer, see? )

In addition, a starís brightness is also affected by other things too, such as how hot and how big it is. Our Sun has a surface temperature of 6000 degrees C, yet there are much hotter and much cooler stars out there, too. As for size, well our Sun is 109 times wider than our own planetÖ Öbut there are stars hundreds of times wider than that, and others a fraction of its size!

So you see, star brightnesses can be very confusing. Best not to worry about it too much!

How far away are the stars ?

Well now, thereís a question. Space, the Universe, whatever you want to call it is so huge that using kilometres to describe distances between objects in it is ridiculous, as ridiculous as using millimetres to describe the distances between cities. Instead astronomers use LIGHT YEARS as their unit of distance.

Now, thanks to writers of bad science fiction (or just bad writers in general) many people think a light year is a length of time, but thatís not true. A light year is the distance that light - the fastest thing in the Universe, with a speed of 230 million metres per second - can travel in a year, which is equivalent to 9.5 million, million kilometres.

Letís make some sense of that. The light from the Sun takes 8 minutes to reach us, so we can say it is 8 light minutes away. The nearest star to the Sun is 4.3 light years away (or 40 million million km away), and stars just keep going and going, getting further and further away the deeper into space you look.

So, when you look at a star say 10 light years away you are really travelling back in time, because you are seeing it as it was ten years ago. Some of the brightest stars are thousands of light years away...

Stars of many colours

So, now you know what stars are, and that they lie at different distances from us.

After a few minutes more of observing from your dark site you will realise that the stars are different colours, as well as brightnesses. There are stars as red as rubies, as blue as sapphires and as golden as amber. Why is this? Simple: a starís colour tells us how hot it is.

Question: which is hotter, a blue blow-torch flame, or a yellow candle flame? Answer: obviously the blow torch - and the same is true of stars, too: blue ones are hotter than red ones.

Our Sunís surface temperature of 6000 degrees C is pretty average, which is why it is a middle-of-the-road orange yellow colour. The red giant star Antares is 700 times wider than our Sun but much cooler, with a surface temperature of just a couple of thousand degrees, hence its colour. In comparison, the mighty blue giant Denebís surface temperature is almost 10,000 degrees!

Now you know why stars are different colours too...

What else can I see ?

Well, although you may never have noticed them before, after looking at the sky from a really dark site for a while you will begin to see the stars arenít just scattered at random all over the sky. Many actually form shapes in the sky like join-the-dots puzzles. Many of these shapes are "asterisms", small, very recognisable parts of larger areas of sky called constellations.

2004-20-a-small_web Pleiades star cluster

The sky is split into 88 of these constellations, and they were named and decided upon many centuries ago by ancient astronomers. They represent all sorts of things -heroes from myths and legends, animals, monsters, ships, you name it thereís a constellation for itÖ

The only problem is very few constellations look even remotely like the thing theyíre meant to be!

You may already know a few star patterns in the sky - Orion, the Plough, perhaps a few others. There are many more just waiting for you to get to know them... but that takes time, good weather and youíll need star charts.

That sounds scary, but donít panic! You can buy several monthly magazines with easy-to-use sky charts inside, and theyíll help you start to find your way around.

Dance of the stars . . . .

Now you know all about what stars are. But have you ever noticed that the stars move? Itís true! If you go outside on a clear night, and spot a bright star in the sky, maybe to the left of a tree or a chimney, half an hour later it will have drifted to the right!

This is because - and it makes sense if you think about it - the Earth turns once every 24 hours, and the stars, like the Sun, appear to wheel around the sky because of that. Like the Sun, stars rise in the east, cross the sky and then set in the west. So a constellation you saw low in the east at sunset will have crossed the sky and will be setting in the west at dawn.

And, because the Earth travels around the Sun, every season we are looking out at a different part of the Universe, and see we see a slightly different sky - with different stars and constellations - every season, too. This means that a bright star visible on one night will most likely not be visible 6 months later. (There are some exceptions: stars around the Pole Star - like those of the Big Dipper - never dip beneath the horizon during the year, they're so close to the pole they just go around and around it.)

And this is the main reason why astronomy is such a challenging and humbling hobby - it takes time, no, it DEMANDS time. The richest man in the world might be able to go out and buy a £10,000 telescope but that doesn't mean he would be able to see everything he wanted to that evening, because the star clusters, galaxies and even planets on his "To See" list might not actually be in the sky. They might be hidden by the horizon for months yet.

Astronomy is definitely a hobby with no short-cuts. But that's part of the fun!

How to spot the planets

Now you can see that some stars are brighter than others, do any look exceptionally bright, so bright they dominate all the others? If so, these are probably not stars at all, but planets, the Earthís sister worlds orbiting the Sun.

Yes, you can actually see five of the Sunís other eight planets with just your naked eye, shining in the night sky like bright stars, because although they are very far away from Earth they reflect the Sunís light back at us. You don't need a telescope!

But how do you tell a bright star from a planet?

Juipter is often the brightest object in the night sky. Jupiter, the brightest Ďstarí in the sky

Luckily, there is a trick. As you know from the nursery rhyme, stars twinkle, but thatís because our atmosphere is constantly shifting about; itís nothing to do with the star itself. Planets twinkle too, but not nearly as much as stars because a) they are much closer to us, and b) are actually tiny discs rather than pin-points of light. So, any particularly bright "star" which is shining with a very steady light, while bright stars around it flash for all theyíre worth, is probably a planet.

Astronomy books and magazines will tell you in advance which planets are visible and where, but two are very easy to identify, but here are a few clues to look out forÖ

If you have ever noticed a lantern-brilliant blue-white star, hanging just above the eastern horizon before sunrise, or above the western horizon after sunset, then you can bet your wage packet that itís the planet VENUS. And if you look up on a dark night and see a bright, unblinking "star" with a vivid red hue then that is almost definitely MARS.

What else can I can see ?

Youíve seen stars, constellations, possibly even a planet or two. What else is there?

Have you ever seen a shooting star? Iíll bet you have - a brief, almost-gone-before-itís-there streak of light across the skyÖ Ö but did you know that shooting stars arenít stars at all? They are really tiny grains of dust burning up as they plunge through the Earthís atmosphere, after drifting around the Sun for billions of years.

On any clear night you can expect to see a shooting star every half an hour or so, but at certain times of the year you can be guaranteed to see many more, when a "shooting star shower" occurs. These happen when Earthís orbit takes it through a thick lane of dust left behind by a comet (more of which in a moment, be patient!). Then, for a short period (usually one night) you can see dozens or even hundreds of shooting stars every hour, all apparently coming from the same part of the sky. The best ones are in mid-August, mid-November, mid-December and the beginning of January.

Occasionally you will see a very faint patch of misty-grey light in the sky, looking like chalk dust smudged onto black card with someoneís finger. Sometimes these smudges develop long, vapour-trail like tails. This is a comet.

Comet Kohoutek Comet Kohoutek

Long ago comets were thought to be terrible fireballs which streaked across the sky bringing death, disease and famine, heralding assassinations and wars. Now we know theyíre nothing of the kind. Comets are actually huge, kilometres-wide dirty icebergs which orbit the Sun in such long, eccentric paths that they are far away from it for up to 99% of their "year".

However, when they near the Sun their icy surfaces melt and gas and dust trapped within them streams off into space forming a tail which can be seen from Earth. You may think that comets are very rare - and most people only hear of the very bright ones which the media alert the public to - but on any particular night you can see up to half a dozen comets, if a) you know exactly where to look, and b) you have a telescope.

After youíve been outside for a while on a clear night you will almost definitely see a "star" glide across the sky. "A UFO!!!" you might screamÖ but the truth is rather less dramatic. The "star" is actually one of the many hundreds of artificial satellites in orbit above the Earth, taking weather photos, transferring phone-calls or eavesdropping on military communications. Although they are very small, we can see them because they are covered in highly-reflective metal panels and solar cells which reflect the Sunís light back down at us; although the Sun may be beneath our horizon, the satellite is so high up it can still see and reflect the Sun, in just the same way that a high-flying airliner often glints like a star in the dawn or dusk sky.

Watching the spacemen . . .

There are literally hundreds of satellites up there, all looking pretty much the same, but occasionally youíll see something much brighter and more impressive-looking than the "bog standard" satellite drifting through the starsÖ what is it?!

Did you know you can see the space station passing over head. Did you know you can see the space station passing over head.

Well, you might be amazed to learn that if youíre in the right place at the right time you can actually see the Space Shuttle and the International Space Station in the sky! They look like very bright "stars", far brighter than normal satellites, And if youíre really lucky youíll catch a Shuttle either approaching or pulling away from the ISS, and youíll see two "stars" flying through the sky together, in formationÖ

But when do you look? Not a problem - there are several great websites which will tell you in advance when and where the Space Shuttle and ISS will be visible from where you are, and you can find Links to them on the Links page on this site.

Ö and that just about completes your tour!

Hopefully you now realise what you have been missing, and will be inspired to go outside the next time the clouds clear and look up at the sky with fresh eyes.

 

You can read Stuartís  guide to what going on in the night sky in the bi-monthly Cumbria Life magazine. Stuart provides more regular updates on his Cumbrian Sky bog. (http://journals.aol.com/stuartatk/Cumbrian-Sky/)

Image Credits: All images NASA

 

[Home] [About Us] [Meetings] [Events] [Resources] [Contact Us]