Space is all around us. There's so much to say about it, it's hard to know where to start. So let's start at the very beginning...

A is for Astronomy and Astrology

Astronomy is the study of space and the things that you find in it, such as stars, planets, and comets. Astronomers look into space using telescopes and other equipment and try and figure out how it works and where it all came from.

Astrology is the prediction of a person's fate based on the belief that their life is influenced by the position of the sun, stars and planets at the exact instant they were born.

The two names are quite similar - just try not to confuse them or you might not get the right idea about this entry.

B is for Big Bang

The Big Bang is the name astronomers use for the start of the universe. The whole of space and everything in it was scrunched up into a tiny ball smaller than a pea. Then suddenly, there was a massive explosion and the pea blew apart. Loads of light and very hot stuff came shooting outwards. Gradually the bits cooled down and formed into gases, mainly hydrogen and helium. Later, these formed into stars and dust and galaxies. The galaxies are still shooting apart as a result of this explosion. The Big Bang happened about 15 billion years ago. There are two possible theories as to what happens next. In the first, the universe continues to expand, gradually slowing down but never actually stopping. The other theory is that of the Big Crunch - where the universe starts to contract again, until it becomes a pea once more.

C is for Constellation

The stars in the sky, as we see them, are dotted around any old way without any definite pattern. If you look carefully, you will see that although the stars are not always in the same place in the sky, they all move together, so that you can see rough patterns in the stars. In ancient times, people thought they saw a group of stars which looked like a swan, another like a dolphin, another like a plough. They called these patterns constellations and made up loads of them. Sometimes it is very hard to see any relation between the group of stars and what it is supposed to look like.

Astronomers use the Latin names for the constellations, so instead of talking about the Great Bear, they say Ursa Major. This is just Latin for 'Great Bear'.

The three easiest constellations in the sky to recognise are Orion the Hunter, the Plough or Big Dipper, and the Southern Cross. The Plough can only be seen if you live in the northern half of the world. The Southern Cross can only been seen from the southern half of the world. Orion can be seen from anywhere in the world.

D is for Darkness

Space is full of darkness. We're lucky enough to live very close to a star, the Sun, so it's bright half the time. Most of space is far from stars so it is very dark. It is also very, very cold.

E is for Earth

The Earth is the planet we live on. It is the third planet from the Sun. It is about 13 thousand kilometres across and is made of rock with a thin coating of water and air. The Earth spins once a day, causing the sun to appear to move around the sky once a day. It also orbits around the sun once a year. The different ways it faces during this year cause the seasons.

The Earth is the only place we know in the Universe where there is life. Life is based on water. If the Earth was any hotter or colder, there would not be liquid water, so there would be no life. It is possible that there are other types of life based on things other than water, but nobody knows for sure.

F is for Force of Gravity

The force of gravity is the glue that sticks the stars and planets together. Without gravity, they would just split up and blow away like dust in the wind. Gravity is a pull between any two things. Each thing pulls the other one. In space, bits of dust pull each other together into clumps. When the clumps get big enough, they make planets. When they get really big, they get hot enough in the middle to go on fire and they turn into stars. Gravity also pulls the planets towards the sun. Because the planets are moving, however, they don't fall into the sun, but go round the sun roughly in circles. In the same way, the Moon goes round the Earth and the Sun goes round the centre of our galaxy.

G is for Galaxy

A galaxy is a big bunch of stars. Some galaxies look like blobs, but the most impressive ones look like big spiral swirls. Our own Sun and all the stars you can see in the sky are in a galaxy which is known as the Milky Way. It is a spiral galaxy. Galaxies can contain from a few hundred thousand up to several billion stars.

H is for Hydrogen

Hydrogen is a type of gas. It is the lightest gas in the universe and by far the most common. Stars are made of hydrogen, which they burn¹ to produce light and heat. The gaps between the stars also have a small amount of hydrogen in them. Big planets like Jupiter have a lot of hydrogen in their air. On Earth hydrogen is not very common as a gas, because in our air it explodes. Hydrogen is one of the basic building blocks of all life on Earth, and is also a major constituent of water.

I is for Io

Io is one of the moons of Jupiter. Jupiter has four big moons, each about the same size as the Earth's moon. Until recently there was not much known about any of these moons. The spacecraft Voyager changed all that by sending back wonderful photos of each of the four moons. Io turns out to be one of the most remarkable things in the solar system. It is covered in volcanoes, which constantly spew out new rock onto the surface, so the patterns of the surface are constantly changing. Io is bright red and black in colour. The other moons of Jupiter are quite interesting too: Callisto, Europa and Ganymede. These four moons are known as the Galilean Moons, as they were discovered by Galileo. You can see them yourself with a good pair of binoculars as four tiny points of light beside Jupiter.

J is for Jupiter

Jupiter is the biggest planet. It is more than twice as big as all the other planets put together. If the Earth was melted down and poured into a Jupiter-sized mould, it would take 1,400 Earths to fill it. But because Jupiter is made mainly of gas, it weighs only 317 times as much as the Earth.

Jupiter's surface is covered with clouds and weather patterns. One thing on the surface is the 'Great Red Spot', which is a hurricane that has been blowing for at least the last 300 years.

Because Jupiter is so big, it has attracted a lot of smaller lumps, which orbit around it as moons. It has four main moons (see 'Io') and a lot of small ones.

K is for Klingons

Most of you will have seen Star Trek. The Klingons were the baddies in the early first series but had become our friends by the time Star Trek: The Next Generation hit the screens.

Although Star Trek is pure fiction, the writers attempt to make it as realistic as possible. It is therefore an interesting introduction to space travel. To make the stories more interesting, the USS Enterprise needs to be able to get from star system to star system quickly in time for the next episode. They invented the Warp Drive, by which they can go pretty fast.

Real space travel is very different. Scientists reckon that the fastest anyone can go is the speed of light, 300 million metres per second. At this speed, it would take about 33,000 years to get to the centre of our galaxy (the Enterprise takes about 40 years to cover the same distance).

Most space travel outside our solar system will therefore be extremely slow. It will take an entire civilisation's lifetime to send a human to another star.

L is for Light

Light is what makes us able to see the stars. It is produced by the Sun and the other stars and reflects off the planets and moons.

Light travels incredibly fast, 300,000 kilometres each second. Space is big, so even at this speed it takes light a full eight minutes to reach us from the Sun. This means that we are not seeing the Sun as it is now but the way it was eight minutes ago.

Nearby stars are so far that it takes light many years to reach us. Astronomers often measure distances in terms of the number of years it takes light to travel the distance. One light-year is the distance light travels in a year. The nearest star after the sun is four light-years away. The centre of our galaxy is 30,000 light-years away. The furthest things visible in the biggest telescopes are 10 billion light years away.

M is for Moon

The Moon is a huge lump of rock which orbits around the Earth. It is so big that its own gravity has pulled it into a ball shape.

The Moon does not produce any light of its own, but reflects the Sun's light like a giant mirror - it is a natural satellite. The Moon is really dark grey in colour; it looks white because the sky against which we see it is so much blacker in comparison.

We know a lot about the Moon because we have sent people to it. 12 people landed on the Moon and another 12 flew around it without landing.

The name 'moon' is also used for small lumps of rock orbiting around other planets. So Jupiter has 63 moons, Mars has two and so on.

N is for Nebula

A nebula is a cloud of dust in space. They appear as smudges in the sky. There is a good one at the sword of Orion. Nebulae look very nice through big telescopes. Some of them have very nice names too, such as the Crab Nebula, the Horsehead Nebula and the Trifid Nebula.

Many nebulae are the birth place for new stars. The dust contracts into balls and eventually forms into stars. Some nebulae are the remains of stars that have exploded. One such is the Crab Nebula, which was a star that exploded in the year 1054. The explosion looked like a new star which was so bright it was visible during the day. After a while it faded away leaving the Crab Nebula.

O is for Orion

One of the easiest constellations in the sky to see is Orion the Hunter. It is visible from all over the world. For thousands of years, people have seen the stars of Orion as a giant with a club in one hand, a lion's skin over his other arm, and a sword hanging from his belt.

The three stars of Orion's belt are unmistakable as they are in a straight line, evenly spaced and all the same brightness. Above the belt, you can see three stars which are his shoulders and his head. Over to the right is the lion's skin. Below the belt are Orion's knees.

Orion's sword is actually a very interesting nebula.

In countries south of the equator, Orion appears upside-down in the sky.

P is for Planets

The Sun has a large number of bits and pieces going around it in orbits. The eight largest of these are called planets. These are divided into:

Four Rock Planets

• Mercury
• Venus
• Earth
• Mars

Four Gas Planets

• Jupiter
• Saturn
• Uranus
• Neptune

There are also dwarf planets such as Pluto, comets such as Comet Halley and other lumps of rock and ice.

There are also planets orbiting around other stars. You can't see them directly because the stars are too far away, but scientists with big telescopes can see that the stars are wobbling, which indicates a big planet orbiting the star.

Q is for Quasar

Quasars are things in space which give off radio waves. We can tell from the kind of waves they give off that they are moving away from us at tremendous speeds. This means that they are either very close to us and all heading away from the Earth, which seems unlikely, or that they are very, very far away and are moving away because the entire universe is expanding. If this is true, then quasars are extremely bright, about as bright as an entire galaxy. The current theory is that they are the cores of galaxies with a huge black hole. Stars falling into the black hole are torn apart, giving off huge bursts of energy in the process.

R is for Rings

Saturn is the sixth planet in our solar system. It is spectacular as it is surrounded by rings. These rings look like a flat disc with a hole in the centre for Saturn. On closer examination they turn out to be thousands of separate rings. The rings are made of tiny lumps of rock, each one in orbit around Saturn.

To see the rings of Saturn from Earth, you will need a good telescope. Spaceships have gone to Saturn and taken photographs, so we have plenty of good pictures.

Robot spacecraft visiting the outer planets have discovered that the other gas planets, Jupiter, Uranus and Neptune also have rings, but they are insignificant compared with the rings of Saturn.

S is for Stars

When you look into the sky at night, you see lots of stars. In the daytime you see the Sun. The Sun is actually a star as well. It is just much closer to us than all the other stars, so it is very bright.

All stars are balls of hydrogen and helium gas. There is so much gas that the gravity pulls it all together. At the centre, the gas is squashed together very tightly. The rubbing together of the gas causes it to heat up so hot that the hydrogen starts to burn in a special process called fusion. This gives off loads of heat and light.

Really hot stars burn with a bluey-white colour. Cooler stars burn yellow or orange, while the coolest, which are still ferociously hot, burn with a red colour. If you look carefully at the stars, you can see differences in colour between them. Vega is bluey-white, while Betelgeuse is a sort of orange.

T is for Telescope

The telescope is an instrument for looking at things that are far away. The simplest telescope is just two lenses in a tube. Extra lenses can be added to sharpen the image. Telescopes made with lenses are called refracting telescopes. You can also make a telescope from a curved mirror and a lens. This is called a reflecting telescope. Most really big telescopes are reflecting.

The telescope was first used by Galileo to look at the sky; he saw four things that amazed him. The first was mountains on the Moon. This led him to believe that the Moon is a world like the Earth. Secondly he saw that there was something odd about Saturn. His telescope wasn't good enough to see the rings properly, but he could see it wasn't just a sphere. He saw the moons of Jupiter. He decided that if these moons went around Jupiter, then there were things that didn't go around the Earth, so the earth can't be the centre of the universe. He decided on a different system where moons go around the planets and planets go around the Sun. He also saw spots on the surface of the Sun and decided that the heavens must not be perfect after all. He was persecuted by the Catholic Church for these beliefs.

U is for Universe

The Universe means everything that we can see and a lot of stuff we can't. It is unbelievably huge and getting bigger all the time. We can tell that all the galaxies are rushing apart from each other. From this we can conclude that once long ago, they were very close together and that an explosion known as the Big Bang caused them all to blow apart.

The force of gravity between all the various things in the universe is pulling them back together and slowing down the expansion. We don't know yet what will happen to the Universe, but it will be one of two things: either there is enough matter in the Universe to stop the expansion and all the galaxies will start to fly back towards each other, eventually colliding in a 'Big Crunch', or they will keep expanding until each galaxy is completely alone in the Universe. If this happens, the Universe will eventually peter out.

V is for Venus

Venus is the bright white planet that you often see just after the Sun has gone down. It is the brightest thing in the sky after the Sun and the Moon and has even been known to cast a shadow.

Because Venus is so beautiful, it is named after the Goddess of Love. It is the only planet named after a woman. It is often known as the Evening Star, or the Morning Star, because it is never up in the middle of the night. However, it is not a star, but a planet.

Venus is the second of the Sun's planets and is the closest planet to Earth. It is often called Earth's sister planet, because it is made of rock like the Earth and is almost exactly the same size. In reality, however, it is totally different on the surface. Venus is completely covered in impenetrable cloud. Due to the large amount of carbon dioxide, the atmosphere traps the Sun's heat. As a result, it is ferociously hot on the surface, about 750°C. This is far hotter than your kitchen oven, hot enough to melt lead.

Because Venus is closer to the Sun than we are, it actually looks like a crescent through a telescope, like a new moon. The only other planet which looks like a crescent is Mercury.

W is for Water

Water is very important because it is the basis for the only sort of life we know. Its presence on Earth has led to life. Very few other planets have any water on them. Mars must have had some long ago, because we can see great canyons which were obviously carved out by floods. The Moon appears to have water buried beneath the surface. Callisto, one of the moons of Jupiter, appears to have a huge ocean of water, perhaps 60 kilometres deep, underneath a crust of ice.

Scientists are very excited by each of these finds, because it is a sign that there might be life on these planets and moons. But so far, no trace of any life has been found.

X is for Planet X

The Sun's eighth planet, Neptune, was discovered in 1846. Astronomers studied the motion of Neptune very carefully and found that it was not completely regular. They decided that there must be another big planet out there disturbing Neptune. They called this planet 'Planet X', because they didn't know anything about it.

For 80 years, they searched for Planet X. While doing this, they found Pluto, which they named the 9th planet; unfortunately, Pluto is not big enough to be Planet X. It's not even big enough to be a proper planet, so eventually in 2006 it was struck off the list of planets. The man who found Pluto, Claude Tombaugh, continued his search until eventually he had searched all the likely places in the entire sky where Planet X might be.

At this stage, most astronomers believe that there is no Planet X. The disturbances in Neptune's orbit turned out to be mistakes in the original observations. When the Voyager spaceships passed Neptune, they gave us much more accurate details of Neptune's position, and it is exactly where it should be. So there is no Planet X.

Y is for Yucatan

Yucatan is not in space; it is a place in Mexico. So what's it doing in a guide to space?

One theory is that a long time ago, about 65 million years ago, a giant lump of rock came down out of space and crashed into the Earth at Yucatan. This caused a huge explosion. Everything in the area was wiped out. Worse still, a whole load of dust went up into the air, making it very dark and cold.

Because of the cold and darkness, no plants could grow so a lot of animals died, including all the dinosaurs. The dust cleared after a while, but the dinosaurs were gone.

Could this happen again? It is possible that another lump of rock could crash into the Earth soon and kill us all. What could we do to stop it? If it is small and we spot it soon enough, we could send a space rocket up to move it before it hits the Earth. If it is big, there may not be much we can do.

Z is for Zodiac

As the Earth goes around the Sun, some stars are hidden by being behind the Sun. There are 12 constellations that are hidden by the sun during the year. The 12 constellations form a circle around the sky which is known as the Zodiac and they are known as the signs of the Zodiac:

• Aries the ram
• Taurus the bull
• Gemini the twins
• Cancer the crab
• Leo the lion
• Virgo the maid
• Libra the scales
• Scorpius the scorpion
• Sagittarius the archer
• Capricornus the sea-goat
• Aquarius the water carrier
• Pisces the fish

These are often just called star signs.

For example, between 21 March and 23 April approximately, the Sun is in front of the constellation Pisces (the Fishes)².

Long ago, when people had only just begun to study the stars, they thought that where the Sun was when you were born had a great influence on the type of person you are - this is astrology. Many people still believe this, but scientists don't, for two reasons:

• There is no reason to believe that the Sun should influence you.
• There is no evidence that the Sun does influence you.

These two together are enough to convince a lot of people that the whole belief in star signs is a waste of time.

Conclusion

Well there you have it! Space explained in 26 steps. Of course Force of Gravity should really have been under G, but there aren't too many interesting things in space starting with F.