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Category The Solar System

What are Quasars?

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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Why Black Hole is called so?

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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What are stars?

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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What is Galaxy?

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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What is Big Bang Theory?

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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What is Universe?

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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HOW MUCH DAMAGE COULD A METEORITE IMPACT DO?

          We know that earth is bombarded by thousands of meteorites every day, none of which does our planet much damage. Any meteorite up to 10m (33ft) in diameter will normally burn up in the atmosphere before it reaches Earth, separating into tiny fragments. If a meteorite larger than this falls to Earth, it can cause considerable damage — impacting with the energy of five nuclear warheads. Approximately once every 1000 years, a larger meteorite does fall to Earth, and several large craters caused by such impacts can still be seen. One such was the nickel – iron meteorite that created the Barringer Crater in Arizona, USA. The meteorite was an incredible 45m (148ft) wide, creating a crater nearly 1.5km (1 mile) in width. However, it would take an impact by an object roughly 5km (3 miles) wide to cause mass extinctions and threaten life on Earth.

          Most meteorites that are found on the ground weigh less than a pound. While it may seem like these tiny pieces of rock wouldn’t do much damage, a 1-lb. (0.45 kilograms) meteorite traveling upward of 200 mph (322 km/h) can fall through the roof of a house or shatter a car windshield. 

          When the Grimsby meteorite landed in Ontario, Canada in 2009, for example, it broke the windshield of an SUV. In another incident, meteorites crashed into the back end of a Chevy Malibu in Peekskill, New York, in 1992, Cooke and Moorhead said. Thankfully, no one was injured during these events. 

          However, the pieces of rock falling from the sky are not even the greatest concern regarding meteor impacts, Cooke said.

          “What causes the most damage is the shock wave produced by the meteor when it breaks apart in [Earth’s] atmosphere,” Cooke said. “So, you don’t have to watch for the falling rocks — you have to worry about the shockwave.”

          For example, the Chelyabinsk meteor — an asteroid the size of a six-story  building that entered Earth’s atmosphere in February 2013 over Russia — broke apart 15 miles (24 km) above the ground and generated a shock wave equivalent to a 500-kiloton explosion, Cooke said. It injured 1,600 people.

          Another major collision was the Tunguska meteorite, which was larger than Chelyabinsk and 10 times more energetic. The meteorite exploded over the Tunguska River on June 30, 1908, and flattened 5000,000 acres (2,000 square km) of uninhabited forest. Because of its remote location, the event is an example of a meteorite that would have gone undetected had it not been so large, Cooke and Moorhead explained. 

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WHY ARE THERE SO FEW CRATERS ON EARTH?

          Unlike many of the planets, moons and smaller bodies in the Solar System, Earth appears to be covered by very few craters. In the early days of the Solar System, Earth was as much a target for meteorites as any other planet, and suffered intensive cratering in the first one billion years of its existence. However, unlike bodies such as Mercury and the Moon, Earth has many geological processes that “hide” craters. Constant weathering and erosion from winds and water wear away or cover up craters. Some may also be hidden by vegetation or lie under the sea, although in the last hundred years, aerial photography and other forms of imaging have given us a clearer view of many remaining craters.

          Impact craters leave quite an impression on the surface of planets and moons — just think of Earth’s moon, which gets its distinctive appearance from millions of encounters of asteroids over the centuries. But Earth is a different story altogether, with only 128 impact craters recorded in the most recent count. That can’t be right, can it?

          He reports that a new study shows that the low number found by past scientists isn’t “just the result of lazy searching”: it’s the surprising truth about a planet that’s astonishingly crater-free.

          The study looked at the ways Earth erosion affects existing craters and concluded that the current count of 70 craters larger than 6 km (3.7 miles) in diameter should be just about right. That’s a rare instance of a complete geologic record, writes Hand — and one that may discourage people on the hunt for new craters.

          But don’t put away your crater-catching gear just yet. The study’s authors note that just because we’ve already found all of the likely large impact craters on Earth don’t mean there aren’t more to discover. The real opportunity, they write, lies in smaller craters: they estimate that more than 90 craters between .6 miles and 3.7 miles in diameter should still be undiscovered and more than 250 between 0.1 miles and .6 miles.

          NASA notes that Earth is equipped with three processes that eat up craters relatively quickly: erosion, tectonics, and volcanism. These forces leave only the largest scars from meteorites or asteroids — unlike, say, the moon, which can’t gobble up craters. Hand writes that the parameters of the study also play a part in the low number — it looks at just surface craters, not those that lie beneath sediment. And the study also didn’t look at volcanic craters, which formed some of Earth’s most distinctive basins and lakes.

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CAN SCIENTISTS PREDICT METEOR SHOWERS?

          Because earth passes through meteor streams at roughly the same time each year, meteor showers can be predicted highly accurately. Astronomers have now even worked out which comets are responsible for each annual shower. Two meteor showers come from the trail left by Halley’s Comet: the Orionids in October and the Eta Aquarids in May. Although meteors in a shower fall to Earth over a large distance, perspective makes them seem to be falling from the same point in the sky, called the radiant.

           Most ‘predictions’ of the rate of meteors per hour during meteor showers are based on both theory and observation. Essentially, a computer model is built containing the trajectories of every known comet – since it is the debris from comets that forms the ‘stream’ of particles we see during a meteor shower.

          This model contains information on the rate that these comets release material, along with the sizes, directions and velocities at which they are released, as well as the gravitational forces that determine their subsequent trajectories through space. The trajectory of the Earth and the conditions of the Earth’s atmosphere are also inputted into the computer model.

          By watching how Earth moves through the meteor stream it is possible to estimate the likely number of meteors that will be visible during a given shower for a given location. But different astronomers use different models. Plus, these models are partly based on difficult measurements of the meteoric particles in the Solar System, so their predictions are often only approximate. But generally, they can be used to reliably predict when a meteor shower is likely to be more or less intense than the average.

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HOW FAST DO METEORITES IMPACT WITH THE EARTH?

          The average meteorite enters the Earth’s atmosphere at around 50km/s (31mi/s), but particles in the atmosphere cause the speeding rocks to slow down. All but the largest meteorites are decelerated to around 150km/h (93mph) by the time they impact. Larger meteorites will not be slowed by atmospheric friction and hit the ground travelling at deadly speed.

          The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the Solar System falls into Earth’s atmosphere creating temporary incandescence resulting from atmospheric friction. A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micrometeoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized.

          Meteor’s come in a range of sizes, from dust-sized which we see as reflected sunlight in the orbital plane of the Solar System (called zodiacal light) to house-sized.

          When a meteor enters the atmosphere friction causes ablation of its surface (i.e. it burns up). If the meteor is small (fist-sized) it vaporizes before hitting the ground. If larger it survives to impact on the ground, although it will be reduced in size during entry into the atmosphere. About 25 million meteors enter the Earth’s atmosphere every day (duck!). Most burn up and about 1 million kilograms of dust per day settles to the Earth’s surface.

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