Our Sun is a star and a very ordinary star. It is average size, temperature, colour and composition.
Planets are objects that orbit a star, our Sun has eight main planets and many other small objects. The main planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus andNeptune. Pluto was originally considered to be the ninth planet but has now been redefined as a Minor Planet. The reason for this was the discovery of many more Pluto like objects orbiting the Sun beyond the orbit of Neptune. The class of 'Minor Planets' now includes the larger named asteroids. Observations of some nearby stars have shown they also have planets.
Most planets have moons orbiting them, Earth has one natural moon, Mars has two small moons, Jupiter has four large moons and about a dozen small ones, Saturn has one large moon, 6 medium sized and many smaller ones. Uranus has four medium sized moons. Neptune has a large moon and one small one. Pluto has one moon almost as big as itself. Mercury and Venus do not have moons.
There are two types of asteroid although both kinds are made of rock, ice and metal (mainly Iron and Nickel). The first group form a ring around the Sun between the orbits of Mars and Jupiter. The second group move in wide elliptical orbits sweeping in close to the Sun and the traveling out sometimes beyond Jupiter and Saturn. Asteroids vary in size from a few meters across up to the largest which may be up to 100km across.
Comets are like dirty snowballs up to 25 kilometers across. They originate in a halo beyond the orbits of the planets. The halo is made of millions of these balls of ice and dust left over when the Sun and planets formed. Sometimes one of these giant snowballs gets moved out of it normal orbit and may fall in towards the Sun. As it moves closer to the Sun gravity begins to pull it in faster. Once inside the orbit of Jupiter, the heat from the Sun begins to melt the ice. The gas produced by the melting ice blown out behind by the radiation from the Sun and develops into a tail. The Comet then swoops around the Sun and back out to the edge of the solar system. Some comets return to become regular comets like Halley's Comet which returns every 76 years.
On any dark, clear night if
you sit back and look up into the night sky, every few minutes
you will more than likely see a streak of light speed across the
sky, this will be a METEOR or shooting star. It is not a star at
all but just a small speck of dust entering the top of the
Earth's atmosphere at very high speed, up to 150 thousand miles
per hour. At this speed the dust is vapourised by the heat and the
surrounding air is also heated and glows much like a florescent
There are two types of Meteor, the first are thought to originate from the large lumps of rock left over when the planets formed, known as ASTEROIDS which orbit the Sun between Mars and Jupiter. Very rarely two Asteroids collide and when they do, chips of Rock and Iron are thrown off and occasionally head towards Earth. These may be a few millimeters across or up to hundreds of metres across. They are quite rare and are seen as individual 'fireballs' sometimes impacting the ground as METEORITES and if big enough may even cause craters.
The second type of meteors originate from comets and are much more common. As a comet approaches the Sun, the frozen gases and water boil off and are blown away by the radiation from the Sun. Dust particles released by the melt are heavier and therefore continue more or less on the same orbit. These particles spread out along the orbit path and may eventually form a complete ring around the orbit. Once a year the Earth may pass through this stream of particles which enter the atmosphere as Meteors. Traveling at between 11 and 76 kilometers per second they burn up in the thin atmosphere at a height of about 100 kilometers.
Aurorea are also known as
Aurora Borealis the Northern Lights or Aurora Australis the
Southern Lights. On a clear night the aurora is visible as waves
of coloured light moving across the sky. Although not common from
the south of the British Isles they are quite common in the far
north of Scotland and even more common nearer to the North Pole.
Aurorea are caused by energetic particles which have been ejected from the Sun, hitting the Earth's atmosphere. Earth has a strong magnetic field so when the particles hit the magnetic field they are deflected towards the poles. As the particles move through the upper atmosphere they ionise the atoms and cause them to glow much like in a neon light.
Aurorea take many shapes and colours and move in different ways. The main colours are red from Nitrogen and green from Oxygen but when the show is faint, the colours are hard to discern, a grey glow may be all that is seen. The colours are caused by the gases in the atmosphere being ionised and depend on how energetic the in coming particles are.
Stars are really large clouds of gas, mainly atoms of Hydrogen. Because they are so large the gas becomes compressed into a spherical shape by its own gravity. As the gas is compressed it becomes very hot, up to millions of degrees in the centre. The heat and pressure in the centre causes the simple Hydrogen atoms to transform or fuse into other heavier elements like Helium, Neon and Carbon this process is called FUSION. The fusion produces huge amounts of energy in the form of radiation which prevents the cloud of gas collapsing and the stable star is formed. Average sized stars like our Sun carry on producing heat and light for about 10 thousand million years (our Sun is halfway through its life and is about 4.5 billion years old). Smaller stars live longer and larger stars have shorter lives. The very largest stars have very short lives and die in massive explosions called a SUPER NOVA.
GROUPS OF STARS
Stars are rather like people in that they form groups. Stars form in groups of billions of stars, in swirling masses called Galaxies. Many galaxies look like huge whirlpools in space. Our sun is part of a galaxy we call the Milky Way which we can see as a hazy band stretching across the sky on a clear night. Within galaxies stars form in groups, from gigantic clouds of gas and dust, so we can see clusters of stars through our telescopes these we call 'Open Clusters'.
When we look up into the night sky some of the stars seem to form groups or shapes. To help us identify parts of the sky we have given these groups names, these are known as CONSTELLATIONS.