Go out on a dark, cloudless night, preferably away from any bright sources of artificial light and look up at the sky. What do you see? The Moon perhaps, stars certainly, some bright some faint. Maybe one of them is a planet but which one and what is the difference between a star and a planet anyway? A brief flash of light streaks across the sky – a meteor! What is it? Where did it come from? Towards the west a point of light moves slowly across the sky before fading in the east, an artificial satellite, perhaps the International Space Station.  Are there other things in the night sky that can only be seen with a telescope? What about unpredictable phenomena such as bright comets and aurorae?

This article is a brief guide for the curious beginner to the inhabitants of the night sky and seeks to answer these questions.

The stars

There is one star that can never be seen at night – the Sun, for indeed the Sun is just another star, basically similar to all the others in the night sky but more impressive because it is many times closer. It lights up our Solar System and is the ultimate energy source of virtually all life on Earth.

Although seemingly a perfect unblemished disk, the Sun’s surface can be marked by cooler areas, sunspots, whose numbers wax and wane on a roughly 11 year cycle.

Examining our star more closely we find it is a hot ball of gas, mostly hydrogen, with a surface temperature of about 6000^oC and a diameter of nearly 1,400,000km. At its centre nuclear reactions are fusing hydrogen into helium and releasing huge amounts of energy while doing so. The Sun has been shining for 4.6 billion years. After another 5 billion years the Sun’s character will change, swelling up to become what is known as a Red Giant so large that it will likely engulf the Earth’s orbit.

As stars go, the Sun is very ordinary and run of the mill. The one thing that distinguishes it from the other stars for us is distance. Our Sun is ‘only’ 149,600,000km away. The next nearest star in the night sky is over 40,000,000,000,000km distant or over 267,000 times further away. Most stars are much further away than this. Once we start talking about distances to the stars these numbers become very big indeed and we need something better to use than kilometres. Astronomers often use the term ‘light year’, the distance light travels in one year or 9,500,000,000,000km. So the nearest star to Earth is 4.25 light years distant.

Looking at the stars in the night sky one thing is clear; they come in all shades of brightness from the very obvious to those at the very limit of visibility. Why is this? Are some stars truly brighter than others or do they just look brighter because they are nearer to us? The answer is both. The closest star to us, Proxima Centauri, is 4.25 light years distant and has an absolute brightness of only 0.005% that of the Sun. Sirius, the apparent brightest star in the sky is 8.6 light years distant and 24 times the brightness of the Sun. Lastly consider Deneb the brightest star in the constellation of Cygnus the swan. This is roughly 2,600 light years distant and has a brightness of perhaps 196,000 suns!

The stars vary greatly in size too. Proxima Centauri is only one third of the Sun’s diameter, Sirius 3.5 times larger and Deneb a full 410 times.

The constellations

Thousands of years ago our ancestors looked up at the sky and tried to make sense of what they saw. Cultures around the world drew on their myths and legends and the environment around them to join the dots of the stars and turn them into patterns with meaning to them – the constellations. The constellations we know today, visible from temperate northern latitudes, come down to us from the classical worlds of ancient Greece and Rome with many originating in Mesopotamia as long as five thousand years ago. The constellations of the southern sky, not visible from Europe, were created as European voyages of discovery brought knowledge of southern stars to northern people. While we continue to use the European names, indigenous peoples invented their own patterns and names thousands of years ago many of which have been passed on and continue to be used in parallel.

It is important to remember that in most cases the patterns are entirely arbitrary and there are no actual relationships between the included stars. Indeed the stars are usually at completely different distances to each other.

Learning the constellations and finding your way around the night sky can be an interesting project and if you would like to give it a go then a pair of linked tutorials starting here may be of help.

The turning sky

Just as the Sun moves across the sky, rising in the east and setting in the west, so do the Moon, stars and planets. The cause of this is simple, the turning of the Earth on its axis. More than this, as the Earth moves around the Sun in its orbit, so different patterns of stars become visible from season to season. In winter, looking south, we find the constellation of Orion the mighty hunter. With the spring comes Leo the lion. Summer finds Cygnus the swan nearly overhead and the autumn skies feature Pegasus the flying horse. Of course, these are just the most obvious constellations of each season, there are many others all around.

The Milky Way

Go outside on a dark clear night in late summer or autumn, look up at the sky and you should see a dim band of light stretching from overhead down towards the south or south western horizon. This is the Milky Way. Seen through a good pair of binoculars or a telescope it is resolved into innumerable faint stars. What we are seeing here is a view of our home galaxy, a giant star city at least 100,000 light years in diameter and containing something like 100 billion stars. Its shape is that of a flat disk with a central bulge and spiral arms. The Sun is embedded in the disc part way from the centre to the edge. From this position looking along the line of the disk we see countless stars. At right angles to this we are looking out of the plane of the galaxy into the spaces beyond and see far fewer stars.

Clusters and nebulae

Is our galaxy just composed of isolated stars or does it have other occupants? The answer is yes it does. Many stars are not alone in space but exist as doubles, triples or even more. Beyond this there are also real clusters of stars. There are two types, open clusters and globular clusters. A good example of an open cluster is the Pleiades or Seven Sisters, visible in the eastern part of the evening sky in autumn as a hazy spot. People with good eyesight can separate this haze into individual stars and with binoculars it is a splendid sight. Open clusters are generally found not far from the plane of the galaxy hence their alternate name of galactic clusters.

The second type of cluster is the globular cluster. These are giant balls of stars with numbers running into hundreds of thousands. They form a halo around the outside of our galaxy.

Lastly there are the nebulae, clouds of gas or dust. These may become visible either by reflecting the light of nearby stars, being excited to glow themselves by stellar radiation or appear as dark clouds silhouetted against a bright starry background. Looking at the picture of the Pleiades, the hazy patches are clouds of dust. The best known gaseous nebula is the Great Nebula in Orion. Roughly 24 light years across, new stars are being born there.

The wider universe

We live in a galaxy composed of stars, gas and dust. A reasonable question is whether that is the entire universe or is there more? The answer is yes, much more. Our galaxy is just one of perhaps 100 billion in the observable universe. They come in all sizes some much larger than our Milky Way, some much smaller. Not all are grand spirals like ours some are elliptical, some irregular. The nearest large galaxy is known as M31, found in the constellation of Andromeda it is some 2.5 million light years away.

Of course the observable universe is much bigger than this; current ideas place its size at around 92 billion light years across.

The Solar System

Having plumbed the depths of the universe let us now return home to our own backyard and look at the Sun’s family.

Our star is orbited by eight major planets and numerous smaller bodies including minor planets and comets. Several of these bodies are orbited by satellites of their own, their moons. For example our own moon orbits the Earth at an average distance of 384,000km.

Before we take a closer look at the solar system’s individual members we can now answer the question posed at the beginning of this article. What is the difference between a star and a planet? A star is a luminous body generating heat and light through the processes of nuclear fusion at its core. A planet however is a much smaller body orbiting our sun and not generating its own light. While thousands of ‘planets’ have now been discovered around other stars these are technically not known as planets but exoplanets. One last point, the only body emitting light in the solar system is the Sun, everything else, planets, moons, comets, etc shine in the sky only by reflecting the Sun’s light. They have no light of their own and were the Sun to be extinguished they too would cease to shine, only the stars would remain to light the sky.

The planets themselves all orbit the Sun in roughly the same plane and as a result appear projected against the background stars in a band of constellations known as the Zodiac. How the planets move is described by Paul Abel here. Many of the planets are visible to the naked eye although some require binoculars or a telescope.

It is important to realise that to the naked eye the planets never appear as disks but as points of light, the images in this article were all made using telescopes.

Should you wish to locate a planet for yourself, the British Astronomical Association regularly publishes “Sky Notes” including details of which planets are visible. In addition there are magazines and software available that will help point you in the right direction.

The inner Solar System

Let’s take a very brief tour of the Solar System. Starting from the Sun the first planet we encounter is tiny Mercury at 4879km in diameter. Being so near to the Sun its temperature can exceed 400^oC. Being closer to the Sun than we are, Mercury like Venus alternates between appearances in the evening and morning skies. However because Mercury orbits so close to our star it is never very high up in the morning or evening twilight and can be challenging to find.

Next up is Venus, slightly smaller than our Earth with a diameter of 12,104km. Permanently shrouded in dense clouds, a runaway greenhouse effect has lifted the temperature on its surface to over 400^oC. Like Mercury, Venus oscillates back and forth between the morning and evening skies and in classical times it was often believed to be two different objects. Apart from the Moon Venus can be the brightest object in the night sky. It is often visible in the twilight hanging like a lamp in the sky well before any stars are visible. Indeed it can become so bright that at times it is visible in broad daylight.

The third planet from the Sun is our own Earth, most important to humanity but in reality only a minor body. It has a diameter of 12,756km and is the only planet known to harbour life.

The Earth has one natural satellite, the Moon which orbits the Earth in just under a month. Its changing apparent shape or phases are a caused by this orbital motion. The Moon’s surface is heavily cratered in many places the result of massive bombardments of meteors, comets and asteroids early in its history. There are also darker areas, less cratered, the result of lava flooding low lying areas and solidifying. These dark areas together with the lighter heavily cratered regions combine to create the familiar “Man in the Moon” appearance seen near full moon.

Beyond the Earth we encounter Mars, the red planet. Only slightly more than half the size of the Earth at 6,792km and with a negligible atmosphere it endures temperatures mostly below freezing. Once thought to be the abode of intelligent life, it is now considered most likely sterile.
 

The surface is heavily cratered, with towering inactive volcanoes and numerous valleys, one as long as the United States is wide. There is strong evidence that water once flowed on its surface but that time is long gone. Mars has two tiny moons, Phobos and Deimos considered to be captured from elsewhere in the solar system.

Orbiting the Sun outside of the Earth, Mars can sometimes be visible all night. Occasionally it can be very bright while at other times it appears as a very ordinary star although its marked ruddy colour can give it away.

After Mars we reach the asteroid belt. Here there are a vast number of pieces of rocky debris ranging in size from tiny grains up to the dwarf planet Ceres which is 950km in diameter. The smaller asteroids can be very irregular in shape. One of the asteroids, Vesta, gets bright enough to be seen with the naked eye but then only with difficulty and at infrequent intervals. Binoculars will show many more but because they look like faint stars knowing exactly where to look is vital.

The giant planets

So far all the objects in the solar system have been rocky worlds but now there is a step change as we reach the realm of the giant planets. Far from having solid surfaces these planets are giant balls of gas possibly overlying a much smaller icy or rocky core.

First up is Jupiter. By far the largest planet in the solar system its diameter is over 11 times larger than that of the Earth. Comprised mainly of hydrogen its atmosphere is in constant turmoil with wind speeds often reaching 360km/hr. Jupiter has an extensive family of moons of which four are bright enough to be seen with binoculars. These four, named Io, Europa, Ganymede and Callisto, were discovered by Galileo when he turned his telescope to the sky in 1609/10. Ganymede is the largest, not only bigger than our own moon but even larger than the planet Mercury!

Jupiter is always bright and is easy to spot as long as it is not too close to the Sun. If you do locate it and have a pair of binoculars then try for the planet’s four brightest moons. They appear as ‘stars’ very close to Jupiter itself. You may not see all of them every time but looking from night to night you will find their positions will have changed.

 

Beyond Jupiter we come to Saturn, probably the Solar System’s most instantly recognizable planet with its iconic system of rings. Despite appearances, the rings are not a solid sheet but are comprised of innumerable small particles all orbiting around Saturn. Like Jupiter, Saturn has a large number of moons some large, some small. The largest of these, Titan, is unique in having a substantial atmosphere albeit one comprised mainly of nitrogen. Titan, like Jupiter’s Ganymede is also larger than Mercury.

Saturn is not difficult to find in the night sky if you know where to look as it is moderately bright.

Moving out from Saturn we reach the first of the so called ice giants: Uranus. Uranus is four times larger than the Earth and has a small family of moons although all are faint. The planet is in principle visible to the naked eye however it calls for good eyesight, a very dark sky and a precise knowledge of where to look. A pair of binoculars makes the job much easier.

The last of the major planets is Neptune which is slightly smaller than Uranus and has one major moon, Triton. Neptune is never visible to the naked eye and a pair of binoculars or a telescope is a must as is a good star chart.

Beyond Neptune we come to a belt of icy bodies, the Trans-Neptunian Objects, of which the largest is the former planet Pluto now demoted to the status of a dwarf planet.

Comets

A comet hanging in the sky like a flaming sword can be a most impressive sight but sadly the majority of comets never become this prominent and are only visible as diffuse smudges in a telescope. Most comets are visitors from the far reaches of the Solar System. They swing in on very elongated orbits, pass close to the Sun and then recede once more into the depths of space.

A comet can be described as a ‘dirty snowball’ comprised of ‘dust’ and ices. When far from the Sun in its orbit it is very cold and effectively frozen solid. Closing in on the Sun it warms up and begins to give off gas and dust. It is these emissions that cause the comet to grow and can create a prominent tail depending on the comet and how close it comes to the Sun. Once past the Sun it cools down once more, enters a state of deep freeze and becomes dormant until it next approaches our star.

Meteors

If you see a brief flash of light streaking across the sky you have almost certainly seen a meteor. A meteor originates as a particle in size from a tiny grain upwards (a meteoroid) orbiting the Sun whose path intercepts the orbit of the Earth. On encountering our atmosphere heat is generated, this not only affects the meteoroid but also the atmosphere causing it to glow in the meteor’s wake. It is this that we see as a meteor in the sky.

The vast majority of meteoroids are tiny and never reach the Earth’s surface. Occasionally a larger one will survive its passage through the atmosphere and reach the Earth’s surface. The resultant rock is now known as a meteorite.

Many meteors result from the debris shed by comets in their journey around the Sun. When the Earth encounters this trail a meteor shower can result. Two of the best are the Perseids seen every August and the Geminids every December.

Satellites

Up until 1957 the Earth had only one satellite, the Moon. Then, with the launch of Sputnik, the first artificial satellite, everything changed. Satellites in orbit around the Earth now number in thousands and several of these may be visible to the naked eye on any given clear night. Generally they appear as a ‘star’ moving steadily across the sky. Some will flash, many will not. The easiest to see is the International Space Station (ISS) which is usually quite bright. Like many satellites, because of the way the orbit works, there will often be a run of several days when the ISS is visible in the evening sky, then a period of invisibility followed by several days of visibility in the morning sky before sunrise. If you want to see the ISS go to heavens-above.com, enter your location and predictions will be presented for a number of satellites including the ISS. There is also an Android app if you prefer and similar apps are available for Apple devices.

Glows in the sky

The most famous and spectacular of the glows in the sky is the aurora. Best seen from higher latitudes and reasonably common in northern Scotland it is caused by matter ejected by the Sun at great speed channelled by the Earth’s magnetic field towards the poles and hitting the upper atmosphere causing it to glow in a variety of different colours. On rare occasions the strength of the Sun’s emission is such that the aurora is driven down to lower latitudes and a grand display can result.

A less spectacular but perhaps more predictable glow comes from Noctilucent Clouds (NLCs). These are clouds that form very high in the atmosphere during the summer months and are still illuminated by the Sun even though it had set a considerable time before. NLCs can be identified by their pearly blue colour, their position above the northern section of the horizon and by the fact that if ordinary clouds are present these show up as dark shapes being too low to be lit by the Sun.

Conclusion

If you have reached this far and were a complete beginner you hopefully now know more about the night sky than when you started and possibly more than most people.

Astronomy can be a very fulfilling and lifetime hobby at whatever level you chose. Maybe you just want to be an armchair astronomer and read about the wonders of the sky. Perhaps you aspire to a telescope and see or photograph the wonders of the universe for yourself. All the images in this article with the exception of the Stellarium constellation chart were made by amateur astronomers who are also members of the British Astronomical Association (BAA). Astronomy remains one of the few sciences where dedicated amateurs can still make a real contribution to scientific knowledge if they choose.