Category The Universe, Exploring the Universe, Solar System, The Moon, Space, Space Travel

BLACK HOLES

Black holes are the strangest objects in the Universe. No-one has ever seen one, but most astronomers are convinced that they exist. They are tiny regions of space surrounded by a force of gravity so strong that nothing, not even light, can escape from them.

All bodies in space exert a force of gravity, the force which attracts other things towards them. The greater an object, the stronger it’s gravitational pull, and the harder it is to escape from it. A rocket launched from Earth must go faster than 40,000 kilometres per hour (its “escape velocity”) to escape Earth’s gravitational pull. The Sun is many thousands of times more massive than Earth, so a rocket would have to travel much faster: more than 2 million kilometres per hour. If there was an object much bigger or denser than the Sun, an escape velocity equal to that of the speed of light may be needed to escape from it.

Where might an object of such high density be found? Stars more than 10 times as heavy as the Sun burn up their fuel in a much shorter time – a few million years, compared to the Sun’s 10 billion years. They swell into massive super giants before blasting apart in supernovas. A supernova’s core compresses in seconds to a tiny, super-dense body called a neutron star. If it weighs more than the three Suns, it squeezes further. An escape velocity of the speed of light would be needed to travel away from it. Any light rays would be pulled back in, so the object is invisible: a black hole.

Imagine a star in space as ball on a rubber sheet. A massive object like a star will “bend” space and anything close to it will fall in towards it. If the ball were so heavy that the sheet stretched into a long, deep tube, the result would be a black hole.

EINSTEIN’S GENERAL THEORY

The great German physicist Albert Einstein (1879-1955) found another way to explain how space, light and matter would behave close to a black hole. In his General Theory of Relativity of 1915, Einstein proposed that the gravitational pull of an object would result in the “curving” of space, in the same way that a person can curve a trampoline. A massive object creates a large “dent” in space into which light and matter would fall. The denser the object, the greater the dent. So the Sun would make only a shallow dent, whereas a neutron star would create a very deep dent. A black hole, the densest object of all, creates a dent so deep that nothing can escape from it.

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STARS

Stars are giant spinning balls of hot gases. Like massive nuclear power stations, they produce vast amounts of energy in the form of heat and light, which they radiate across space as they shine.

They may look like tiny points of light in the night sky, but many stars are incredibly big. Betelgeuse, in the constellation of Orion, is 800 times the size of the Sun, our local star. Stars vary enormously according to the amount of light they emit. Some of the most powerful give off more than 100,000 the light of the Sun, while others are 100,000 times weaker.

Stars are born when clouds of dust and gas in space, known as nebulae, compress together under the force of gravity to become dense “blobs”, called protostars. It is not certain why this happens. Maybe the pressure of an exploding star nearby at the end of its life triggers the process.

After a star has formed it becomes a stable “main sequence” star. The Sun is a typical star of average brightness. More massive stars, like Rigel (also in Orion), glow blue-white, while at the other end of the scale, a white dwarf, the collapsed core of an old star, is no bigger than the Earth.

A star begins its life as a dense mass of gas and dust called a protostar (1). The core becomes so hot that nuclear reactions start deep inside it. Gas and dust are blown away (2), although some remain in a disc surrounding the new star. Planets may form here (3). The star is now a main sequence star (4). When the fuel it uses to produce energy runs out, the core collapses and the star swells into a red giant (5). A massive star will become a supergiant that will blast apart in a mighty explosion called a supernova (6). It ends its days as a neutron star or a black hole (7). A red giant will puff away into space, leaving behind a white dwarf.

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GALAXIES

           Galaxies are gigantic collections of stars. The galaxy in which the Sun is situated, the Milky Way Galaxy, is a vast spiral of about 200 billion stars measuring about 100,000 light years across. There are billions more galaxies in the Universe, most of which are elliptical (oval) in shape. There are also others that have irregular shapes.

            The Milky Way has a bulge at its centre, the nucleus, where older red stars are concentrated. Four giant arms radiate out from the nucleus. These contain younger blue stars as well as areas of gas and dust – the raw material for the creation of new stars. The whole spiral spins at a speed of about 250 kilometres per second.

            The Milky Way Galaxy closely resembles the Andromeda Galaxy, which lies 2.25 million light years away. The Sun is situated on one of the spiral arms about halfway out from the nucleus. Here are mostly yellow and orange young-to-middle aged stars.

            The Horsehead Nebula is really a gigantic cloud of dust and gas that has taken on a familiar shape. It is one of many clouds in our Galaxy where stars start to form.

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BIG BANG

Many astronomers believe that the Universe began life in a single momentous event. This was an incredibly hot, dense explosion called the Big Bang, which took place about 15 billion years ago. During this explosion, all matter, energy, space – and time itself – were created.

In the first few millionths of a second, the particles that make up atoms, the building blocks of all matter, were formed. It took about 100,000 years for the first atoms, those of the gases hydrogen and helium, to come together. By this time, the searing heat of the Big Bang had cooled, space had expanded and the gases began to spread out. Gradually, however, gravity drew the gases together, leaving vast regions of empty space in between.

About a billion years after the Big Bang, the clouds of gas started to form into galaxies. Matter inside the galaxies went on clumping together until stars were created. Our own Sun was born in this way about 5 billion years ago. Its family of planets, including our Earth, was formed from the debris spinning round the infant Sun. With billions and billions of stars and planets forming in the same way across the Universe, it seems almost certain that life will have also evolved elsewhere. Will we on Earth one day make contact with these alien life-forms?

The expansion of the Universe is slowing down. Some astronomers think that gravity may eventually bring the expansion to a halt, then collapse all matter once more to a single point in a “Big Crunch”. Others believe that there is not enough material in the Universe to do this and that the Universe will carry on expanding forever.

Many scientists think that all matter in the Universe will eventually collide: the “Big Crunch”. Vast amounts of invisible “dark matter” in the Universe may exert sufficient gravity to halt its expansion and cause the galaxies to compress together.

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UNIVERSE

Everything that we can think of and everything else that exists – all belong to the Universe. From grains of sand to tall buildings, from particles of dust to giant stars and planets, from microscopic bacteria to people – all are part of the Universe. It even includes empty space.

The Universe is unimaginably vast: billions upon billions of kilometres wide. Distances in the Universe are so great that we have to use a special measure to record them. This is a light year, or the distance that light, which moves at a speed of about 300,000 kilometres per second, travels in one year: about 9,460,528,405,000 kilometres. The nearest star to Earth (after the Sun), Proxima Centauri, is 4.2 light years away. The most distant objects we know in the Universe are more than 13 billion light years away from Earth.

Nearly all the matter in the Universe is contained in galaxies, enormous masses of stars, has and dust. There may be about 100 billion galaxies, each containing hundreds of billions of stars. Galaxies are grouped into giant “clouds” of galaxies, called superclusters. These are spread round the Universe like a net, made up of strings and knots. In between there are gigantic empty spaces.

The superclusters are, themselves, made up of smaller clusters of galaxies. One of these, a cluster of 30 galaxies or so, is called the Local Group. It contains the Milky Way Galaxy, the vast spiral of stars to which our own local star, the Sun, belongs.

Astronomers have discovered that all galaxies are rushing away from one another. This means that, a long time ago, they were once all close together. So the Universe had a definite beginning – and may have an end.

The Universe is composed of many galaxy superclusters, themselves made up of clusters of galaxies. One of these contains the Milky Way Galaxy, a spiral-shaped mass of about 200 billion stars, one of which is our own Sun, parent to a family of nine planets.

The third planet from the Sun is Earth, orbited by the Moon. Earth is the only world in the Universe where life is known to exist, but we may discover others one day.

It is possible that the Universe will carry on expanding forever. In this sequence, the Universe is created in an immense explosion called the Big Bang. It expands rapidly, with all the galaxies moving away from one another as the Universe inflates like a balloon.

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