My Science School

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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The Solar System is everything that orbits our star — the Sun over 60 moons and millions of asteroids, meteoroids and comets. Pluto is the furthest planet from the Sun, but the Solar System does not end there. Surrounding the planets is a vast sphere of comets —the Oort Cloud. Objects beyond this are pulled away from the Solar System because the Sun’s gravity is not strong enough to hold them.

The Solar System consists of the Sun, and everything bound to it by gravity. This includes the 8 planets and their moons, the asteroids, the dwarf planets, all the Kuiper belt objects, the meteoroids, comets and interplanetary dust. Since the gravitational effects of the Sun are thought to reach out almost 2 light-years away – almost half the distance to the next star – there could be any number of objects out there, as part of the Solar System.

There are separate regions in the Solar System. First, there’s the Sun, of course. Then there are the inner terrestrial planets: Mercury, Venus, Earth, and Mars. Then comes the asteroid belt; although, not all the asteroids are located in this region. The largest dwarf planet, Ceres, is located in the asteroid belt. Then come the outer gas giants: Jupiter, Saturn, Uranus, and Neptune. Then comes the Kuiper Belt, which includes 3 more dwarf planets: Pluto, Makemake, and Eris. Beyond the Kuiper Belt is thought to be the Oort Cloud, which could extend out to a distance of 100,000 astronomical units (1 AU is the distance from the Sun to the Earth).

Between the planets are smaller objects which never formed a planet or moon. This can range from microscopic dust, up to asteroids hundreds of kilometers across. Beyond the orbit of Neptune, much of this material is icy.

The solar wind emanating from the Sun blasts through the Solar System, interacting with the planets, and pushing material out into interstellar space. The region where this solar wind blows is called the heliosphere, and where it stops is called the heliopause.

The immediate neighborhood around the Solar System is known as the Local Interstellar Cloud. It has high-temperature plasma that suggests that there were nearby supernovae. The closest star to the Solar System is the triple star system Alpha Centauri.

Well, Earth is located in the universe in the Virgo Supercluster of galaxies. A supercluster is a group of galaxies held together by gravity. Within this supercluster we are in a smaller group of galaxies called the Local Group. Earth is in the second largest galaxy of the Local Group - a galaxy called the Milky Way. The Milky Way is a large spiral galaxy. Earth is located in one of the spiral arms of the Milky Way (called the Orion Arm) which lies about two-thirds of the way out from the center of the Galaxy. Here we are part of the Solar System - a group of eight planets, as well as numerous comets and asteroids and dwarf planets which orbit the Sun. We are the third planet from the Sun in the Solar System.

For thousands of years, astronomers and astrologers believed that the Earth was at the center of our Universe. This perception was due in part to the fact that Earth-based observations were complicated by the fact that the Earth is embedded in the Solar System. It was only after many centuries of continued observation and calculations that we discovered that the Earth (and all other bodies in the Solar System) actually orbits the Sun.

Much the same is true about our Solar System’s position within the Milky Way. In truth, we’ve only been aware of the fact that we are part of a much larger disk of stars that orbits a common center for about a century. And given that we are embedded within it, it has been historically difficult to ascertain our exact position. But thanks to ongoing efforts, astronomers now know where our Sun resides in the galaxy.

For starters, the Milky Way is really, really big! Not only does it measure some 100,000–120,000 light-years in diameter and about 1,000 light-years thick, but up to 400 billion stars are located within it (though some estimates think there are even more). Since one light year is about 9.5 x 1012 km (9.5 trillion km) long, the diameter of the Milky Way galaxy is about 9.5 x 1017 to 11.4 x 1017 km, or 9,500 to 11,400 quadrillion km.

Astronomers can gauge the movement of a star using a technique called the Doppler Effect. All stars and galaxies emit electromagnetic radiation. The wavelengths of any form of electromagnetic energy are affected by movement — the radiation emitted by an object moving towards an observer is squeezed, moving towards the blue end of the spectrum where wavelengths are shorter (blueshift). The wavelengths of an object moving away are stretched, and 'move towards the red end of the spectrum (redshift). Most of the stars and galaxies in the Universe have redshifted, meaning that everything is drifting apart.

A few years after Albert Einstein had developed his famous (and by now very well tested!) theory of General Relativity (GR) in 1915 he applied it to the entire universe and found something remarkable. The theory predicts that the whole universe is either expanding or contracting. There really isn't any other alternative. To have the universe staying static is like a pencil balanced on its point... possible, but very, very unlikely and not liable to last for very long.

In 1929 the astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He expected that about equal numbers would be moving toward and away from us. After all, the Earth isn't a particularly special place in the universe.

Since the time of Hubble we have observed millions of galaxies with better equipment and verified his results. With the exception of a small handful of galaxies close to us, every galaxy is moving away from us.

And in fact, the farther away a galaxy is the faster it is moving away from us. This fits in very well with Einstein's predictions. The galaxies seem to be receding from us because the entire universe is getting larger. The space in between the galaxies is stretching! And the farther away a galaxy is the more space there is to stretch so the faster the galaxy appears to move away from us.

Over the past half-century astronomers have observed many other facts about the universe that all point to the fact that the universe is expanding. While a very inventive person might be able to explain away one or at most two of these discoveries, the expansion of the universe is the only theory that can explain all of them at once. And with each passing year the evidence piles up higher!

Although we know the universe is expanding, nobody knows for sure what it is expanding into. Some scientists claim that it is not expanding into anything because nothing exists outside the Universe. Instead, space itself is stretching to accommodate the expanding matter. The Universe has no outside edge and no centre because the force of gravity distorts everything within it.

There is no edge to the universe, as far as we know. There’s an edge to the observable universe—we can only see so far out. That’s because light travels at a finite speed (one light-year per year), so as we look at distant things we’re also looking backward in time. Eventually we see what was happening almost 14 billion years ago, the remnant radiation from the Big Bang. That’s the Cosmic Microwave Background, which surrounds us from all sides. But it’s not really a physical “edge” in any useful sense.

Because we can only see so far, we’re not sure what things are like beyond our observable universe. The universe we do see is fairly uniform on large scales, and maybe that continues literally forever. Alternatively, the universe could wrap around like a (three-dimensional version of a) sphere or torus. If that were true, the universe would be finite in total size, but still wouldn’t have an edge, just like a circle doesn’t have a beginning or ending.

It’s also possible that the universe isn’t uniform past what we can see, and conditions are wildly different from place to place. That possibility is the cosmological multiverse. We don’t know if there is a multiverse in this sense, but since we can’t actually see one way or another, it’s wise to keep an open mind.

Okay, so we don’t actually think there is an edge to the universe. We think it either continues on infinitely far in all directions, or maybe it is wrapped up on itself so that it isn’t infinitely big, but still has no edges. The surface of a donut is like that: it doesn’t have an edge. It’s possible the whole universe is like that too (but in three dimensions—the surface of a donut is just two-dimensional). That means you could set off in any direction into space on a rocket ship, and if you traveled for long enough you would come back to where you started. No edges.

But there is also a thing we call the observable universe, which is the part of space that we can actually see. The edge of that is the place beyond which light hasn’t had time to reach us since the beginning of the universe. That’s only the edge of what we can see, and beyond that is probably more of the same stuff that we can see around us: super-clusters of galaxies, each enormous galaxy containing billions of stars and planets.

That depends on what you mean by the edge of the universe. Because the speed of light is finite, as we look farther and farther out in space, we look farther and farther back in time — even when we look at the galaxy next door, Andromeda, we see not what’s happening now, but what was happening two and a half millions of years ago when Andromeda’s stars emitted the light that our telescopes are only now detecting. The oldest light we can see has come from the farthest away, so in one sense, the edge of the universe is whatever we can see in the most ancient light that reaches us. In our universe, this is the cosmic microwave background — a faint, lingering afterglow of the Big Bang, marking when the universe cooled down enough to let atoms form. This is called the surface of last scattering, since it marks the place where photons stopped ping-ponging around between electrons in a hot, ionized plasma and started streaming out through transparent space, all the way across billions of light-years down to us on Earth. So you could say that the edge of the universe is the surface of last scattering.

Although nobody can be sure how the Universe began, most scientists believe that it was horn from an enormous explosion 13 billion years ago. This explosion, called the “Big Bang”, was the point where space and time came into existence and all of the matter in the cosmos started to expand. Before the Big Bang, everything in the Universe was compressed into a minuscule area no bigger than the nucleus of an atom. The Big Bang was an unimaginably violent explosion that sent particles flying in every direction. A process called cosmic inflation caused the Universe to expand into an area bigger than the entire Milky Way in less than a second. Moments later, the temperature began to decrease, and the Universe began to settle down. Stars and galaxies began to form roughly one billion years after the Big Bang.

Initially, the universe was permeated only by energy. Some of this energy congealed into particles, which assembled into light atoms like hydrogen and helium. These atoms clumped first into galaxies, then stars, inside whose fiery furnaces all the other elements were forged.

This is the generally agreed-upon picture of our universe’s origins as depicted by scientists. It is a powerful model that explains many of the things scientists see when they look up in the sky, such as the remarkable smoothness of space-time on large scales and the even distribution of galaxies on opposite sides of the universe.

But there are things about this story that make some scientists uneasy. For starters, the idea that the universe underwent a period of rapid inflation early in its history cannot be directly tested, and it relies on the existence of a mysterious form of energy in the universe’s beginning that has long since disappeared.

“Inflation is an extremely powerful theory, and yet we still have no idea what caused inflation or whether it is even the correct theory, although it works extremely well,” said Eric Agol, an astrophysicist at the University of Washington.

For some scientists, inflation is a clunky addition to the Big Bang model, a necessary complexity appended to make it fit with observations. This wouldn’t be the last addition.

“We’ve also learned there has to be dark matter in the universe, and now dark energy,” said Paul Steinhardt, a theoretical physicist at Princeton University. “So the way the model works today is you say, ‘OK, you take some Big Bang, you take some inflation, you tune that to have the following properties, then you add a certain amount of dark matter and dark energy.’ These things aren’t connected in a coherent theory.”

The Universe contains quite literally everything — from you and me to the most distant stars. It is everything and anything that exists, occupying an unimaginably vast area. Distances in space are so immense that light from the furthest galaxies takes over 10 billion light years to reach Earth, even though light travels fast enough to go round the Earth several times every second. Everything that you can see in the night sky lays our Universe, from the Sun to far-off gas clouds like the Eagle Nebula (right).

The Universe is everything we can touch, feel, sense, measure or detect. It includes living things, planets, stars, galaxies, dust clouds, light, and even time. Before the birth of the Universe, time, space and matter did not exist.

The Universe contains billions of galaxies, each containing millions or billions of stars. The space between the stars and galaxies is largely empty. However, even places far from stars and planets contain scattered particles of dust or a few hydrogen atoms per cubic centimeter. Space is also filled with radiation (e.g. light and heat), magnetic fields and high energy particles (e.g. cosmic rays).

The Universe is incredibly huge. It would take a modern jet fighter more than a million years to reach the nearest star to the Sun. Travelling at the speed of light (300,000 km per second), it would take 100,000 years to cross our Milky Way galaxy alone.

No one knows the exact size of the Universe, because we cannot see the edge – if there is one. All we do know is that the visible Universe is at least 93 billion light years across. (A light year is the distance light travels in one year – about 9 trillion km.)

The Universe has not always been the same size. Scientists believe it began in a Big Bang, which took place nearly 14 billion years ago. Since then, the Universe has been expanding outward at very high speed. So the area of space we now see is billions of times bigger than it was when the Universe was very young. The galaxies are also moving further apart as the space between them expands.

The idea of the “Big Bang” was first suggested in the 1920s by an astronomer named Edwin Hubble. He discovered that the Universe was expanding and suggested that it must have been much smaller in the past. The most convincing argument for the Big Bang lies in the presence of cosmic back-ground radiation. This is an echo of the energy released by the Big Bang, and was detected in 1965 by two astronomers. Scientists believe that the only possible source of this radiation is the dying heat of the Big Bang.

The Big Bang theory may be nice but it has to pass the judgment of observation. Nature and experiments is the final judge of the correctness of scientific ideas. Though some details of the Big Bang still need to be perfected, the general scheme of an early hot universe with a definite beginning is accepted by most astronomers today. Even so, we have to be open to the possibility that future observations could show it to be wrong. The observations given below are sometimes said to be “proof” of the Big Bang theory. Actually, the observations are consistent with the Big Bang theory, but do not provide proof. Recall from the discussion that scientific theories cannot be proven to be correct. As of now, the Big Bang theory is the only one that can explain all of these observations.

The galaxies (or galaxy clusters) are systematically moving away from us such that the farther away galaxies are moving faster away from us. As a result of General Relativity this means that space itself is expanding carrying the galaxies with it. Both the Big Bang Theory and its major competitor, the Steady State Theory, could explain it. Recall that the Steady State Theory used the perfect cosmological principle while the Big Bang uses the cosmological principle.

The cosmic microwave background radiation can be explained only by the Big Bang theory. The background radiation is the relic of an early hot universe. The Steady State theory could not explain the background radiation, and so fell into disfavor.

The amount of activity (active galaxies, quasars, collisions) was greater in the past than now. This shows that the universe does evolve (change) with time. The Steady State theory says that the universe should remain the same with time, so once again, it does not work.

The number of quasars drops off for very large redshifts (redshifts greater than about 50% of the speed of light). The Hubble-Lemaitre Law says that these are for large look-back times. This observation is taken to mean that the universe was not old enough to produce quasars at those large redshifts. The universe did have a beginning.

The observed abundance of hydrogen, helium, deuterium, lithium agrees with that predicted by the Big Bang theory. The abundances are checked from the spectra of the oldest stars and gas clouds which are made from unprocessed, primitive material. Even better observations are those made of light from very distant quasars that have passed through gas in regions of the universe where are no stars that could have contaminated the gas. The intervening intergalactic primordial gas imprints its signature on the quasar light giving us the composition of the primordial gas. All of those places have the predicted relative abundances.

How big is the Universe? Many astronomers think it has no end, so its real size cannot be measured. But it is possible to measure the diameter of the observable Universe by calculating the distance between the farthest known objects in all directions.

The light from the most distant galaxies has travelled 15 to 20 million galaxies light years before it reaches the Earth. So the diameter of the Universe, as far as we can see, is as much as 40 million million light years, or 378 million million million million kilometres.

 

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The stars in any double-star system, such as Cygnus X-1 and HDE 226868, swing around their common centre of gravity. If the stars are equally massive, their centre of gravity lies halfway between them. If not, it lies closer to the heavier star. So a double-star system forms a natural balance which allows astronomers to ‘weigh’ stars. By studying the motion of the star HDE 226868, astronomers found that the centre of gravity lay so close to the star that it suggested its companion must be half the weight, or mass, of the star itself.

HDE 226868 is a type of the star called a blue super giant. It is 20 million miles (32 million km) across and shines 50,000 times more brilliantly than the Sun. A blue super giant is about 20 times heavier than the Sun. That is, it has 20 times the mass of the Sun. So if its invisible companion, the black hole Cygnus X-1, is half this weight, it must weigh as much as ten Suns.

The Sun itself weighs as much as 300,000 Earths or 1989 million million million million tons. Astronomers calculate this figure by using the theory of gravity. Careful experiments in the laboratory have revealed the gravitational pull between two large lead spheres of known masses. This force depends partly on the distance between them. This experiment can be ‘scaled up’ so that the distance between the spheres becomes the distance of the Earth from the Sun. It can then be deduced how massive the Sun must be in order to exert the gravitational pull needed to keep the Earth and the other planets in orbit around it.

 

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In the 17th century, the English physicist Sir Isaac Newton recognised that there were problems with the traditional refracting telescope, which used glass lenses to focus light from a star.

The lenses produced a fringe of false colours around the star. This happened because when a beam of light coming through when a beam of light coming through glass is bent, its waves, being of different lengths, are bent at, different angles. Blue light, for example, which has short waves, is bent (or refracted) more sharply than red light, which has longer waves.

So newton designed a reflecting telescope which collected and focused light by means of two mirrors. (These mirrors were made from an alloy of tin and copper, known as speculum metal.) the front of the mirror which collected the light was curved, rather like a shaving mirror. As a result, it could focus light just like a lens.

Large modern telescopes all use mirrors to collect light, although they have grown in size from Newton’s 1in (25mm) mirror to  Soviet monster 236in (6m) in diameter.

 

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