My Science School

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.

The creation, according to Archbishop James Ussher, took place at 8 PM on October 22 in 4004 BC. The Irish cleric made his calculation in the mid-17th century after study of the ages of the old Testament of patriarchs, long genealogies and other Biblical details.

His view was challenged in 1785 in the Scottish naturalist, James Harden, declared that the formation of mountains and the erosion of river beds must have taken millions, not thousands, of years.

But it was not until the discovery of radioactivity by the French physicist Antony Henri Becquerel in 1896 that an accurate idea of the Earth’s age was made possible.

Scientists now accept that the earth’s crust solidified around 4700 million years ago. This calculation has been made possible by a study of the decay of various radioactive minerals.

When rocks are formed by the cooling and solidification of volcanic lava, radioactive elements are trapped inside. These elements decay at a precise rate, defined as half-life - the time it takes for half the radioactivity to decay.

Careful study has determined the half-lives of individual elements. By measuring the amount of any radioactive element in the sample of rock, the process of decay can be used as if it were a clock which started ticking when the rock was formed.

It is not the precise quantity of the radioactive element left that matters, because that depends on how much there was originally. What is important is the ratio between the quantity of radioactive material and the substance into which it changes. The older the rock, the lower the radioactive material it will contain and the greater will be the proportion of its decay products.

In examining rock samples, several different dating systems can be used. A common one is the decay of the radioactive element potassium 14, a process with the half-life of 11,900 million years. The decay of uranium into lead (half-life 4500 million years) is also used. In the case of the earth, about half its original uranium has decayed into lead. So the age of the earth is about the half-life of uranium.

 

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Five of the planets in the solar system Mercury, Venus, Mars, Jupiter and Saturn are so bright that astronomers have known about them for thousands of years. But in the past couple of centuries, astronomers have found three more distant and fainter planets: Uranus, Neptune and Pluto. These are also indications of a 10th planet beyond Pluto. The discovery of these and new planets requires both mathematical calculations and luck.

Until 1781, no one suspected that there for planets beyond Saturn, so no one was actually looking for them. Then on March 13, the amateur astronomer William Herschel found Uranus, while looking for pairs of stars. He knew it was not a star because it had a visible disc, just like the Moon shows when it is full. As astronomers tracked its motions, they decided it had to be a planet.

After this largely accidental discovery, astronomers began to wonder if there might be another planet, even farther out. This suspicion was reinforced when they discovered that Uranus did not orbit the sun at a constant rate. It seemed that the planet was feeling the gravitational pull of a more distant and unknown planet.

Two brilliant mathematicians- John Couch Adams, in Cambridge, and Urbain Leverrier, in France calculate independently where this new planet would be. On August 31, 1846, Leverrier sent his prediction to Berlin Observatory and the astronomers there identified a star as the new planet, now called Neptune.

By the end of the last century, it was suspected that both Uranus and Neptune were being pulled by the gravity of the planet father out still. This time it was an American astronomer, Percival Lowell, who calculated where this planet X should be. In 1930, Clyde Tombaugh, working at the Observatory founded by Lowell, detected a faint speck of light that moved from night to night. It was indeed a new planet, close to Lowell’s calculated position, but much fainter than Lowell had predicted. This planet was called Pluto.

But many astronomers believe that Pluto is too small to affect the giant planets Uranus and Neptune. In 1978, astronomers and the US Naval Observatory found a moon orbiting Pluto. The motion of this moon revealed Pluto’s gravity, and it is far too weak to pull on Uranus and Neptune.

 

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Astronomers from Europe are at the moment considering building a telescope 10 times more powerful than any that exist the very large telescope. With it, they hope to look farther into the universe than ever before. Even so, no one really expects to see the edge of the universe.

Modern studies of the universe are based on Einstein’s general theory of relativity. This theory states that matter has a gravitational field which distorts space and time so that space becomes curved and time runs fast or slow. The gravity of matter also bends light.

In testing the effects of the general theory, scientists have found that it accounts for the motion of planets circling the sun and stars orbiting other stars.

Accepting that the theory can be applied to the universe as a whole, cosmologists also accept one final production of Einstein’s theory that the universe has no edge. The theory in fact says that there are two possibilities for the universe. One is that it curves round on itself, like the surface of the planet. Although it has no edge, it is finite. A space traveller setting off in one Direction and never changing course would eventually arrive at the starting point. This is a closed universe.

The other possibility is that the universe is infinite, that space goes on for ever in all directions. In this open universe, however far you travelled you would always come across new regions of space.

Whichever possibilities correct depends on the amount of matter in the universe. If there is enough matter, its gravity will bend space around so that the universe is closed. In this case, the gravity is strong enough eventually to halt the expansion of the universe, and draw galaxies together into a big crunch.

The most recent estimates of the amount of matter indicate that there is not enough matter to close the universe. The universe is there is likely to be infinite in size, with no end. This also means that the universe will keep on expanding for ever.

 

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In the 1920s the American astronomer Edwin Hubble made and astounding discovery about galaxies: they all moving away from the earth at speeds that depend on their distance - the father away they are, the faster they are receding. This is known as Hubble law and it occurs because the universe itself is expanding, which means that all the galaxies are rushing apart from one another.

It is quite easy to measure a galaxy’s speed, by looking at its light and seeing how the wavelength changes according to the Doppler effect. Hubble’s law tells astronomers how to calculate the distance from the speed. So if a distant galaxy is found to be receding at, say, 3,000,000 km/h, by multiplying Hubble’s constant (about 13, when calculating in kilometer per hour and light years) by the galaxy’s speed in kilometer per hour, they can work out that it is 40,000,000 light years away. In this way, astronomers have measured distances to galaxies that lie a staggering 12,000 million light-years away.

 

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A particularly bright kind of star acts as a beacon for measuring the distance to the near galaxies. These stars are called ‘Cepheid variables’, which change in brightness in the regular way.

Astronomers cannot measure the time it takes a Cepheid to flash from maximum brightness, down to minimum, and back to maximum. This time is called its period. Brightness Cepheid flash more slowly than fainter ones, so once Cepheid period has been established, its brightness can be deduced. If a Cepheid is found with a period of two weeks, for example astronomers can say that it is 4000 times brighter than the sun. By investigating the apparent dimness of Cepheid in distant galaxies they can tell how far away the galaxies life.

 

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To establish the distance of stars that are even farther away, astronomers use the stars temperature and brightness. They can measure a star’s temperature surprisingly easily: a bluish star is hot, around 20,000°C; or white or yellow star has a medium temperature and orange or red stars are cool about 3000°C.

The hotter a star, the brighter it is. A star with a temperature of 10,000°C, for example, is 40 times brighter than the sun (which has a temperature of 5500°C). So if a star of 10,000°C is found which appears very dim, then it must be a long way off in space, its brightness diminished by its great distance. Before astronomers can use this relatively simple method they need to know the relationship between brightness and temperature, and the distance from earth. This is why the first use methods such as Parallax, on nearby stars. After measuring the brightness of those stars they can then use what they know as a guide to ascertain the relative brightness of more distant stars.

The measurement of star brightness allows astronomers to measure distances to any star in the Milky Way, some lying as such as 100,000 light years away.

 

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For stars more distant than 300 light years, astronomers need a different technique. One method involves finding out the direction a Star is moving in, and its actual speed.

To establish the direction, it is much easier to work with the cluster of stars than with a single star. Many stars belong to clusters that consist of hundreds, or even thousands, of stars moving through space. Perspective makes the star in each cluster appear to converge. The angle of their converging lines reveals the direction the cluster is heading in space - towards earth, away from Earth at a 45° angle, and so on.

A Star’s speed can be deduced from its light. The stars motion towards or away from Earth changes the wavelength of the light that it emits - so that it becomes blue if it is coming towards and red if it is moving away (a phenomenon called the Doppler effect).

By combining the rate of change in the star’s spectrum with the direction of movement of the cluster, astronomers can work out its real speed through space, and hence calculate the distance to the cluster.

 

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The stars lie millions of times farther away than the sun, so astronomers have to use different techniques to establish their distance. The most important is the method of Parallax, which involves measuring the angle of the movement of a star between two points and relating it to the Earth’s orbit.

A simple experiment illustrates the method. Hold a finger in front of your face; in relation to the background. Now close that eye and open the other. The finger seems to have moved. The nearer the finger is to your face, the greater distance it seems to move.

In astronomy, the finger is the nearby star whose distance is being measured. Astronomers observe its position relative to very distant stars, looking at it from two different positions in the Earth’s orbit. By measuring the angle of the stars apparent movement between these two positions, also known as the Parallax angle, and knowing the diameter of the Earth’s orbit, astronomers can calculate the distance.

The Parallax angle is measured in arc seconds. One arc second is 1/3600 of a degree in the sky, or roughly 1/2000 the apparent size of the moon. The distance to a star in light-years is 3.26 divided by the Parallax angle. The result is given in parsecs, which is the unit of distance that corresponds to a parallax of one second of arc, or 3.26 light years. Using this method, astronomers have found the distance to hundreds of the nearest stars. For example, the nearest star to the sun is a faint one called Proxima Centuari, which lies 4.22 light years away or 1.2 parsecs. The brightest star in the sky, Sirius, is 8.6 light years away, or 2.64 parsecs.

 

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