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The Beginner's Sky
Sunday, 2019, December 8 - 11:26 |
Introduction

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

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.
WARNING: Never stare at the Sun either with or without a telescope, binoculars or other optical aid. Severe eye damage or even blindness can result.
Examining our star more closely we find it is a hot ball of gas, mostly hydrogen, with a surface temperature of about 6000oC 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.

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

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

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.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

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.

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 400oC. 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 400oC. 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 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.

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

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.

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 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

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
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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.
Whatever your intentions the BAA can help you. There are a series of tutorials aimed at the person just starting out. These are available irrespective of whether you become a member or not so feel free to dive in and start your journey. When you are ready, the BAA will be pleased to welcome you as a new member and support you in whatever direction this amazing hobby takes you.
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