How do Stars Explode?


Exploding Stars



In the year 1054, a very bright star suddenly appeared in the sky. It was so bright that it could be seen in the daytime. Then it slowly began to fade away. After a time, it disappeared. What kind of star was this?



The disappearing star was a star that had exploded. Certain kinds of stars can explode. A small star called a white dwarf may suddenly flare up and become much brighter. Such a star is called a nova. After a time, the star’s brightness fades away.



When a very large star called a supergiant begins to die, it explodes, too. This explosion sends a gigantic cloud of glowing gas out into space. This kind of explosion is called a supernova.



A star that becomes a supernova may become a billion times brighter. The bright star seen in the sky in 1054 was a supernova.



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What are Asteroids?


ASTEROIDS



Asteroids are small, mostly rocky, irregular-shaped bodies. They are found orbiting the Sun in a band filling the 550-million-kilometre gap between Mars and Jupiter. The largest, Ceres, measures just under 1000 kilometres across, but only a handful have diameters greater than 100 kilometres. About 4000 have been recorded, but there are many thousands more too small to be identified.



Astronomers believe that, during the formation of the Solar System, Jupiter’s strong gravitational pull caused nearby planetesimals to smash into one another rather than build up into another planet. This left the belt of fragments we call the asteroids.



The asteroids have continued to collide with one another since their formation, producing smaller fragments called meteoroids. These have occasionally crashed on to Earth’s surface (when they are known as meteorites). It is feared that one day a large meteorite may devastate Earth, causing climatic change sufficient to wipe out many life-forms.



            Most asteroids are rocky, indicating they come from the outer layers of a former minor planet. But some are metallic – they come from the core of such a planet.



            A close-up view of the irregular shaped objects that make up the asteroid belt between Mars and Jupiter. From study of asteroid fragments that have fallen to Earth, scientists have dated the age of the Solar System to 4.6 million years ago.




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What are comets?

COMETS



Comets are potato-shaped lumps of dust measuring only a few kilometres across, but accompanied by (when near the Sun) tails of has or dust that stretch for hundreds of millions of kilometres across space. The lump of dust is fused together by frozen gases and water ice. Like all other objects in the Solar System, comets orbit the Sun, although their orbits are often very elliptical (elongated ovals), looping in towards the Sun from distant reaches of the Solar System. When a comet approaches the Sun, part of its ices melt and the gas and dust escape, forming a surrounding cloud, or coma. As it rounds the Sun, the coma is swept back into two tails, a straight gas tail and a broader, curved dust tail, always pointing away from the Sun.



Sometimes, small pieces of debris break off from comets. Great showers of these fragments, called meteors, sometimes come quite close to Earth. Millions of tiny particles burn up in Earth’s atmosphere. Commonly known as shooting stars, they appear to us as split-second streaks of light in the night sky.



FAMOUS COMETS



The English astronomer Edmund Halley (1656-1742) was the first to realise that comets were orbiting objects. He once made a famous prediction: a comet that he observed in 1682 would return to the skies in 1758. Halley believed that comets recorded in 1531 and 1607 were simply earlier sightings of the one he saw in 1682. Halley did not live to see his prediction come true. Halley’s Comet, as it has been known ever since, was duly sighted on Christmas Day 1758 and has reappeared every 75 to 76 years. When Halley’s Comet appeared in March 1986, the space probe Giotto flew within 600 kilometres of it, sending back pictures and sampling the gases and dust particles given off by it.



A sighting of a comet is always a great event. The 1997 appearance of Comet Hale-Bopp was the most spectacular of recent years. Comets can also be destructive if they pass too close to a planet. In July 1994, drawn in by gravity, fragments of Comet Shoemaker-Levy smashed into Jupiter, creating massive fireballs on impact.



            On 30th June 1908 there was a huge explosion in the Tunguska region of Siberia, Russia. Trees in an area about 100 km across were felled by the blast, but no crater was found. The Tunguska fireball may have been a comet exploding at an altitude of about 6 km.




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Do we have some more Moons also, other than Earth’s natural satellite?


MOONS



Moons, also known as satellites, are relatively small worlds that orbit the planets of the Solar System. Earth has one moon, known simply as the Moon, but other planets have many more - Saturn, for example, has at least 18 moons. Moons are very varied in size and form. Many have unusual landscape features that intrigue astronomers.



Moons are created in different ways. Some are the result of fragments of rock or ice being pulled together by gravity to form a globe. Others are asteroids that have been “captured” by a planet’s gravitational force.



All seven of the moons illustrated here larger than the smallest planet, Pluto, while the largest moons, Ganymede and Titan, are even bigger than Mercury, the second smallest planet. Jupiter’s four largest moons are all in the top seven. They are called the “Galileans” after the Italian scientist Galileo Galilei who first discovered them with one of the first telescopes in 1610. Ganymede has an icy surface with cratered plains and areas showing strange “grooved” patterns.



Titan, Saturn’s largest moon, is the only moon to have a thick atmosphere, made mainly of nitrogen. Beneath its continuous cloud layer, there may be a sea of methane.



Callisto, Jupiter’s second largest moon, is heavily cratered. Measuring 600 kilometres across, its most prominent crater, called Valhalla, is surrounded by a series of ripples. Io, the third of Jupiter’s Galileans, with its crust a vivid mixture of yellows, oranges, reds and blacks, looks a little like a pizza. In fact it is peppered with active volcanoes and lakes of molten rock.



Our own Moon is the fifth largest moon in the Solar System, although it would take 81 Moons to make up a world the size of Earth. The Moon’s lava plains indicate past volcanic activity, but there are no active volcanoes there today.



Next in order of size comes Europa, the fourth Galilean and an object of great interest amongst astronomers. Looking like a cracked egg, its surface consists of ice sheets that are continually melting and re-solidifying. It is by no means impossible that, beneath those ice sheets, there is a warm ocean of liquid water. Could it be that life has also evolved on Europa and that there are life-forms swimming in its oceans? Future space probe missions may find out.



Triton is Neptune’s largest moon. Its surface is the coldest place known in the Solar System. At -235°C, the temperature is low enough to freeze nitrogen. Triton was photographed in stunning detail by Voyager 2, the last of its close encounters, in 1989.



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Will you add some facts about Planet Pluto in my knowledge Bank?

PLUTO



Pluto is the smallest, coldest and outermost planet in the Solar System. It was the last to be discovered, identified in 1930 by the American astronomer Clyde Tombaugh. He compared photographs of part of the sky taken six days apart and noticed that a pinprick of light had moved slightly against the background of stars. Pluto was the only outer planet not visited by Voyager 2, so astronomers still know little about it. Some even propose that Pluto is really a comet and not a planet at all.



Pluto has a very elongated orbit, ranging between 7400 and 4400 million kilometres from the Sun, bringing it inside the orbit of Neptune for part of the journey. Pluto’s moon, Charon, is just over half its size and lies only 19,640 kilometres away from it. Both spin in a direction opposite to that of the other planets except Venus.



Pluto is denser than the icy moons of Uranus and Neptune, suggesting that it has relatively large, rocky core.



Pluto’s surface is probably an “icescape” of frozen nitrogen, carbon monoxide and methane. There may be craters made by collisions with rock and ice fragments. Seen from Pluto, the Sun looks no more than a bright, distant star. It still provides just enough heat to evaporate some of the surface frost and create an extremely thin atmosphere. Charon, Pluto’s nearby moon, features prominently in the sky.



Thousands of icy objects may exist in the outer reaches of the Solar System. They may form either a belt or a cloud. This could be the birthplace of comets.



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Will you add some facts about Planet Neptune in my knowledge Bank?

NEPTUNE



Neptune was discovered by German astronomer Johann Galle in 1846. Its largest moon, Triton, was recorded a few days later. Besides that, very little was known about Neptune until the space probe Voyager 2 visited it in 1989.



A bright blue globe, Neptune almost completely lacks surface features. At the time it was photographed by Voyager, a storm system, called the Great Dark Spot (which later disappeared), could be seen racing in a direction opposite to the planet’s rotation. Winds on Neptune blow at more than 2000 kilometres per hour.



Like the other gas giants, Neptune has a system of rings. There are four extremely faint rings, composed of dark, icy fragments.



VOYAGER 2



The greatest journey by a space probe so far undertaken was made by Voyager 2. Between 1979 and 1989, it flew close by Jupiter, Saturn, Uranus and Neptune, transmitting superbly clear pictures of the planets and their moons. Voyager has since sped away from the Solar System, although it continues to send back signals - 20 billion times weaker than those of a watch battery!



Voyager is playing its part in the search for life in other solar systems. Should aliens ever come across the space probe they will find an audiovisual disc on board. If they play it, they will hear, among other things, the sounds of whales, baby crying and greetings in 55 languages.



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Will you add some facts about Planet Uranus in my knowledge Bank?


URANUS



Uranus was discovered in 1781 by William Herschel, an amateur German astronomer living in England. More recently, astronomers found that Uranus is tilted 98° from the vertical, meaning that it orbits the Sun almost on its side. So for much of the 84-year-long journey, both poles face long periods of continuous daylight, followed by continuous night.



            Uranus’ relatively small, rocky core is surrounded by a slushy ocean of water with some ammonia. Its thick atmosphere is composed mainly of hydrogen.



            Uranus has a family of 11 faint rings, none more than 10 km wide, each made up of pitch-black blocks, measuring only a few metres deep. They circle Uranus’ equator.



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Will you add some facts about Planet Mars in my knowledge Bank?


MARS



Although Mars is much smaller than Earth, the two planets have a number of similarities. The Martian day is only a little longer than ours and its angle of tilt means that Mars has four seasons, just as we do on Earth. Daytime temperatures at the equator in midsummer can sometimes reach 25°C. Thin clouds of water vapour or early morning surface frosts can also sometimes be seen. Like Earth, Mars has volcanoes, mountains, dried-up river beds, canyons, deserts and polar icecaps.



For these reasons, Mars is thought to be the only other planet where life may once have existed. However, analysis of the Martian soil by space probes Viking 1 and 2, which touched down on the planet in 1976, and Pathfinder in 1997, failed to find any sign of past or present life.



Mars is a barren planet. Its reddish colour comes from iron oxide dust (similar to rust). From time to time, large dark regions appear on the surface. These are areas of bare rock, exposed when storms remove the dusty covering. The Martian landscape features some dramatic landforms. The Solar System’s highest mountains and its deepest canyon, Valles Marineris, are found on Mars.



Mars has quite a low density and a very weak magnetic field. This suggests that it has only a relatively small ball of iron at its core.



 A number of valleys and channels have been carved into the Martian plains. From the evidence of sediments – muds and silts deposited by water – it seems likely that there were once rivers, lakes and even seas on Mars. The only water left on the surface today is frozen in the polar icecaps. The rest may have been lost to space due to Mar’s weak gravity, or hidden from view as a deep-frozen layer beneath the surface.



Picture Credit : Google



 


Will you add some facts about Planet Moon in my knowledge Bank?


MOON



The moon is neither a star nor a planet. It is a ball of rock that travels around Earth, taking about 27 days to complete the circle. It is the brightest object in the night sky, although the light it “shines” is reflected from the Sun.



The Moon may have formed when a large object or planetesimal collided with the newly-formed Earth more than four billion years ago. The impact “splashed” into space vast amounts of debris that later came together to form the Moon.



            A completely barren world, the Moon’s surface consists of cratered highlands and wide plains. The Moon’s internal structure is similar to Earth’s; its crust is thicker and not divided into tectonic plates.



            With neither air nor liquid water, it is impossible for plants or animals to live on the Moon. The barren lunar landscape is pitted with craters, blasted out by meteorites crashing to its surface. Scattered debris has left streaks radiating from some craters. The Moon also has wide, smooth lava plains. Early astronomers thought these were seas. They are still called by the Latin name for sea, mare.



PHASES OF THE MOON



The shape of the Moon appears to change from one night to the next. This happens because, as it travels round Earth, it spins only once, so the same face remains pointed towards us at all times. It is our view of the sunlit part that changes. When the face pointed towards us is turned away from the Sun, we cannot see the Moon at all: a New Moon (1).When it is turned towards the Sun, we see a complete disc: a Full Moon (5). In between, it passes through crescent (2), quarter (3) and gibbous (4) phases, and back again (6-8).



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Will you add some facts about Planet Saturn in my knowledge Bank?


SATURN



All four gas giants have rings, but Saturn’s, visible from Earth through even a small telescope, are broad, bright and magnificent. As detailed photographs taken by Voyager 2 show, the rings are made up of billions of blocks of ice and rock, ranging in size from boulders as large as houses down to tiny fragments the size of snowflakes. They are only a few tens of metres thick. Some astronomers think that the rings are the fragmented remains of a moon that was smashed apart by a passing comet.



Three rings can be made out from Earth. The outer ring (A ring) is separated from the other two lying inside it (B and C) by a gap called the Cassini Division. Voyager 2 spotted fainter rings beyond A ring. It also revealed that each ring was, itself, divided into thousands of ringlets.



Saturn has a large family of moons, many of which are small, irregularly shaped bodies with some even sharing the same orbits.



Swirling clouds and storms can sometimes be seen as ripples on Saturn’s globe. Saturn rotates very quickly, producing a distinct bulge at its equator. It is the least dense of the planets: if a large enough bathtub could be found, Saturn would float in the water!



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Will you add some facts about Planet Jupiter in my knowledge Bank?


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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Will you add some facts about Planet Venus in my knowledge Bank?


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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Will you add some facts about Planet Earth in my knowledge Bank?


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.




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Will you add some facts about Mercury in my knowledge Bank?


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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What are Planets?


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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