Here's something I wrote in 1992.

Comets

Most of us saw Halley's Comet in 1986. It was a bright spectacle, white and ancient.

Comets are a chunk of matter, ice and other frozen chemicals. They circulate around in the "Oort Cloud", far out beyond Pluto. There are millions of different shaped comets, varying in size.

The Oort Cloud goes around the sun in a circular fashion. Sometimes a star's gravity might push the Oort Cloud, causing a few to fall out of their previous orbit. The stray comet either gets thrown into outer space, or might start spinning towards our Solar System.

If it does spin to us, it will head for the sun. When it reaches within the sun's heat (approximately past Jupiter), a tail forms from the evaporating ice and frozen gases contained in the comet, always pointing away from the sun. Millions off tonnes of liquid and gases are shed when the comet reaches and leaves the sun's heat. When this happens, the comet gets a bit smaller.

Normally comet swings out near the Oort Clout (on its outward orbit) and comes plunging back to the sun. But if it comes too close to a planet, it may actually fall into the orbit of that planet, usually Jupiter.

Comets have an unceremonious death. They either plunge straight into the sun, unable to swing past it, or just lose all their gases and turn into a rock, like an asteroid, as the frozen gases get boiled away.

Some names of comets are: Comet Encke, Faye, Ikeya-Seki, Tempel 2, Halley, Holmes, and Mrkos.

In the past, people thought comets meant that something bad would happen soon. Even recently in 1973, a book proclaimed that the world would end on 31 January 1992, just because Kohoutek was passing the sun. Some astronomers still believe that a comet's dust falling to the earth causes influenza and other viruses.

Many coincidences have occurred when comets have just passed such as 1519 when the Aztec Emperor, Moctezuma, saw a comet and offered no resistance to the Spaniards, in 1066 King Harold was killed during the battle of Hastings, and in 1688 when a comet was accused of causing a sneezing sickness in the cats of Westphalia. This persuaded many people to believe that it was impossible to predict when a comet would come.

This was until Edmund Halley looked into history books and deduced that three comets, in 1531, 1607, and 1682 were the some one. He then went to predict that the comet would come in 1758. It did come, but 18 years after his death.

After that, beliefs began to change. A new theory was set. Things began to get clearer. People began to actually want to see comets.

Stars

How stars are made

Stars are born and eventually die. The middle of their lives are good, strong, and steady.

Their life starts in a cloud of particles, or a nebula. The cloud begins to collapse under its own gravity. When it gets squashed enough, the molecules start combining. First the molecules are mostly hydrogen. The reactions inside the star turn the hydrogen into helium. Our own sun formed from a nebula. The cloud of matter contracted and a bright yellow-orange star is formed. Around it, planets were already forming, The inner planets were bombarded by many meteors and asteroids, some making the planets bigger because they stuck on. The outer planets were made mostly out of gas, not able to contract in time.

The sun, when it was at its fullest, began to blow away the excess matter using its solar wind. There might be stars like the sun which have planets around them too.

How stars die

Stars have an untimely death. They start shrinking, as the hydrogen runs out, because the gravitational forces start to pull the star in on itself. Then the star starts to burn the helium. The helium makes the star expand and become a red giant. Then the gravitational forces act again. The tug of war continues, with the star swelling and getting smaller.

Suddenly, the helium will run out, so the star will contract. It will contract so quickly that it will explode. The remains called a nebula will drift away, and perhaps become another star or stars.

If the star was very heavy before, it might turn into a black hole. Or a pulsar, a neutron star, or a plain white dwarf.

White dwarfs

White dwarfs are the remains of medium sized dead stars. They still shine, but not too brightly. Eventually, even the light fades away, and the once proud star winks out.

Pulsars and neutron stars

If a star is 1.5 times more massive than the sun, it would turn into a pulsar or a neutron star.

Atoms are made of neutrons, protons, and electrons. Neutrons have no charge, protons a positive charge, and electrons a negative charge. Usually the electron shell (the orbiting electrons) keeps the atoms apart.

But if a huge force is exerted on a few atoms, the shell will be crushed and the electrons and proton will be forced to join. When they join, they cancel each other out, though keeping their original mass. The size is also decreased dramatically.

This was first suggested by Fritz Zwicky and Walter Baade in 1934. A neutron star is only a few kilometres across, so how could Zwicky and Baade prove that they existed? When the atoms of a star get squashed, huge amounts of heat are released as X-rays.

In the 1950s, their prediction could be tested. Before it could not be detected because X-rays could not get through the atmosphere. It was thought that there was a neutron star in the Crab Nebula. The first time it could be tested was when the moon passed in front of the nebula in 1964.

If there was a neutron star in the Nebula, the x-rays coming from it would be quickly turned off by the moon. But it was cut slowly, so there was no neutron star.

This was not the end of the story, though. In 1931, Karl Jansky discovered radio waves coming from space. It was found that in the Crab Nebula something was sending out regular pulses of radio waves. Jocelyn Bell, a student of Anthony Hewish, found what is now called a pulsar, short for a pulsating star. This pulsar turned every 0.033099 seconds. It gets 36 billionths of a second longer each day.

Actually, a pulsar is a rotating neutron star sending out radiation. The reason why the moon cut the X-rays off slowly was because the X-rays were coming from the debris and gas surrounding it, and out-competed the neutron star.

Black Holes

When a huge star, larger than 3.2 times the mass of the sun collapses, it usually turns into what astronomers call "Black Holes". It is called that because light cannot escape it, so it is black and dark, and also because it is a 3-D "hole". If you can imagine, a round circle in a piece of cardboard rotated quickly, you may understand how it is a hole.

From above and below (strangely, black holes have an "up" and down"), you can definitely tell it is a sphere. But how large is it? It depends greatly. If the sun had the mass of the smallest possible black hole and exploded into a black hole, the size would be 5.8 kilometres across its diameter. This was first calculated by the German astronomer, Karl Schwarzchild, so it was called the "Swartzchild Radius".

As mentioned in the chapter about neutron stars, the atoms are smashed so the electrons and protons are made to join and turn into neutron stars. But with black holes, the gravity is so strong that it squashes the electron shells so much that nothing can stop them from squashing together into virtually nothing.

There is one essential fact about gravity which will make you feel tall. The closer you are to a centre of an object, the stronger the gravity is. Say you were on the Sun (you'd be dead by then but...). The gravity is 28 times more than it is than on the earth. If the sun suddenly shrank but kept all its mass, you would be closer and so the gravity would be stronger. That is exactly what happens on a black hole. Black holes have an exceptionally high rate of tidal effect (what it is called). So if you were caught in a black hole, just in the last second before you would be pulled into atoms (the gravitational and atomic forces are so strong they break you into molecules, to atoms, and then to virtually nothing), you would be stretched 17 kilometres (10 miles) tall!

We find black holes by looking at x-ray pictures of space. Nothing we know of can get out of a black hole, so what use are x-rays? Actually, we don't detect x-rays which have gone inside a black hole. We detect them before they go inside. So a lot of x-rays means there probably is a black hole there. Also, x-rays are released when the atoms are separated.

Mini Black Holes

When the big bang happened (if it did), things were flung out into nothing. Some of the matter were tightly packed planets which turned into mini black holes. The mini black hole was first suggested by Stephen Hawkings, an English scientist, in 1971. He also said that black holes can slowly "evaporate". They lose their mass by some gravitational energy changing into particles and escaping, carrying a bit of mass from the star or planet.

Unfortunately, with normal black holes, they gobble more mass than they evaporate. When mini black holes eventually disappear, they go with a big bang and send out a a special type of gamma rays. So far, we have had no signals of those gamma rays.

Galaxies

Stars are arranged into neat clusters, some huge to some containing only a hundred or so stars. These are the galaxies. A galaxy has many shapes, some having arms which spiral out from the centre, and some with no shape at all.

Edwin Hubble divided the types of galaxies into groups. These groups were: Spirals (classified as 'S'), Ellipticals (E), and barred spirals (SB). Other types added later were the Irregular galaxies and Lenticular galaxies.

Ellipticals were made since they quickly used up all their matter and changed it into stars. Spirals and barred spirals were created when some of the matter quickly turned into stars, but the rest were left in a disk around it. In time the leftovers too became stars.

Galaxies are further put together into groups. Our galaxy belongs in the Local Group. Larger groups are named clusters. These again are put together into the huge super clusters, of which the Virgo Cluster is the centre of one.

Spiral galaxies usually have the older, red stars near the middle, since they were formed first. Also, many stars are in the centre, so it bulges.

Galaxies are the largest possible grouping of stars close together.

Our very own "Milky Way" is the galaxy which holds our Sun. Different impressions of our galaxy can be created by looking at different wavelengths of light coming from it.

The universe

There are many many more things out there which we don't yet know about, and many more things which I have not covered. Many fantasies have been built up around the stars: Star Trek, Star Wars... because the universe is larger than anyone can comprehend.