Category Modern Science

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.

Picture Credit : Google

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.

Picture Credit : Google

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.

Picture Credit : Google

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.

Picture Credit : Google

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.

Picture Credit : Google

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.

Picture Credit : Google