Our Mysterious Universe

What mysteries will be revealed as humans turn their attention to the vastness of space? Will our robot space probes find traces of life on other planets and moons? Will ever more powerful telescopes provide answers to the origins of the cosmos? Will the huge radio aerials now scanning the skies find evidence of extraterrestrial civilisations? Our planet Earth is just one tiny speck in the infinity of space. It seems inconceivable that there should not be life anywhere else.

Is the universe finite ?

So far, astronomers still cannot say for certain whether the universe is finite or infinite. Most support the latter theory, but the universe could also be finite. Even if this were true, however, the universe would not have any boundary. It would curve in on itself so that - to put it very simply - it would be shaped like a ball. And just like a ball the universe would have no spatial limits, but its volume would be finite.

How far can we see into space ?

The most distant object visible to the naked eye is the andromeda Galaxy, some 2.5 million light-years away from Earth. It is a spiral galaxy similar to the Milky Way- the galaxy in which we live. While the Andromeda Galaxy may seem very remote, using modern telescopes astronomers can see galaxies that are an incredible 13 billion light-years away. At this distance, however, observers come up against an insurmountable barrier. This is because every time we look into space we are simultaneously looking back into the history of the cosmos. When we look at a galaxy 13 billion light-years away, we are seeing it as it looked when light from it left on its journey, 13 billion years ago. At that time - close to the origin of the universe - the first galaxies had only just been formed. Before that, there was nothing to see - which means that we cannot see any deeper into space.

How long is a light year ?

Although statements like 'it took light-years' are frequently bandied about, the unit of measurement 'light-year' does not measure a period of time, but instead denotes a very great distance. A light-year is how far light travels in one year. Since light moves at a speed of around 300 000 km/sec., this is about 10 trillion kilometres. Given the huge scale of the universe, a light year is therefore an ideal unit for measuring the distances between the stars and galaxies.

How is distance measured in space ?

Many different methods are used to measure distance in space. Apollo 11 astronauts left a laser reflector on the Moon, making it possible to measure to the centimetre the precise distance between the Moon and the Earth.

The distances of other bodies in our solar system, such as the planets and asteroids, can be measured by radar, that is by reflecting radio waves off them.

However, this method will not work with stars, which are much further away. For these, astronomers use a technique that measures tiny differences in the position of a star when it is observed from opposite sides of the Earth's orbit, that is at intervals six months apart. Since the diameter of the Earth's orbit is known, the change in angular position of a star at either side of the orbit can be used to calculate its distance. Known as the 'parallax method', this works for objects at distances of up to about 1000 light-years.

For still more distant objects, such as galaxies, astronomers must use indirect methods. One of these involves Cepheid Variables, a type of star which has the convenient property of varying in brightness at regular intervals. We know that the length of the intervals depends on a star's luminosity, which means that by measuring a Cepheid Variable's brightness it is possible to calculate how far away it - and the galaxy that it is a part of- is from us.

Does the Universe have a centre ?

When it was discovered that nearly all galaxies are moving away from us, scientists wondered whether the Milky Way might be situated at the centre of the universe. Plausible as the idea may seem, it is incorrect. In reality, galaxies scarcely move, it is the space between them that stretches - just like the surface of a balloon when it is inflated. Every point moves away from every other point, but no one point is the central point on the balloon's surface. This would also be true if the universe were not curved like the balloon's surface but infinite. Although we may find this hard to imagine, even a universe of this kind would have no centre.

How big is the Universe ?

According to the latest findings, the universe came into being about 13.7 billion years ago. In theory, therefore, we cannot receive any radiation more than 13.7 billion years old. Or, to put it another way, no light that reaches us from deep space can have travelled from more than 13.7 billion light-years away. Of course, this. does not mean that the universe comes to an end at this distance. The universe could be very much larger than the area we are able to observe through our telescopes, and it could even extend to infinity.

Will the universe keep on expanding for ever ?

In theory there are three possibilities as to what may happen to the universe in the future. The expansion of the cosmos could be held back by gravity, come to a complete standstill and then go into reverse. The universe would then collapse in upon itself causing a Big Crunch. The second possibility is that expansion could slow down but continue, in which case it would take an infinite amount of time to come to a standstill. The third possibility is that expansion will accelerate over time. Until 15 years ago astronomers were convinced that one of the first two scenarios was most likely. Then they made the unexpected discovery that stellar explosions in very distant galaxies proved that expansion was in fact increasing. Researchers don't yet know why this should be the case, but it seems that expansion is driven by a mysterious 'dark energy' that fills our cosmos.

How loud was the big bang ?

The Big Bang was not a 'normal' explosion - it did not even take place in existing space. Rather, the Big Bang created not only matter and energy but also space and time. This means that there was no place from which the Big Bang could have been seen or heard from the outside. Observers (or listeners) would have found themselves right in the middle of the Big Bang - and would have been instantly vaporised due to its extremely high temperatures. Despite this, the American astrophysicist John Cramer once took the trouble to produce a computer simulation of the sound of the Big Bang. He compared the result to 'a large jet plane 100 feet (300 m) off the ground, flying over your house'. But his description is some what misleading. In order to make the sound of the Big Bang audible, Cramer had to scale up the low-frequency sound waves by 100 000 billion billion times. The actual sound of the Big Bang is imperceptible to the human ear.

Are there other universes out there ?

According to string theory - a relatively new theory which attempts to combine all the forces known to physics into a single unified whole - there may be any number of universes in existence. However, each of them might be governed by different natural laws. For this reason most of these universes could not sustain life, since to find a combination of natural laws that would allow life to exist would be exceptional.

However rare such a combination may be, any universe containing an intelligent observer would inevitably be an exceptional universe of this kind. Some scientists believe this is one way of explaining why it is that the natural laws in our universe seem designed to allow life to exist - because we are here to observe it. Or it may be merely a matter of chance that this is how it is.

How old is the Universe ?

The most accurate measurement of cosmic microwave background radiation so far was made by the US satellite Wilkinson Microwave Anisotropy Probe (WMAP), which found our universe to be 13.7 billion years old. By comparison, Earth came into being some 4.5 billion years ago.

What are galaxies ?

Stars are not evenly scattered across the universe. Instead they form large systems called galaxies, which can contain several hundred billion stars each. Galaxies exist in a variety of different shapes and sizes, from gigantic elliptical galaxies, through to spiral galaxies like our own Milky Way, and small, irregular galaxies such as the Large and Small Magellanic Clouds.

What are clusters of galaxies?

Galaxies themselves are not distributed randomly or evenly through the cosmos. They collect in groups and clusters. For example, our Milky Way belongs to the Local Group of galaxies which comprises about 30 star systems. Other galaxies are located in clusters which have several thousand members. One example is a cluster within the constellation Virgo, which is made up of more than 2000 galaxies.

How many galaxies can we see? 

Between September 2003 and January 2004, the Hubble Space Telescope took the longest ever exposure of the sky in the history of astronomy-more than 270 hours. Based on the number of galaxies visible in this spectacular image-dubbed the Hubble Ultra Deep Field - scientists estimate that with the technology available today some 50 billion galaxies can be observed.

How far is it to the nearest galaxy?

It is currently thought that the Canis Major dwarf galaxy in the constellation of the Great Dog is the galaxy closest to us. It is only 25 000 light-years away from our solar system and 42 000 light-years away from the centre of the Milky Way. This small system appears to be disintegrating as the gravitational pull of the much larger Milky Way is tearing it apart and absorbing its stars.

Are light rays always straight ?

Light rays are subject to the effects of gravity. If a ray passes close to a star or galaxy its path is curved by an effect known as 'gravitational lensing'. This can produce some interesting effects. For example, scientists can observe quasars (see right) which would otherwise be hidden from us behind a much closer galaxy. The light curves around the intervening object, allowing us to see it. Depending on how objects are situated in relation to one another, the light from distant bodies can even be broken up into spectacular arcs or rings. The diversion of light also causes variations in brightness. If a star visible from Earth passes in front of another star, the brightness of the more distant star becomes momentarily more intense. If the star in front is also being orbited by a planet this can lead to a further increase in brightness. Thanks to this phenomenon, astronomers have discovered several planets orbiting other stars.

What are Quasars ?

Quasars are among the brightest objects in the universe, radiating more energy than entire galaxies. After speculating for years about the mysterious nature of quasars, astronomers now know that they are located at the heart of some galaxies. They draw their energy from enormous black holes which have masses billions of times greater than that of our Sun. Like cosmic vacuum cleaners these gravitational monsters suck up matter from their surroundings, releasing unimaginable quantities of energy in the process.

How many Stars are there in the sky ?

With the naked eye and under favourable conditions - a moonless night well away from city lights - some 3000 stars can be seen in the sky. But one look through even a modest telescope reveals that many more stars exist, especially in the shimmering strip of the Milky Way. In fact, our galaxy alone contains about 200 billion stars.

Who named the constellations ?

From the earliest times people arranged conspicuous stars in the night sky into patterns in which they believed they could see figures of living beings, gods or objects. For example, the 12 well known constellations which make up the zodiac date back to the Babylonians. Most of the other constellations in the northern sky have their origins in Ancient Greece. Two thousand years ago, Eratosthenes and Ptolemy wrote of 48 constellations, most of the names of which were taken from Greek mythology. More recently, more constellations previously unknown to Europeans have been introduced, especially in the Southern Hemisphere, so that the heavens are now home to creatures such as the Bird of Paradise and the Chameleon as well as all manner of technical equipment such as the Air Pump and the Chemical Furnace. In 1928, the International Astronomical Union finally drew up the list of 88 constellations and their boundaries, which is still in use today.

How far away are the constellations?

Constellations are not true associations of stars; they merely help us find our way around the sky. Our ancestors arranged groups of bright stars more or less arbitrarily to form images. For this reason there is no answer to the question of how far away a constellation is. The stars in a constellation are situated at varying distances from Earth. For example, the seven brightest stars in the Great Bear are between 55 and 93 light-years away, and the star with the designation Kappa in the Great Bear's front paw is 200 light-years away.

What are Stars ?

Stars are nothing other than suns. Just like our own Sun, they too are huge balls of incandescent gas, inside which nuclear fusion transforms hydrogen into helium. This process releases enormous amounts of energy, which is why stars shine and radiate heat. It is only because stars are so far away that they appear in the night sky as no more than tiny points of light. In reality, many stars are considerably larger than our Sun.

Why do stars twinkle?

As starlight travels towards an observer on Earth it has to pass through the Earth's atmosphere where the light is distorted by layers of air of differing temperature and thickness. Because these layers of air are also in constant and turbulent motion, from the observer's point of view the stars appear to flicker. Seen with the naked eye they seem to twinkle, while through a telescope they appear blurred. Basically the effect is the same as when the air above a hot road surface in summer seems to shimmer. When stars are observed from space or from the Moon, which has no atmosphere, they do not twinkle. However, there are some stars whose brightness really does vary. although these variations occur very slowly, over periods of days, weeks or even months.

What makes up the zodiac ?

When we talk about the zodiac, we are referring to constellations through which the Sun moves in the course of the year. These constellations are mostly named after animals and cover an area about 14° wide running above and below the Sun's apparent annual path across the heavens. Even though we cannot see these stars by day, they are still there, hidden by the brightness of the Sun. Originally there were 12 signs of the zodiac, but since the adoption of constellation boundaries defined by the International Astronomical Union in 1930, the Sun now passes through a 13th constellation - Ophiuchus, the serpent bearer. The Sun crosses through Ophiuchus in moving from Scorpius to Sagittarius, but Ophiuchus is not included in the traditional signs of the zodiac.

Do the stars influence our destiny?

Astrologers claim that our destiny is influenced by the position of the Sun, Moon and the planets in relation to the stars. However, numerous research projects - some carried out according to criteria laid down jointly by astrologers and astronomers - have shown that such connections do not exist. When a daily horoscope in the newspaper seems unusually accurate it is because it is so cleverly composed that almost any reader can identify with it. However, it would be unfair to accuse astrologers of cheating the public. Many are simply trying to offer sensible advice about dealing with everyday problems. Nevertheless, their work is based on a false doctrine and unscientific principles.

Why do stars appears pointed in photographs ?

Some photographs, taken by large telescopes, show the brightest stars with points radiating from them. In reality, of course, stars do not have points. They are huge spherical balls of gas, just like the Sun, and because they are so far away they appear as pinpoints of light in even the most powerful telescopes. On Earth, atmospheric turbulence blurs the images of stars, but this does not affect photographs taken from space. Instead, the size of the star's image is governed by light being bent at the edge of a lens, which scatters it into a small disc. At the same time, supports for the secondary mirror inside a telescope also diffract the light- and this is what produces the radiating points.

Why is the Sun yellow while the stars are white ?

The light receptor cells - known as cones - in the human eye, are considerably less sensitive than the cells responsible for distinguishing light and darkness - known as rods. As a result, in dim light we are barely able to discern colour - as the saying goes: at night all cats are grey. Therefore, most stars appear to us to be white - only in the very brightest stars can we see traces of colour. For example, the star Antares in the constellation Scorpius appears to be reddish. The colour of a star enables astronomers to gauge its temperature. Red stars are cooler than the Sun, while blue stars are very hot.

Does the Milky way look like a Catherine-wheel ?

The Milky Way is the galaxy that contains our solar system. It is a spiral galaxy and contains about 300 billion stars. Viewed face-on, our galaxy looks like a gigantic catherine wheel, while side-on on it looks like a flat disc. Since we, the observers, are within this disc, we see most stars as making up a bright strip that extends across the sky, which is where the name Milky Way originally came from. In fact, all the stars visible with the naked eye are part of the Milky Way.

How old is the Milky Way?

The oldest objects in the Milky Way are the so-called globular star clusters, spherical clusters of hundreds of thousands of stars. The most recent calculations reveal globular star clusters to be about 13 billion years old, although determining their age is difficult and the figures are still uncertain. The Milky Way itself could be as much as 800 million years younger. According to the latest findings, the universe came into being 13.7 billion years ago and the first stars lit up 400 million years after the Big Bang. This suggests that the Milky Way cannot be older than 13.3 billion years.

Can stars be bought ?

There are, in fact, many suppliers offering stars for sale, especially on the Internet, and the sale is usually linked to the right to name the star. Purchasers even receive a certificate that appears to place an official stamp on the whole transaction. In reality, it is pure fantasy and no more than a money-making scheme. The stars belong to no one and neither the purchase nor the name given to the star will be recognised by any official organisation.

How do newly discovered celestial bodies get their names?

Only the astronomers' professional organisation, the International Astronomical Union (IAU) can decide on names, and there are strict rules surrounding the naming of celestial bodies. For example, comets are always named after their discoverer. In the case of asteroids, the discoverer is entitled to suggest a name but it must be approved by a committee of the IAU. These days, most stars are merely given a catalogue number which usually contains a set of celestial coordinates indicating the body's position in the sky.

Can we see planets orbiting other stars ?

Astronomers have discovered more than 200 planets orbiting other stars, although they have only actually seen four, and even then the experts are not sure that these discoveries are really planets and not just very small stars.

In cases such as these, astronomers are faced with several problems. Normally, stars are millions of times brighter than their planets, and, moreover, planets, as seen from Earth, are very close to their stars. For these reasons, most extrasolar planets - or exoplanets - are very hard to see, and can only be detected by indirect means. For example, a large planet's gravity may be strong enough to pull its star from side to side as it orbits around it. Alternatively, a planet may pass in front of its star at regular intervals, causing the star's light to fluctuate enough for the differences to be detected by instruments on Earth.

Where in the Universe is life possible ?

Earth is the only known example of an inhabited world. Life on Earth is based largely on the existence of liquid water. Scientists used to believe that there could only be liquid water on planets that orbited within a very narrow band around their star. This was because heat from the star would need to be great enough to melt ice, but not hot enough to evaporate all surface water. Within our solar system, only Earth fulfils these conditions, although in the distant past Mars might have met them as well.

However, we now believe that on one of Jupiter's moons there is an ocean lying beneath a thick layer of ice. The ice is melted by the tidal energy of the huge planet close by, and might be able to harbour life. This discovery means that the area around a star in which a populated world could exist has expanded considerably. Of course, we must not exclude the possibility that quite different life forms exist that can get by without water.

Could we discover life on another planet?

Future developments in telescope technology could enable us to capture images of planets similar to Earth, and precise investigation of the light from such planets could show us whether they harbour life or not. The presence of life changes the atmosphere of a planet by affecting its chemical balance. For example, a high oxygen level, as found in the Earth's atmosphere, can be a sign of an inhabited world. Perhaps, within a few decades, we will be able to detect intelligent life on distant worlds if, like us, the extraterrestrials have polluted their atmosphere with artificially produced chemicals.

What sort of object is our Sun ?

The Sun is a gigantic, glowing ball of gas made up mainly of hydrogen and helium. Its diameter is just over 100 times that of the Earth and its mass is 330 times greater. The Sun contains 98% of all of the mass contained in our solar system, which it dominates with its immense gravity.

Why does Sun shine ?

Unlike the planets and our Moon, which merely reflect the Sun's light, the Sun emits its own light. This is because it has a surface temperature of around 6000° C and therefore glows brightly. Most solar radiation has a wavelength near the centre of the visible region of the electromagnetic spectrum - which explains why we see it as being yellow. This is obviously not a coincidence, since most creatures on Earth have evolved to perceive the region of the spectrum in which the Sun gives its greatest amount of light. In addition, the Sun emits radiation in other areas of the electromagnetic spectrum, such as radio signals and X-rays as well as infrared and ultraviolet rays.

How does our Sun generate its energy?

Towards the centre of the Sun the temperature rises rapidly, reaching a maximum at the core of around 15 million degrees Celsius. In addition, the Sun's core is under immense pressure - 250 billion times normal atmospheric pressure on Earth. In these extreme conditions the atomic nuclei of hydrogen atoms collide with each other at great speed and fuse to become helium nuclei. During this nuclear fusion, energy is released which travels outwards from the interior of the Sun and is radiated from the Sun's surface. The released energy's journey from the Sun's centre to its surface takes around 10 million years. If the energy supply at the centre of the Sun were to be used up today (and somehow the Sun didn't collapse) it would take us 10 million years to notice.

What does the interior of the Sun look like ?

Energy-producing nuclear fusion only occurs in the Sun's inner core. Although this contains about half the Sun's matter, it takes up only 1.6% of its volume - hence the inner core's radius is about a quarter of the size of the Sun's total radius. The energy initially travels outward in the form of radiation through a section of the Sun which is known as the radiative zone. About 100 000 km beneath the surface, the temperature drops sufficiently to cause the gas to absorb the radiation. This is why the Sun's external layer - the convective zone - bubbles like a boiling cauldron. The matter, heated by radiation, rises to the surface where it cools and then sinks back down to the edge of the radiative zone.

How old is the Sun ?

The most ancient rocks that scientists have discovered so far, both on the Earth and on the Moon, are about 4.5 billion years old. Since the Sun and its planetary system - which includes the Earth and its Moon - were formed at much the same time, the Sun must also be 4.5 billion years old.

This means our Sun is less than halfway through its life span. We know this, because astronomers have been able to calculate that the Sun's energy store will last for about 11-12 billion years, based on the Sun's mass and the amount of energy it emits. Hence we can expect the Sun to shine for another 7 billion years or so.

Will the Earth be swallowed up by the Sun one day ?

In about 5 billion years, when the hydrogen in the sun's core has been exhausted, the Sun will swell up to become a red giant (see p. 27). Astronomers are not entirely sure just how large the Sun will eventually become, but it is likely that in this phase of its development the Sun will only consume the inner planets Mercury and Venus. However, even if Earth is spared total annihilation - life on our planet will no longer be possible. A gigantic and hostile Sun will shine with 2300 times its current intensity, and will cause the Earth's crust to melt into an ocean of lava.

What do we know about the life of the solar system ?

Our solar system is a busy place. In addition to the large planets, there is an immense throng of smaller celestial bodies. These include dwarf planets, moons, asteroids and comets, as well as innumerable smaller lumps of rock - the meteoroids - which are mainly debris left by passing asteroids and comets. They range in size from fragments as small as grains of sand to boulders several metres across.

What is the difference between stars, planets and moons ?

Stars are hot celestial bodies which generate energy in their gaseous core and so shine of their own accord. A few years ago it was still impossible to say with certainty whether or not there were other stars with planetary systems. Recently, however, planets have been found orbiting more than 100 nearby stars, so it seems safe to assume that many stars in the universe are probably at the centre of planetary systems.

Unlike stars, planets do not emit their own light, they merely reflect the light of the star around which they orbit. We are only able to see the planets in our solar system because they reflect the Sun's light. Hence the brightness of a planet in the sky depends not only on its size, but also on its position relative to the Sun and how well it reflects the Sun's light.

Finally, moons are those celestial bodies that do not orbit a star, but travel around a planet instead. Some of the 140 or so moons in our solar system are even larger than planets. Ganymede, for example - Jupiter's largest moon (and the largest moon in the solar system) - has a diameter of 5262 km, which is considerably larger than Mercury, which has a diameter of only 4880 km.

How many planets orbit our Sun ?

There are now only eight planets in our solar system, namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Little Pluto was thought of as a planet until 2006, but by then astronomers had came across increasing numbers of similar celestial bodies beyond Neptune's orbit, at least one of which was larger than Pluto. Scientists were therefore faced with having to make a decision. Should the largest of those small bodies also be listed as planets - or should Pluto lose its status as a planet? Following a period of intense discussion, they decided to take the latter course. Pluto, and other small, but still spherical, celestial bodies have since been referred to as dwarf planets.

Do other planets have seasons ?

We have seasons on Earth because of the tilt of the Earth's axis in relation to its orbit. This causes the Sun to shine more on the Northern Hemisphere for one-half of the year and then more on the Southern Hemisphere for the other half. Other planets can have seasons for the same reason, provided their axes of rotation are also tilted in relation to their orbits. Mars' axis, for example, has a 25° tilt, which is almost the same as Earth's at 23.5°. However, seasonal differences on Mars are greater because of Mars' eccentric orbit and it is possible to see seasonal fluctuations in Mars' polar ice deposits.

Uranus is an extreme example. Its axis has a 98° tilt, and hence the Sun shines on the northern pole for half a Uranian year (84 Earth years) and then on the southern pole for the second half. The unlit pole is left in complete darkness.

What laws govern the movements of the planets ?

In the early 17th century, the German astronomer Johannes Kepler formulated three laws to describe the movement of the planets within the solar system. The first law covers the shape of a planet's orbit, which is always elliptical, with the Sun at one of the foci of the ellipse (see diagram 1 below). The second law describes a planet's speed. An imaginary line joining the Sun and a planet sweeps out equal areas during equal intervals of time. We can deduce from this that when a planet is closest to the Sun (at perihelion) it travels faster than when it is furthest away (at aphelion). Finally, the complicated third law compares the orbits of various planets. Using the orbital period (T) of a planet and the semi-major axis of its orbital ellipse (a) - see diagram 1 below-if we divide T2 by a³ we get a constant that is the same for every planet. This is useful because, if we know the dimension of the semi-major axis of at least one planet - Earth for example - then it is possible to calculate the size of other planets' orbits when we know only their observed orbital periods. Kepler's three laws turn out to be a natural consequence of the law of universal gravitation, formulated by Isaac Newton and published in 1687.

Are the planets' orbits stable?

Astronomers have been unable to answer this question with any degree of certainty. Although the Sun is the dominant body in the solar system, and therefore the major factor in determining planetary orbits, the planets also attract each other with their much smaller gravitational forces, causing minor orbital fluctuations. Long term climatic trends on Earth, such as ice ages, are among the consequences of this kind of orbital fluctuation. It is reassuring to know that planetary orbits have remained more or less stable since the formation of the solar system. However, not even computer simulations can help us predict whether this will be the case forever.

Where are the planets to be found in the sky ?

Unlike stars, which barely change their positions relative to one another over human lifetimes, planets are in constant motion, which is why our ancestors called them wandering stars. This constant motion is, of course, due to their steady progress along their orbits around the Sun. The orbits of all the planets - including Earth - are located in almost the same plane known to astronomers as the ecliptic. This is why we find all the planets in the sky close to the line of the ecliptic.

Because the Earth also travels around the Sun, and because each planet travels at a different speed, strange effects can occur when observing distant planets over weeks or months. A planet can appear to slow down and even move backwards across the sky before reverting to its original direction of travel. Of course, the planet doesn't actually change direction. The effect is caused by the fast moving Earth catching up and overtaking one of the slower moving outer planets.

Why is Mercury so rarely visible from Earth ?

Mercury is the closest planet to the Sun in our solar system. Since its average distance from the Sun is only 58 million km (compare with 150 million km between the Earth and the Sun), its position in the sky is also very close to the Sun. Seen from Earth, Mercury cannot move any further from the Sun than 28°, and most of the time it is even closer than that, or even directly in front of or behind the Sun. From Earth the planet is nearly always obscured by the Sun's brilliant light. Mercury is only visible for most people when the Sun is not in the sky at the same time - which is briefly at dawn and dusk. Then Mercury can be seen low on the horizon either rising or setting just before or just after the Sun.

What are the evening star and the morning star ?

Like Mercury, Venus is an inner planet, which means that it's orbit inside Earth's orbit. This is why, when you look up at the sky, you will never see Venus and the Sun at opposite horizons. Thus Venus cannot be seen in the sky in the middle of the night, but only in the evening or in the morning, when its prominent, brightly shining disc is popularly known as the evening or morning star - both names identifying the same celestial object. However, Venus cannot set in the evening as the evening star and then rise again in the morning as the morning star (or vice versa). Venus' position is either to the west or east of the Sun, so that it can only ever be either the morning star, or the evening star.

Why is it so hot on Venus?

The temperature on Venus is about 460°C. However, this is not due solely to the fact that Venus is much closer to the Sun than the Earth. Rather, the conditions observed on Venus are a consequence of a powerful greenhouse effect. Some 96% of the planet's dense atmosphere is made up of the greenhouse gas carbon dioxide. Some scientists believe that enormous volcanic eruptions occurred on Venus about 800 million years ago, during which almost the entire surface of the planet was covered with fresh lava. It is possible that large quantities of carbon dioxide were released during this event, which could have triggered, or at least intensified, the greenhouse effect observed today.

Why is Mars red ?

Even when viewed with the naked eye, Mars is a conspicuous object in the sky, not simply because it is so bright, but because of its strong red colour. The cause of this colouring is the dust that covers almost the entire surface of Mars, and which contains large amounts of iron oxide. Simply, Mars is rusty. Planetary scientists suspect that a large proportion of the iron arrived on Mars in meteorites. However, for the iron on Mars to have been oxidised, surely there must have been free water on the red planet at some time in the past? This is what scientists used to believe, but recent studies have shown that the very small quantities of water in Mars' thin atmosphere are quite sufficient - with the help of powerful ultraviolet rays - to make the iron go rusty. This does not necessarily mean, however, that there have never been large amounts of free water on Mars any time in the past.

Is there, or has there ever been, life on Mars?

Many geological structures found on Mars may have been caused by water - twisting valleys that have the appearance of river courses, stream structures around craters and mountains, the coastlines of large lakes and even the suggestion of dust-covered ice floes. Current findings indicate that 4 billion years ago Mars may have had a thicker atmosphere as well as large expanses of open water - there may even have been an actual ocean in the Northern Hemisphere.

Life could have emerged at that point and primitive life forms could have been preserved to this day deep beneath the Martian surface or in active volcanic regions. In the 1970s, biological experiments on board the American Viking space probes provided contradictory results that continue to be debated to this day. For this reason, plans are afoot for future Mars probes to employ more sophisticated methods in the search for traces of life.

What are Martian canals?

In 1877, the Italian astronomer Giovanni Schiaparelli thought he had seen some thin, straight lines on the surface of Mars. He called these 'canali', which was translated into English as 'canals' and so triggered a debate lasting many years surrounding the possible existence of an intelligent civilisation on Mars. However, Schiaparelli himself believed that the 'canali' had natural causes. The wealthy Bostonian businessman Percival Lowell was fascinated by the canals, so much so that he built his own observatory in Flagstaff, Arizona. Between 1894 and 1916 he mapped 437 canals in total. Lowell was convinced that the canals were an artificial irrigation system devised by the intelligent inhabitants of a dying world. In reality, however, the Martian canals were merely an optical illusion. The human brain has a tendency to interpret a row of dots as a line. Photographs taken by the first American Mars probes in the 1960s finally cleared the matter up- there are no canals on Mars.

How are asteroids different from comets ?

Asteroids small bodies that are made up chiefly of rocks are small that are made up Sun in the wide asteroid belt, which lies between the orbits of Mars and Jupiter. Comets, on the other hand, are made up of a loose mixture of rocks and ice, and for this reason they are often called 'dirty snowballs'. However, recent research has shown that rocks are the main constituent of most comets.

Astronomers think that far beyond Neptune's orbit our solar system is surrounded by a shell made up of several billion comets, known as the Oort Cloud. Although the largest asteroids can be several hundred kilometres in diameter, most comets are only a few kilometres across. Comets only attract the attention of the general public when gravitational forces at work in the Oort Cloud send them on a path that takes them into the inner solar system. As they approach the Sun, volatile constituents vaporise and form a gas and dust tail which can be more than 100 million km long and clearly visible from Earth. 

What makes the Earth and the Moon special ?

There are eight planets in our solar system, and most have moons, but there is something special about the Earth and its Moon. The fact that there is life on Earth is one important factor that makes it special, but there is another - astronomical - reason as well. No other planet has such a large moon relative to the planet's size. This is why some scientists do not consider the Earth and its Moon to be a typical planet-moon system, but prefer to think of it as a 'double planet'.

How was the Moon formed ?

It is probable that the large moons of Jupiter and Saturn were formed from the solar nebula, at the same time as their planets. The smaller moons, on the other hand, are more likely to be captured asteroids. The history of the formation of Earth's Moon is quite different. According to current theories, our Earth collided with another, Mars sized, proto-planet 4.5 billion years ago, shortly after the Earth had been formed. This caused a large part of the Earth's crust and most of the other planet - to be hurled into space. This matter initially formed a disc around the Earth, and it was out of this material that the Moon eventually formed. The newly formed Moon was probably only between 20 000 km and 30 000 km away from Earth and, because of its proximity, caused massive tidal fluctuations on the surface of the young planet. These tidal forces acted to ensure that, firstly, the Earth's rotational speed slowed down, and secondly that the Moon moved further away from the Earth. The distance between Earth and Moon is now 384 400 km on average, and this distance continues to increase by 3.7 cm a year.

Why do we always see the same side of the Moon ?

The Moon turns once on its axis in the time it takes for it to orbit the Earth, which is why the same side always faces us. This characteristic of the Moon's orbit-known as synchronous rotation - came about as a consequence of the tidal effect of the Earth on the Moon. The Moon does not have any oceans or seas, and hence no water to ebb and flow, but the Earth's gravitational field does cause the surface of the Moon to rise and fall, and this effect has gradually slowed the Moon's rotation to a point where it now matches its orbital period. Although we always see the same craters and seas, thanks to the elliptical shape of the Moon's orbit, our angle of vision does shift slightly, which means we are able to see just over half- about 59%-of the Moon's surface from Earth. Nobody knew what the far side of the Moon looked like until space probes were able to photograph it.

Why does the Moon have phases ?

Because the Moon orbits the Earth, the angle between the Moon and the Sun changes constantly as seen from the Earth. If the Moon, as seen from the Earth, is exactly opposite the Sun, we see the half on which the Sun is shining, and we have what is termed a full Moon. If, on the other hand, the Moon is at right angles to the Sun, it is only half lit when viewed from Earth, and we have a half Moon. If the Moon is in that part of its orbit where it is in the same line of sight as the Sun, it will show us its unlit side, and we have a new Moon.

The term new Moon, as used by astronomers, is not identical with the appearance of the new Moon, which is of great importance, especially in the Islamic calendar. This is when the thin lunar crescent first appears, and it occurs between one and four days after an actual new Moon, depending on location and time of year.

What is lunar eclipse ?

At a full Moon, the Moon (as seen from Earth) is exactly opposite the Sun, with from Earth his shadow therefore falls in the direction of the Moon, which is why, when the three celestial bodies are aligned, the Moon can sometimes cross Earth's shadow. When this happens, the Moon goes dark as it is eclipsed. We speak of a total eclipse of the Moon when the Moon travels completely into the Earth's umbra- the darkest part of its shadow. An observer on the Moon would then see the Sun as being completely obscured by the Earth. However, an observer on the Moon would see the Sun only partly covered by Earth if the Moon enters the Earth's penumbra - its partial shadow. Because some sunlight still reaches the surface of the Moon when any part of it is in the Earth's penumbra, this is known as a partial eclipse of the Moon.

Why don't lunar eclipses happen every month?

The Moon's orbit is at an angle of 5.1° to the Earth's orbit. This is not much, but is sufficient to ensure that the Moon can travel up to 37 000 km above or below the centre of the Earth's shadow. A lunar eclipse is only possible when the full Moon crosses the Earth's orbit, and this only happens twice a year. Unlike solar eclipses, which can only be seen by observers watching from small portions of the Earth's surface, a lunar eclipse can be seen from anywhere on the side of Earth facing the Moon.

Why is the Moon red during a total eclipse?

Even during a total lunar eclipse the Moon does not become fully black, but instead it glows a dark and ghostly copper colour. This is because the Earth's atmosphere refracts (bends) some sunlight so that it falls into the shadow area on the Moon. For an observer on the surface of the Moon this would look as if the Earth were surrounded by a red glow. Because blue light is scattered more strongly in the Earth's atmosphere than red light-the reason we have red sunsets and sunrises - it is mainly red light that reaches the Moon.

What dangers threaten us from outer space ?

Earth is an island of life in the hostile ocean of space. The cold vacuum of space is full of dangerous radiation and high energy particles, not to mention objects of all shapes and sizes - from specks of dust to rocks, asteroids and comets - all hurtling around in every direction. It is reassuring to know that the Earth's magnetic field and its atmosphere protect us from many of these dangers.

Are exploding stars a danger for Earth ?

If a star were to explode in our immediate vicinity, the radiation released would represent a grave danger to life on Earth. However, we are fortunate, since the closest star with sufficient mass to explode as a supernova at the end of its life is Betelgeuse, which is more than 400 light years away in the constellation Orion. Given the distance, the Earth's magnetic field and its atmosphere should be able to provide us with adequate protection when such an explosion occurs.

Far more dangerous would be the explosion of a high-mass star made from primordial gas (with a low heavy element content). This is believed to lead to the creation of gamma ray bursts - bundles of intense, high energy radiation. If these were to strike Earth, the ozone layer could literally be torn away - even if the explosion itself took place many thousands of light-years distant - and for years afterwards life on Earth would be exposed to dangerous ultraviolet radiation. However, according to current scientific opinion, potentially dangerous gamma ray bursts of this kind only ever occur in young galaxies. Fortunately, at the grand old age of 10 billion years, our Milky Way is far too old for such an event.

Why do we need satellites ?

Satellites are far from an indulgent luxury exclusively for the use of the rich nations. The everyday lives of almost everyone on the planet would take a change for the worse if satellites were suddenly to disappear. They have become the foundation of modern telecommunications. Without satellites there would be no global positioning system (GPS), on which ships and aircraft now depend. Satellites provide data for weather forecasts and monitor everything from forest fires to marine pollution. They also play a vital role in science. Without telescopes in space, astronomers would have a greatly restricted view of the cosmos.

Who built the first rockets ?

Invented by the Chinese and brought to Europe by Mongolian warriors, rockets driven by black powder have been part of the arsenal of European armies for centuries. The British army, for example, fired more than 25 000 rockets when it attacked Copenhagen in 1807. Further developments in rocket technology were driven forward by three men: Konstantin Tsiolkovski in Russia, Robert H Goddard in the USA and Hermann Oberth in Germany. At the forefront of their endeavours was the search for a better fuel black powder was not powerful enough to propel a rocket into space. In 1926, Goddard built his first liquid fuel powered rocket. It only flew for 46 m, but this approach proved to be the way forward. The first ballistic rocket - the V2- was developed during the Second World War for German armed forces by Wernher von Braun and his team.

When was the first satellite sent into space?

In October 1957, the Soviet Union used a rocket to launch the first man-made satellite into an orbit around the Earth. This caused some dismay, especially in the USA where there had been a general conviction that the Americans were ahead in the race to conquer outer space. For 57 days, Sputnik! sent out its famous 'beep-beep' signal before burning up in the atmosphere. It took the Americans until January 1958 to launch their first satellite. Explorer 1 was the first real scientific satellite, and counted among its achievements the discovery of the Van Allen radiation belts - bands of energetic charged particles that girdle the Earth.

How many satellites are there now ?

More than 5800 satellites have been sent into space since 1957. About 3100 satellites currently orbit the Earth. although only about 1000 of these are still operational - the remainder should really be regarded as space junk. Nowadays, space agencies try to give satellites targeted crash landings at the end of their useful lives, but at the beginning of the space age this kind of precautionary measure had not been considered. After several years, low orbiting satellites crash of their own accord because the friction caused by the extremely thin outer atmosphere of the Earth gradually slows them down.

What is a geostationary satellite?

The speed of a satellite in a high orbit is less than it is in a low orbit, and the distance that a satellite has to travel on its path around the Earth also increases with its altitude. All of this means that the orbital period of a satellite increases with increasing altitude. At an altitude of 36 000 km, the orbital period is precisely 24 hours. For a satellite in an orbital path that takes it along directly above Earth's equator, this means that it will always be positioned in its orbit above precisely the same spot on the surface of the planet, since the Earth also completes one full revolution in 24 hours. This kind of orbit is called a geostationary orbit. Geostationary orbits have obvious advantages for telecommunications satellites, since the fact that the satellite is always in the same place means that the dishes sending and receiving signals can point at a fixed position.

Why don't satellites fall out of the sky ?

Satellites travel around Earth without extra propulsion. They remain in orbit despite the Earth's gravitational pull because they travel so extremely rapidly. If there were a satellite that orbited very close to Earth's surface, it would have to travel at 7.9 km/sec., or 22 000 km/h. The need for speed reduces with increasing altitude. At a height of 400 km - at about the same altitude as the orbit of the International Space Station - the speed has fallen to 7.7 km/sec.

Is space junk dangerous ?

A large proportion of all the junk in space was produced by the almost 200 explosions that have taken place in the Earth's orbit in the last 50 years. These include the controlled explosions of old rocket stages and the destruction of leftover fuel. Also among the debris are dead satellites set adrift, tools lost by astronauts, paint flakes and much more. American and European radar systems have registered more than 10 000 pieces of junk that are larger than 10 cm and fly at altitudes of up to 2000 km above the surface of the Earth. Nobody knows precisely how many less easily registered particles of between 1 cm and 10 cm there are, but the experts fear there may be more than 100 000 of them. All these objects flying around pose a permanent danger to satellites, manned spacecraft and space stations, because at velocities of 50 000 km/h, a 1-cm-long piece of metal is capable of punching a hole through the pressure hull of a spacecraft. In July 1996, the French spy satellite Cerise was hit by some debris and badly damaged. Nobody knows how many satellites have been lost due to collisions with space junk.

Why do we need telescopes in space ?

There are many disadvantages to observing the sky from the surface of the Earth. Turbulence in the atmosphere causes images to become blurred, which is why large, Earth-bound telescopes are not able to fully exploit their theoretical resolution. Space telescopes, such as the Hubble Space Telescope, can provide clear images because they are orbiting 569 km above the Earth and its atmosphere. The atmosphere - especially the water vapour it contains - absorbs a large part of the radiation that reaches Earth from space. It is only by means of visible light, and those radio wavelengths between a few millimetres and several metres, that astronomers can get a reasonably clear look into space from the ground. To study using other types of radiation -infrared, ultraviolet, gamma rays and X-rays - their instruments are best moved into space.

How does GPS work ?

The global positioning system(GPS) is made up of many satellites that are constantly transmitting to Earth very precise time signals produced with the aid of atomic clocks. Because the satellites are positioned at various distances from the receiver, it is possible to calculate the position of the receiver from the satellites' orbits and the time delays experienced in receiving the signals - provided the signals are received from at least four satellites. There is a problem, however. The GPS satellites travel through space at 14 000 km/h. Because of their speed, the clocks on board the satellites - as predicted by Einstein's theory of relativity - run slower than clocks on Earth. There is also a second relativistic effect caused by the Earth's gravitational field being a little weaker in orbit than it is at ground level, and this causes the clocks on board the satellites to run a little faster than those on Earth. Unfortunately, these two effects do not cancel each other out, and the navigation system has to make precise allowances for the temporal distortions caused by gravity and motion. Since the divergence amounts to several kilometres a day, these corrections are essential if GPS is to get us to where we want to go.

Why bother sending people into space ?

This question lies at the centre of a sometimes heated debate - with supporters and opponents of manned space travel both making convincing cases for their respective points of view. Unmanned space probes are much cheaper than manned spacecraft because they do not require life support systems. Also, exploration of the solar system would not have been possible without robot space probes. However, there are many areas where humans are superior to machines, however clever the machines seem to be.

Who was the first human in space ?

Four years after the 'Sputnik shock' of 1957, the USA had to accept another humiliating defeat in the space race. In April 1961, the Soviet Union sent the first human being into space aboard the Vostock 1 rocket. The cosmonaut Yuri Gagarin and his space capsule circumnavigated Earth only once, and after a flight lasting 1 hour and 48 minutes he landed safely back on Earth in southern Russia. The flight was entirely remote controlled and Gagarin would only have needed to intervene in the event of an emergency. It wasn't until February 1962, after two successful sub-orbital flights, that the USA succeeded in sending their own astronaut John Glenn into orbit.

Why are astronauts in space weightless ?

Weightlessness does not mean that there is no gravity, only that the gravity is not perceptible. In fact, the gravity in an orbit close to Earth is not much less than it is on Earth's surface. If we were able to construct a tower tall enough to reach that height, a person standing on the top of the tower would not be weightless. We feel our weight because the ground pushes up on us, balancing the force of gravity. However, there is no ground for an astronaut floating in space, and so he or she does not feel the force. This is also true if the astronaut is in a satellite, since this does not provide any firm ground either as it orbits the Earth with the astronaut. Gravity is nevertheless present, and keeps the spaceship and its astronaut in Earth's orbit, just as it does the Moon.

How many people have been to the Moon ?

The US Apollo space project sent a total of only 12 people to the Moon in the years between 1969 and 1972. First to reach the Moon, on 20 July 1969, were the Apollo 11 astronauts, Neil Armstrong and Edwin Aldrin. On 14 December 1972, the commander of the Apollo 17 mission, Eugene Cernan, became the last human being to have stood on the lunar surface. Although the Americans sent nine manned flights to the Moon, only six landed. Apollo 8 was the first manned spacecraft to circumnavigate the Moon. During the flight of Apollo 10, tests were carried out on the lunar module while in orbit around the Moon, but the astronauts did not land. Apollo 13 was severely damaged on its outward flight to the Moon and as a result the astronauts had to abandon their plans for a landing as they struggled to survive. After Apollo 17, the lunar mission program was cancelled for financial reasons, although the original plan had been for another three manned landings.

What do we need a space station for ?

The conditions for performing extended experiments in weight lessness under constant human supervision exist only in a space station. It is possible to carry out some experiments on board unmanned satellites, but having astronauts work on processes allows much more flexible and hence more useful experiments. In the long term, some scientists hope this will lead to the manufacture of new materials and medicines in space, although the sceptics believe that production in space would be too expensive and so of little interest to industry. However, space stations are indispensable when it comes to preparing for flights to other planets. This is because the effects on the human body of long periods in space can only be researched by actually having people spend extended periods in space - and the only practicable place for that is aboard a space station.

What can astronauts do to protect themselves from radiation in space ?

Protecting astronauts from cosmic radiation during long space missions a trip to Mars for example - would require a shield of matter with a density of around 1 kg/sq cm, which would involve something like a 10-m-thick layer of water. The enormous weight of such a shield would vastly increase the costs of a mission to Mars, which is why space travel experts are searching for alternatives. It may be possible to produce an extremely strong magnetic field to act as a shield, but the strengths required would call for the installation of superconducting electromagnets aboard spacecraft.

How long would it takes to travel to Mars 

With the rockets currently in use, a flight to our neigh bouring planet would take around six to seven months - provided the launch took place when Mars and the Earth were suitably aligned. This latter factor is a major problem facing a manned trip to Mars. The astronauts would have to wait on Mars for anything up to 18 months before Earth would again be in a suitable position for the return flight. This means that a manned Mars mission would require a total travel time of at least two and a half years. The journey could be completed much faster with something like a powerful nuclear engine. This would not only cut travel time to Mars to just two months, but it would also reduce the need to take into consideration the relative positions of the two planets. However, a nuclear engine that would enable this kind of extremely rapid trip has to date only made it as far as the engineer's drawing board.

What is the future for space travel ?

Forecasts are difficult to make. In the 1960s, after the Apollo Moon flights, plans for a permanent, manned Moon base were already being made, and flights to the entire solar system seemed to be within the reach of astronauts. But then the Apollo program was cancelled, and for many decades manned space travel was limited to flights into Earth's orbit. In the 1970s, many believed that the space shuttle ushered in the era of reusable spacecraft. However, the shuttles have proved to be too complicated and too dangerous, and NASA is now returning to an approach that makes use of expendible rockets. Nevertheless, if current plans are carried through, astronauts will return to the Moon by the end of the next decade, and if all goes well, the first manned flights to Mars may follow in about 20 years.

Why isn't it possible to fly faster than the speed of light?

The speed of light in a vacuum (299 792 km/sec.) is an absolute limit imposed by the laws of physics. Imagine a spacecraft travelling through space at half the speed of light. If an astronaut aboard this craft shone a beam of light from the front of the ship, would a stationary outside observer see the beam spreading out from the spacecraft at one and a half times the speed of light? Surprisingly, the answer is no. Whether measured from inside or outside the spacecraft, the speed of light remains the same. This apparent contradiction can only be resolved with the help of Einstein's theory of relativity, according to which time appears to pass more slowly and distances are shortened at high speeds. Since time and space aboard the rapidly moving spacecraft appear relativistically distorted, a measurement of the speed of light made by the astronaut will give the same result as one made by the stationary outside observer. Another consequence of the theory of relativity is that the mass of an object appears to increase with its velocity, so by the time a craft reached the speed of light its mass would be infinitely large.

How long would it take to journey to the stars?

Today's technology may allow speeds of perhaps one-hundredth that of light, which means that a journey to the nearest stars would take several centuries. But if it ever becomes possible to fly at velo cities approaching the speed of light, then journey times could be greatly shortened. For an astronaut aboard a spacecraft travelling at 99.9% of the speed of light, only 4.5 years would pass on a journey to a star 100 light-years away. For everyone else left at home on Earth, however, 100 years would pass and the astronaut would return to find a world quite different to the one he or she left 200 years earlier.

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The life and death of stars 

Although the stars seem constant to us, there is in fact a relentless cycle of life and death going on in space. Astronomers can identify regions where new stars are being born as well as the remains of stars that have exploded. The stars appear unchanged to us only because they take an almost inconceivably long time-millions and billions of years - to develop. Spectacular and rapid events like a supernova - a star exploding - are very rare. The course of a star's life and its eventual death depend on the amount of mass it accrued during its formation.

How are the stars and planets formed?

The life of a star begins with a gigantic cloud of cool hydrogen gas with an initial temperature no more than a few degrees above absolute zero (-273.15° C). The cloud gradually collapses under its own gravity and clumps of material begin to form inside it, causing it to collapse at ever increasing speeds, while the temperature of the gas rises steadily. The cloud eventually starts to glow, giving off thermal radiation which can be detected by infrared telescopes. The internal density and temperature ultimately increase to a point where nuclear fusion takes place, in which hydrogen is converted into helium, and so a new star is born. Typically, many stars of different sizes emerge from one large gas cloud. Sometimes, star clusters made up of thousands or even millions of stars are created all at once. A rapidly rotating cloud of gas and dust (the remnants of the original cloud) floats around many of the young stars, and inside this cloud there are further clumps of matter which have the potential to develop into planets and other, smaller celestial bodies. It is thought that planets may be formed around most stars.

How old are the oldest stars?

The first stars in the cosmos were formed 400 million years after the Big Bang. Some stars are very frugal in using their supply of fuel, which means that even today we are still able to see stars belonging to the very first generations. This is especially the case with globular clusters, where we find stars that are more than 13 billion years old, although new stars are being formed all the time. One of the best-known starbirth regions is the Great Orion Nebula, which is 1600 light-years away and visible with the naked eye as part of the constellation Orion. The Hubble and Spitzer telescopes have revealed that many young stars in the Orion region have discs of matter rotating around them in which planets may be forming,

What determines how long stars live and how they die?

The duration of a star's life and the manner of its death depend on its mass. This is because large, high-mass stars use up their energy reserves more rapidly than smaller, low-mass stars. This is the reason why large stars shine brightly for no more than a few million years, while smaller stars can live for billions of years. Stars produce their energy as a result of a process known as nuclear fusion - during which hydrogen is converted into helium. However, the time inevitably comes when all the hydrogen at the core of a star has been used up.

What are red giants and white dwarfs? 

Once a star's hydrogen reserves are used up, the star enters the final phase of its life. It swells up to become what is known as a red giant. Our own Sun will one day turn into a red giant, and in so doing may even devour the Earth, along with several more of the inner planets of the solar system. Further fusion processes occur in the core of the enlarged star over millions of years, during which, for example, helium is converted into carbon and oxygen. When all of a star's stock of potential nuclear fuel has been exhausted, it collapses and turns into a white dwarf. At this stage the star is only about the same size as Earth, and it takes billions more years for it to cool down.

What is a supernova? 

When a star has a mass of more than eight times that of our Sun its end is considerably more dramatic than that of most regular stars. In its final stages, the dying star hurls its outer layers into space with a mighty explosion which is known as a supernova. At the same time, the star's interior collapses to form a small, extremely dense body, which can takes the form either of a neutron star or a black hole.

What is a neutron star?

The matter in a neutron star is as densely packed as the matter in an atomic nucleus. It is so tightly packed, that a tiny piece of a neutron star the size of a pinhead would weigh more than one million tonnes. The immense pressure in the interior of this kind of object pushes the electrons into the atomic nuclei's protons - leaving nothing but neutrons behind. Neutron stars have a diameter of only about 20 km but contain about the same mass as our Sun.

Can we see black holes?

If the mass of a star's collapsing interior is more than about three times that of our Sun, its gravity becomes so strong that even the neutrons cannot halt the process. Inexorably, the star continues to collapse until it turns into a black hole - the name given to a region of space where gravity is so powerful that nothing, not even light, is able to escape. This is why these gravity holes are 'black'. However, we are still able to detect black holes because their colossal gravity draws in surrounding matter which, as it descends into the black hole, becomes very hot and begins to glow - thereby revealing the black hole's position. Black holes don't only occur when a star dies. There are also monster black holes at the centres of galaxies which have a mass of millions if not billions of times that of our Sun. These are thought to have originated in the early stages of the universe, and there is even one of these super-massive black holes in the centre of our Milky Way.

The planets - a journey through the solar system

The eight planets in the solar system differ a great deal from one another. There are the inner planets Mercury and Venus, closest to the Sun; the gas giants Jupiter, Saturn, Uranus and Neptune; mysterious Mars; and the only planet that certainly harbours life, our Earth.

Do all planets have moons?

There are only two planets in our solar system that do not have a moon, and these are Mercury and Venus. The number of moons each of the other planets has varies. While Earth only has only one Moon, Jupiter is orbited by at least 63 satellites. Astronomers keep discovering more, so the actual number may be even larger.

Do other planets have atmospheres?

Mercury is the only planet in our solar system that doesn't have an atmosphere. However, the atmospheres of other planets are quite different from Earth's. On Mars, for example, the atmosphere is very thin and made up chiefly of carbon dioxide, while on Venus the surface atmospheric pressure is about 90 times that on Earth.

Does water exist on other planets?

So far, scientists have found liquid water only on Earth. Some suspect that there may be frozen water in the perpetually shaded regions around Mercury's poles. Frozen water is also present near the poles on Mars, and it is highly probable that some deposits of water are hidden under that red planet's surface.

How tall is the tallest mountain in the solar system?

At more than 24 km high and 600 km wide, the extinct volcano Olympus Mons on Mars holds the record. Unlike Earth, Mars lacks plate tectonics, so sections of its crust do not move and volcanic hot spots stay in the same position. It is because they have remained in one place that Martian volcanoes have risen to great heights.

Does vulcanism occur on other planets?

Astronomers have discovered old volcanoes on Venus, as well as on Mars. So far, no active vulcanism has been observed on any other planet, although it has been detected in Jupiter's moon lo. However, atmospheric gases on Venus indicate that there may be active volcanoes on that planet as well.

Why are the solar system's outer planets so different from the inner planets?

Planets are formed within rotating discs of gas and dust that surround young stars. With Sun-like stars, rocky Earth-like planets form in the hot, inner area of the disc. Gas giants and ice planets, on the other hand, are formed further out, in cooler parts of the disc.

Why are Jupiter and Saturn so large?

In the inner solar system, the young Sun's radiation blew away any leftover gas at an early stage, but it was preserved for a longer period in the outer areas. This is why those planets that formed in the outer regions of the solar system had more time to attract the gas present in their surroundings, and so grew for much longer.

What is the Great Red Spot on Jupiter? 

For more than 300 years, a reddish oval spot has been observed in Jupiter's cloud-streaked atmosphere. This is the Great Red Spot, a huge cyclonic storm which is about twice the size of the Earth.

Is there an ocean on Jupiter's moon Europa?

Magnetic field measurements indicate that an ocean about 100 km deep is concealed under a 1-km-thick layer of ice on Jupiter's moon Europa. It may be that the giant planet's gravity heats the inside of the moon by 'squeezing' it thoroughly, maintaining any water in a liquid form. The American space agency NASA is currently working on plans to send a space probe to this moon in an attempt to find out exactly what is happening there.

What are the rings around Saturn made of?

Saturn's rings are made up predominantly of ice particles, but they also contain dust and rocks. The size of the particles in the rings ranges from less than 1 mm to several metres. Some astronomers think that Saturn's rings were formed as a result of a collision between one of the planet's moons and a wandering asteroid.

Do other planets have rings as well?

The planets Jupiter, Uranus and Neptune have also been found to have rings. However, these are not as prominent as the rings around Saturn, and they consist chiefly of dust and ice particles from the various planets' moons.

How was Neptune discovered?

The observed orbit of Uranus was found to deviate slightly from the path predicted by calculations. The reason had to be a more distant planet, and it was possible to calculate the position of this eighth planet on the basis of the orbital disturbance. The new planet was first observed in 1846, and given the name Neptune.

Are there any undiscovered planets in our solar system?

In addition to Pluto, there are many more dwarf planets beyond Neptune's orbit - and more are being discovered. However, it is unlikely that another large planet will be discovered further out in the solar system because astronomers have not observed any more inexplicable disturbances in the orbits of known planets and comets.

Where does the solar system end?

The solar wind meets the gas between the stars at a distance of about 150 times that between Earth and the Sun. This region is often called the boundary of our solar system. However, the Sun's gravitational influence extends even deeper into space. It is assumed that our solar system is surrounded by a cloud of comets which stretches up to a light-year into space.

Missiles from outer space

The scene is Siberia; the date 30 June T1908. Suddenly a massive explosion rips apart the silence hanging over plains near the Stony Tunguska River, and a huge pillar of smoke and fire rises to the sky. The thunder of the explosion is heard hundreds of kilometres away, and seismic tremors are registered all around the globe. Trees across an area of more than 2000 sq km are snapped like matchsticks and beneath the centre of the explosion a forest fire rages.

This real catastrophic event was probably caused by a 30-m-wide asteroid exploding 10 km above the Earth's surface. Fortunately, the affected region was almost uninhabited so, although several reindeer herds fell victim to the explosion, only two people died. Had the asteroid exploded above a city it would have been a tragedy of major proportions. The force of the blast was comparable to 10-20 megatons of TNT - the equivalent to 50-100 times the explosive power of the Hiroshima atom bomb.

Astronomers have calculated that an event like the one at Tunguska involving an asteroid is likely to happen somewhere on Earth every couple of hundred years. This is because of the innumerable asteroids and comets that are travelling through our solar system, one of which could at some point crash into the Earth.

Are comets dangerous?

In medieval times, the appearance of a large comet in the sky caused fear and alarm because they were considered to be the harbingers of plagues and wars. Even as recently as 1910, when Earth crossed the tail of Halley's Comet, many people feared that the world was about to end because astronomers had detected substances like sulphur and cyanide in the comet's tail. Today we know that the matter in a comet's tail is much too thinly distributed to pose any kind of danger to us. A comet is only dangerous if it crosses Earth's path. In 1994, the comet Shoemaker-Levy crashed into Jupiter, a vivid demonstration that such catastrophes are not events restricted to the past, but that they could happen at any time. It is extremely unlikely that humankind would have survived if Shoemaker-Levy had collided with the Earth rather than Jupiter.

What happens when an asteroid or comet hits Earth?

Boulders from space as small as 1 m across can cause explosions that are comparable to the blast that resulted when the atom bomb was dropped on Hiroshima. On average, events of this kind are registered once a year by reconnaissance satellites circling the Earth. Fortunately for us, the shockwaves do not reach the Earth's surface because the explosions occur in the uppermost layers of the atmosphere. Although many small fragments of rock from space strike the Earth as meteorites, an asteroid needs to be at least 50 m across in order to have a chance of reaching the ground or even the lower levels of the atmosphere. The consequences of an event such as that taking place above a populated area would be devastating, but an asteroid landing in the sea close to a coast, would also have dramatic consequences as the huge waves it would generate washed ashore. However, a global catastrophe which affected the whole of humanity would require an asteroid with a diameter in excess of 1 km. The enormous amount of dust such a strike would inject into the upper layers of the atmosphere would be particularly destructive. Such an event would lead to a global winter that would last for several months, resulting in the worldwide collapse of agriculture and other food production.

Did an asteroid cause the extinction of the dinosaurs?

There have been several mass extinctions in Earth's history. The best known occurred at the transition between the Cretaceous and the Tertiary periods, 65 million years ago, when around one-third of the planet's animal and plant species, including the dinosaurs, disappeared. Many experts agree that this event was caused, or at least triggered, by the impact of an asteroid or comet. An impact crater up to 300 km across, formed at the time in question, has been found on Mexico's Yucatan Peninsula.

Can humans protect themselves from asteroids and comets?

Experts disagree about whether it would be possible to defend the Earth from rogue comets and asteroids. Unlike weapons experts, who see this as a new business opportunity, astronomers remain unimpressed and maintain that protective measures may not always be necessary. Depending on the size of the body involved, an impact in a large ocean, far from a coast - or in an unpopulated land area - would probably not pose a major threat to life. If calculations predicted an impact by an Earth-bound asteriod in a heavily populated area, scientists believe there would still be plenty of time to prepare a defence. They expect to have warning of an approaching asteroid several decades in advance of an impact and there are, in fact, already programs in place to record all potentially dangerous asteroids in order to make long term forecasts of their trajectories. Comets are more of a problem, however, because their emergence from the outer reaches of the solar system cannot be predicted. Their orbits are also hard to calculate because surface eruption of gas can change their course.

What sort of protective measures could be taken if a cosmic boulder were actually to threaten Earth? Blowing it up, as in the 1998 movie Armageddon, would not be possible with the means at our disposal today. Nor would it be advisable, since thousands of fragments crashing into the Earth would be even more destructive than an intact asteroid. However, given sufficient advance warning, a small course correction would be enough to divert an asteroid and make it miss our planet. This might be achieved by attaching a solar sail or a rocket engine to the surface of the oncoming threat.