My Science School

JUPITER

Jupiter is the largest planet in the Solar System. Large enough to contain more than 1300 Earths inside it, Jupiter is more massive than all the other planets combined. Along with Saturn, Uranus and Neptune, Jupiter is known as a “gas giant”, because it is mostly made of gas with no solid surface at all.

The colourful patterns of red, brown, yellow and white on Jupiter’s surface are produced by the chemicals sulphur and phosphorus in the swirling atmosphere. Jupiter’s extremely quick rotation is probably responsible both for separating the clouds into different colour “zones” (the lighter bands) and “belts” (the darker bands), and for the continual storms. The Great Red Spot, its most famous feature, is such a storm. The quick rotation also causes Jupiter to bulge at its equator, so that it measures 7500 kilometres less from pole to pole.

Jupiter has a system of rings consisting of dark grains of dust. The four largest of its moons are bigger than the planet Pluto. The beautiful, ever-changing patterns on Jupiter’s globe are violent winds.

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VENUS

About the same size as Earth, Venus is shrouded in thick, unbroken clouds made of droplets of deadly sulphuric acid. Because its cloud cover reflects the light of the Sun from its surface, Venus is a very bright object in the night sky.

Some 25 kilometres thick, the clouds prevent most sunlight from reaching the surface. But another kind of radiation from the Sun, called infrared, does get though and Venus’s dense atmosphere stops it from escaping. The result is a constant surface temperature hotter than the melting point of lead and the hottest in the Solar System. If any space explorer landed on Venus, he or she would be simultaneously incinerated, suffocated by the unbreathable carbon dioxide air, dissolved by acid and crushed by air pressure about 90 times that on Earth.

Venus spins slowly on its axis, actually taking longer to complete one rotation than to orbit the Sun. Relative to all the other planets except Pluto, it spins backwards.

            Venus is covered by thick clouds. They race round in the planet in just four days. The interior of Venus is similar to that of Earth, although its metallic core is much larger than Earth’s.

            Beneath the clouds, Venus’s barren surface features tens of thousands of volcanoes (some possibly still active) surrounded by vast lava plains. Lava flows have cut channels in the ground that look as if they may have been carved by rivers. Odd, dome-shaped volcanoes, or “pancakes”, as they have been described, have formed where lava has oozed to the surface, and then cooled as it spread out in all directions.

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EARTH

Our own planet, Earth, is the largest of the four inner planets. Third in order of from the Sun, 71% of its surface is taken up by oceans. Water is also present as droplets or ice particles that make up the clouds, as vapour in the atmosphere and as ice in polar areas or on high mountains.

Liquid water is essential for the existence of life on Earth, the only body in the Solar System where life is known to be present. Earth’s distance from the Sun - neither too close nor too far - produces exactly the right temperature range. The atmosphere traps enough of the Sun’s energy to avoid temperature extremes. It also screens the harmful rays of the Sun and acts as a shield against bombardment by meteoroids.

Earth’s magnetic field is generated by electrical currents produced by the swirling motion of the liquid inner core. The magnetic field protects Earth from the solar wind.

Earth’s outer shell, made up of the rocky crust and partly-molten upper mantle, is divided into about 15 separate pieces, called tectonic plates. Volcanoes and earthquakes occur where plate edges meet.

            When Earth lies directly between the Sun and the Moon it casts its shadow on the Moon. This is called a lunar eclipse.

            In contrast to the barren landscapes of the other planets, much of Earth’s is covered by vegetation, including forest, scrub and grassland. Different climates determine the types of plants and animals that live in different places. Large areas show the important influence of humans: for example, farmland, roads and cities. Land areas are continually sculpted by the weather and moving water or ice.

MERCURY

          Mercury, the closest planet to the Sun, is the second smallest planet in the Solar System. Because it is so near the Sun, it can be seen from Earth only with difficulty - low in the dawn or twilight sky close to the Sun.

          Mercury’s surface looks quite similar to that of our Moon. Bare and rocky, it is covered with craters, the result of continual bombardment by meteorites during the first billion years of its existence. Originally molten, Mercury’s surface shrank as it cooled after the bombardment eased, resulting in “wrinkles” - long mountain chains. With no winds or water to erode the rocks, Mercury’s landscape has remained the same ever since.

           Mercury’s orbit has an unusual shape All the other planets, except Pluto, have nearly circular orbits, but Mercury’s is elliptical - more like an oval. At its closest, Mercury is 46 million kilometres from the Sun, 70 million kilometres away at its most distant.

            Mercury has great extremes of temperature. Where it faces the Sun, it can exceed 400°C, but during the long nights (lasting about 59 Earth days) and with no atmosphere to keep the heat in, temperatures can plummet to - 170°C.

            Mercury’s surface is made up of thousands of craters, as well as mountains and lava plains.

            Mercury, the densest planet apart from Earth, has a large metal core made of iron and nickel, surrounded by a thin rocky shell.

            The landscape of Mercury is dominated by thousands of craters. The huge Sun burns with a fierce heat – turning to severe cold when this face of the planet is turned away from it. Large boulders falling from space have produced craters in Mercury’s surface measuring many kilometres across, some with smaller craters inside. Because there is hardly any atmosphere, Mercury’s skies remain black even during the day.

            When a meteorite strikes the surface of Mercury, it punches a saucer-shaped crater in the ground. Debris is blasted out in all directions, creating long streaks.

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THE PLANETS

A planet is a large object in orbit around a star. It can be made of rock, metal, liquid, gas, or a combination of these. Planets do not produce light, but reflect the light of their parent star.

In our own Solar System, there are nine planets, including Earth, orbiting the Sun, our parent star. Observations of other stars made by astronomers using powerful telescopes indicate that they, too, have planets. There could therefore be billions of other planets in the Universe.

The Earth is the largest of the four inner, or “terrestrial”, planets: Mercury, Venus, Earth and Mars. They are, as the scale illustration demonstrates, dwarfed by the four “gas giants”, Jupiter, Saturn, Uranus and Neptune, so called because they have comparatively small rocky cores surrounded by thick layers of liquid and gas. Pluto fits into neither category, being a small, outer planet made of ice and rock.

The diagram shows the relative distances of the planets from the Sun. Pacing out their positions would give an even better idea of the huge distances between them. If the Sun were a football, Mercury would be pinhead 10 paces away from it. Earth (the size of a peppercorn) is a further 16 paces on from Mercury, with the Moon a thumb’s length away from Earth. Another 209 paces would bring you to Jupiter (a large marble), while Pluto lies 884 more paces distant. To reach the nearest star, Proxima Centauri, you must walk another 6700 kilometres!

EXPLORING THE PLANETS

Because the giant planets lie so far from Earth, it would take too long for people to travel to them. So space probes have been launched to “fly by” every planet except Pluto and send back pictures. Voyager 2 made the greatest journey. Space probe Cassini visits Saturn in 2004.

THE PLANETS FORM

The Solar System began life as a cloud of gas and dust drifting across the Milky Way Ga1axy. It is thought that a supernova may have sent shock waves racing across space, striking the cloud and somehow causing it to collapse under its own gravity.

Within 100,000 years, the collapsed cloud became a swirling disc, called a solar nebula. Under pressure from gas and dust spiralling inwards, the centre became hotter and denser and began to bulge. It would soon evolve into the infant Sun.

Away from this central furnace, particles of dust began to clump together like snowflakes, first into small fragments of rock, then becoming large boulders. Over millions of years, some grew into blocks several kilometres across, called planetesimals. These eventually started to collide with one another, building up like snowballs to become the four rocky inner planets, Mercury, Venus, Earth and Mars, and the cores of the four gas giants, Jupiter, Saturn, Uranus and Neptune.

The solar wind stripped away any remaining dust and gas, including the atmospheres around the four inner planets. The giant planets lay beyond the solar wind’s fiercest blast, so they were able to hold on to their thick blankets of gas.

Jupiter’s gravitational pull caused nearby planetesimals to destroy one another rather than build up into another planet, leaving a belt of rock fragments, known as asteroids, still orbiting the Sun, as they do today.

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SOLAR SYSTEM

The solar system consists of the Sun and an array of objects that orbit it. These objects include the nine known planets, their 64 known moons, asteroids, comets, meteoroids and huge amounts of gas and dust. The Sun’s great size relative to the other objects in the Solar System gives it the gravitational pull to keep them permanently in orbit around it.

The planets orbit the Sun in the same direction (anticlockwise in this illustration) and in elliptical (oval-shaped) paths. Pluto’s orbit is the most elliptical of all the planets. For part of its journey around the Sun, its orbit actually lies inside that of Neptune. All the planets, and most of their moons, travel on approximately the same plane, with the exception of Mercury and, once again, Pluto, both of which have tilted orbits.

Constantly streaming away from the Sun in all directions is the solar wind, made up of electrically-charged particles (parts of atoms).Travelling at more than 400 kilometres per second, it produces electric currents inside a giant magnetic “bubble” called the heliosphere. The heliosphere protects the Solar System from cosmic rays arriving from space. Its edge, some 18 billion kilometres from the Sun, marks the true boundary of the Solar System.

EARLY ASTRONOMERS

Thousands of years ago, in the time of the ancient civilizations of Egypt and China, people thought that the Sun and Moon were gods, the Earth was flat and the sky was a great dome suspended above it.

In later years, astronomers from ancient Greece proved that the Earth was round. Many believed that the stars were fixed to a great sphere that rotated around the Earth each day. One Greek astronomer, Aristarchus, proposed that the planets, including Earth, orbited the Sun, a star, but most astronomers of this time thought that the Sun, Moon and planets all travelled in circular paths around Earth, the centre of the Universe. Ptolemy, who lived in the 2nd century AD, observed that, while the stars moved across the night sky along regular paths, the planets appeared to “wander” from theirs. He proposed that they each moved in their own small circles, called epicycles, as they orbited Earth.

The Polish priest and astronomer, Nicolaus Copernicus, challenged Ptolemy’s view of the Solar System, declaring that the Sun lay at the centre of a system of orbiting planets. Only the Moon orbited the Earth. Copernicus wrongly believed that the planets’ orbits were perfect circles and that they moved in epicycles. It was left to the German astronomer Johannes Kepler (1571-1630), who showed that the planets moved in elliptical, rather than perfectly circular, orbits. The shapes of their orbits also explained the “wandering” that so perplexed earlier observers, thus disproving the idea that the planets moved in epicycles.

The Italian astronomer Galileo (1564-1642) was the first to use a telescope. From his observations of the moons of Jupiter in orbit around that planet, and the changing shape of Venus as it orbited the Sun, he concluded that Copernicus had been correct: the planets do orbit the Sun.

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THE SUN

The Sun is an ordinary star. To us on Earth it is of crucial importance since no life could exist without it, but it is simply one of billions of stars in the Milky Way Galaxy, itself one of billions of galaxies in the Universe. For a star, the Sun is below average size - some astronomers classify it as a “yellow dwarf”. Yet it is massive when compared to the planets. The Sun contains more than 99 per cent of all the matter in the Solar System. Its diameter of 1,400,000 kilometres is more than 100 times that of Earth.  

The Sun is a spinning ball of intensely hot gas made up almost entirely of hydrogen (three-quarters of its mass) and helium. It produces massive amounts of energy by “burning” about four million tonnes of hydrogen every second.

INTERNAL LAYERS

At the centre of the Sun is the core, a region of incredible pressure (200 billion times that on the Earth’s surface) and intense heat - about 15 million °C. This is the Sun’s nuclear furnace, where the energy that keeps it shining is released. Hydrogen atoms fuse together to form helium. Energy from this reaction flows out from the core through the radiative zone to the convective zone. Here, in a continuous cycle, hot gas bubbles up to the surface before sinking down to be reheated again.

THE SURFACE OF THE SUN

The Sun’s outer shell, the photosphere, is only about 500 kilometres thick and, at 5500°C, much “cooler” than at the core. It is in a state of constant motion, like water in a boiling kettle. Hundreds of thousands of flaming gas jets, called spicules, leap up to 10,000 kilometres into the Sun’s atmosphere, known as the chromosphere.

Invisible lines of magnetic force that twist around the Sun’s globe are the cause of many extraordinary features. Huge arches of fire, called prominences, can be held up above the Sun by magnetism. Flares, sudden, massive explosions of energy, burst forth when the magnetic field shifts. Where magnetic field lines erupt through the photosphere, there are dark, cooler areas (about 4300°C) known as sunspots.

Beyond the chromosphere lies the corona, the Sun’s hot, shimmering outer atmosphere. This is visible from Earth only during a total solar eclipse.

DEATH OF THE SUN

When the Sun’s fuel of hydrogen starts to run out, it will grow into a much bigger and brighter star, called a red giant. It will eventually shed its outer layers into space. All that will remain of the Sun itself will be, at first, a small, extremely dense star (a white dwarf), before it eventually cools and wastes away (a black dwarf).

            By coincidence, the Moon and Sun appear to be the same size in the sky. So when the Moon passes between the Earth and the Sun, it may block out our view of the Sun, a solar eclipse. During a total eclipse, an event only rarely witnessed, the Moon covers the Sun’s surface entirely and the corona shines out from behind a black disc. For a short while, dusk falls. In a partial eclipse, part of the Sun still remains visible.

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CONSTELLATIONS

Constellations are areas of the sky, divided up for the purpose of identifying stars, galaxies and other objects in the heavens. Years ago, before telescopes were invented; early astronomers grouped the stars together into patterns, imagining their shapes to look like gods, heroes and sacred beasts from popular legends. The 88 constellations that exist today include 48 known to the ancient Greeks, who inherited some from the Babylonians.

            A line running from two stars in the constellation Ursa Major (great Bear) points to the Pole Star, almost exactly due north. Years ago, seafarers used this observation for navigation.

            Orion, a hunter in Greek myths is an easy constellation to spot. Three stars in a diagonal line form his belt, while others make up his dagger and shield. The belt stars point down towards Sirius, the brightest star in the night sky. In Greek myths, Centaurus was half man, half horse.

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QUASARS

Incredibly powerful, massive black holes may, astronomers think, be found lurking at the centres of galaxies. There could even be one at the centre of our own Milky Way Galaxy. Astronomers have detected a ring of fast-moving, hot gas swirling around the centre. The ring of gas is probably in the grip of a powerful gravitational pull - most likely, astronomer’s suspect, to be the work of a black hole.

The activity at the centre of our Galaxy is as nothing compared to that of quasars. These objects look like stars, but they lie at incredible distances from us: the farthest quasars are 13 billion light years away. To be visible at that distance means they must be giving off immense amounts of energy. Quasars are the centres of extremely violent galaxies containing super-massive black holes, weighing up to 100 billion Suns. The brilliant light comes from the disc of hot gas and dust spiralling into the black hole.

            Black holes are invisible, but it is possible to detect them by studying their effects, astronomers observing a star called Cygnus X-1 saw that it was giving off enormous amounts of energy (a sure sign of violent activity in the Universe). They discovered that this huge, hot blue star was being dragged around in a circle by an unseen object with a huge gravitational pull. That unseen object, astronomers now believe, is a black hole, which is tearing gas from the star. The gas forms a whirling disc before plummeting into the black hole. As it falls, it travels faster and faster until it moves almost at the speed of light itself. Close to the hole, the gas becomes so hot it emits massive amounts of energy.

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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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