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Who needs pyramids and hanging gardens when you can have quasars and supernovae. When it comes to wonders, the Universe holds the aces.

Spanning 30 billion light years, the Universe is big. But that's not all it's got going for it. Long before the seven wonders of the ancient world were built, the cosmos forged its own set of crowning marvels, which will still be here long after the ephemeral efforts of mankind.

Born in the Big Bang's searing fireball 15 billion years ago, the juvenile Universe expanded and cooled. The first objects to condense from the hot smog were unimaginably bright galaxies called quasars. These evolved and cooled to become the galaxies, 50 billion of which, each containing hundreds of billions of stars, fill the modern day Universe.

Galaxies are brimming with celestial exotica - mysterious back holes and super dense stellar corpses known as white dwarfs and neutron stars. Galaxies also play host to the most violent explosions of the modern day Universe - supernovae and gamma-ray bursts, re-enacted by the Universe as if in homage to its fiery history.

Virtually every stage of the Universe's 15 billion year history can be observed given a powerful enough telescope. This is because of the time it takes for light to travel from a distant galaxy. For example, you can see the Universe as it was ten billion years ago simply by looking out to a distance of ten billion light years. The look-back time effect offers an invaluable resource for astronomers trying to piece together the Universes evolution from primaeval fireball into the structured collection of planets, stars and clusters of galaxies we see today. Increasingly powerful telescopes both on Earth and in orbit, such as the Hubble Space Telescope, are helping by letting astronomers peer further out into space, and so further back towards the cosmic history books elusive opening pages.

The Big Bang

Our cosmos emerged from a superheated cloud of subatomic particles 15 billion years ago.

Cosmic Background Radiation map from COBE

Cosmic Background Radiation map from COBE

NASA

The birth of the Universe was marked by the biggest explosion in cosmic history. Before the Big Bang, there was nothing, absolutely nothing. No space, no time. That all changed about 15 billion years ago, when something happened that made the three dimensions of space and one of time that we see today sprang into existence, while also giving rise to all the matter in the universe.

It might sound like a con. If the Universe emerged from nothing, then where did all this matter come from. Actually, it's completely above board. In addition to its matter, which has positive energy, the Universe also has gravity, which has negative energy. Einsteins famous E=mc2 equation, which basically says that matter and energy are the same things, reveals that the gravitational energy exactly balances the matter-energy, making its total net energy zero. So the Universe really is made from nothing.

Flaring into existence, the primordial soup rapidly expanded and cooled. At just over three minutes old, the Universe had cooled to a relatively chilly billion degrees Centigrade and the first atomic nuclei, hydrogen and helium, formed. Three hundred thousand years later - still a blink of an eye on cosmic scales - the Universe became cool enough to allow electrons to latch on to these nuclei, making the first atoms. These then coalesced into today's galaxies, stars, planets and later, lifeforms.

Physicists and astronomers aren't sure exactly what triggered the Big Bang. But recently researchers at the University of Princeton have suggested that the Universe may have created itself. They say that wormholes - tunnels through space and time - in our universe can lead to new baby universes. The team also argued that because there are no existing laws of physics to prevent time travel that there is nothing to stop one of these new universes from being our own.

The Death of a Star

When a star reaches the end of its life, the result is a spectacular explosion called a supernova.

When it comes to stars, the bigger they are the harder they fall. Stars are massive spheres of gas, which live out their lives burning hydrogen into helium by nuclear reactions in their cores. But when the nuclear fuel eventually dries up, its time for catastrophe.

A very violent fate indeed is in store for stars heavier than about eight times the mass of the Sun. These stars ultimately fo supernova, exploding so intensely that the light from the blast temporarily outshines the combined light from over 100 billion normal stars.

Multiwavelength X-ray, infrared, and optical compilation image of Kepler's Supernova Remnant, SN 1604.

Multiwavelength X-ray, infrared, and optical compilation image of Kepler's Supernova Remnant, SN 1604.

NASA

Supernovae leave behind a remnant object - either a neutron star, a pulsar or a black hole. The Crab Nebula in the constellation of Taurus is the debris from a supernova which was recorded in 1054 by Chinese astronomer. In 1987, astronomers saw a supernova explode in the Large Magellanic Cloud, a small galaxy orbiting our own Milky Way. it was the first supernova visible to the naked eye since 1604. Now, with the aid of powerful telescopes, astronomers, astronomers are seeing supernovae in galaxies as far away as nine billion light years, which went off when the Universe was less than half its present age.

Lighter stars do not go supernovae. Instead, they inflate to become cool bloated stars known as red giants, hundreds of times the size of the present-day Sun. When the Sun becomes a red giant, it will swell up sufficiently to engulf Mercury. After about a billion years the star flings off its outer layers, blasting away the remains of any surrounding planets into a diffuse cloud of gas and dust called a planetary nebula.

Planetary nebulae appear as large circular disks which led astronomers to initially confuse them with planets, hence the name. Lying at the centre of a planetary nebula is a white dwarf star.

Black Holes

Crank up a gravitational field high enough and the result is a gap in space where the known laws of physics break down - a black hole.

Black holes are probably the most audacious phenomena in the Universe. If a star becomes sufficiently small or heavy that its gravitational attraction is strong enough to prevent even light from escaping from its surface, the resulting object is called a black hole. The idea is an old one. In 1783 the Reverend John Michell predicted the existence of what he called 'dark stars' using Newtons theory of gravity. But black holes weren't taken seriously until Einstein's General Theory of Relativity, a new far-reaching theory of gravity was published in 1915. The actual term 'black-hole' wasn't coined until 1967, by Princeton physicist John Wheeler. The first real black hole, Cygnus X-1, was detected in the early 1970s. Because black holes are black, you can't see them directly. But if a black hole orbit around another star, its strong gravity will pull material from the star and heat it to such high temperatures that it emits X-rays, which can be detected. Although this was just what astronomers saw in Cygnus X-1, whether the object was a black hole still wasn't clear. So much so, in 1974 Stephen Hawking bet Kip Throne, of Caltech, that the source of the X-rays was not a black hole. In 1990, however, Hawking conceded defeat and handed over his wager - a years subscription to Penthouse.

A supermassive black hole is is so dense that within a certain radius, its gravitational field does not let anything escape from it, not even light.

A supermassive black hole is is so dense that within a certain radius, its gravitational field does not let anything escape from it, not even light.

Sky & Telescope Magazine

Astronomers now believe that massive black holes - some weighing up to three billion times the mass of the Sun - lie at the centres of many galaxies, including our own Milky Way. Smaller black holes, such as Cygnus X-1 form when very heavy stars die in supernova explosions.

Great Spirals

Spiral galaxies are the cover girls of the cosmos. One hundred thousand light-years across and only a thousand light years thick, they contain hundreds of billions of stars. Our own Milky Way galaxy is a spiral, in which the Sun lies about two-thirds of the way out from the centre. Just as the planets orbit the Solar System, the Sun orbits around our galaxy once every 225 million years.

Spiral arms are vast shock waves which sweep around the galaxy, roughly once every two billion years in the case of the Milky Way. The waves squash clouds of gas and dust lying in the place of the galaxy disk, causing them to fragment and collapse into new stars. The light from each new generation of stars illuminates the waves, forming the galaxies attractive spiral pattern. Because the spiral arms revolve much slower than the stars in the galaxies central disk, every hundred million years or so the Sun passes through one. Some scientists have suggested that these passages could explain some of the mass extinctions of life on the Earth that punctuate the fossil record.

NGC1300 Barred Spiral Galaxy

NGC1300 Barred Spiral Galaxy

NASA/ESA

The bright regions of spiral galaxies are only the tip of the iceberg. Studies of their motions show that the parts we can see make up only about 10 per cent of their total mass - spirals are embedded in a vast cloud of unseen, or dark, matter. Detecting this dark matter directly is one of the ongoing goals of experimental physicists.

Spirals aren't the only galaxies in the Universe. Roughly 60 per cent are elliptical galaxies - oval-shaped, spiral-less galaxies, weighing anything up to a thousand million times the mass of the Sun.

Gamma-Ray Bursts

The biggest bangs since the Big Bang, gamma-ray bursts (GRB's) pack up to 100 times the blast energy of a supernova and come from galaxies at the edge of the Universe 12 billion light years away.

Cycle of pulsed gamma rays from the Vela pulsar.

Cycle of pulsed gamma rays from the Vela pulsar.

Goddard Space Flight Center

A single ray of gamma radiation carries 100 million times the energy of a ray of visible light. A GRB emits billions upon billions of gamma rays every second, and roughly one GRB goes off every day.

Despite their high energies, gamma rays can't penetrate the atmosphere of our planet because of its chemical makeup. Consequently, GRBs weren't detected until mankind ventured into space.

The first burst was seen by a group of satellites collectively called Vela, designed as watchdogs for the tell-tale gamma rays from clandestine nuclear weapons tests. In 1973, after analysing nearly ten years of data from the satellites, a team of scientists at Los Alamos National Laboratory found that most of the bursts detected hadn't come from Earth but came instead from outer space. The race was then on to find out what they really were.

Over twenty-five years on, researchers are still working on the problem. One leading idea is that colliding neutron stars - two neutron stars orbit around one another, slowly spiralling together until they eventually merge, releasing huge amounts of energy which we see as a GRB. To date, scientists have suggested over 200 theories to explain how GRBs are generated, even including exhaust from alien warp drive engines.

GRB have also been proposed as one possible cause of mass extinctions, such as the death of the dinosaurs. If one went off within 3000 light years of Earth it would bombard the planet's surface with ten times the lethal dose of muon articles. On a brighter note, this barrage would also turn much of the Earths atmosphere into nitrous oxide, laughing gas, and we'd literally die laughing.

Quasars

Size doesn't matter. The brightest objects in the Universe, quasars emit the quasars emit the equivalent output of 1000 normal galaxies; from a region just one millionth the size of our own Milky Way. Laying billions of light years from the Earth, they were the first objects to form in the Universe after the Big Bang.

in 1960, Jodrell Bank radio observatory detected a tiny distant radio source, named simply 3C48. Later astronomers using the five-metre optical telescope at Mount Palomar looked at 3C48, expecting it to be a galaxy. To their surprise they found instead that it looked more like a star, earning it the name quasi-stellar object, later contracted to jus quasar. Over the following years, another half dozen quasars were detected.

An artist&squo;s impression of a growing quasar.

An artist&squo;s impression of a growing quasar.

NASA

In 1963, Dutch astronomer Maarten Schmidt discovered that quasars were situated billions of light years from Earth, and to have observed their brightness at such distances, they must be the brightest objects in the Universe. Other astronomers discovered that the brightness of some quasars changes over timescales of about a day. To do this in such a coherent way, the central power source must be no more than a light day across - tiny compared to a galaxy.

Figuring out how so much power could come from a region so small was a problem. Such a large energy demand would require a million billion solar masses of conventional chemical fuel or a billion solar masses of more efficient nuclear fuel. Astronomers eventually discovered that quasars are instead powered by gravity. If you drop something in a gravitational field it accelerates. A solar mass of material per year falling onto a compact 100-million solar mass black hole generates enough energy in this way to explain the quasar power source perfectly.

Many galaxies seen today, the Milky Way included, have black holes at their centres and are thought to be 'dead' quasars.

Superdense Stars

When stars end their lives they leave behind a remnant object, often so dense that just a teaspoon full of its constituent material weighs as much as a mountain.

White dwarfs squeeze a solar mass of material into a sphere the size of the Earth. Formed during the death of Sun-like stars, they are squashed to around a million times the density of water. They can be found at the centre of a planetary nebula.

Supermassive Stars are large enough to reach Saturn

Supermassive Stars are large enough to reach Saturn

Neutron stars, formed in supernovae are even denser. Made entirely of neutrons they also weigh about a solar mass, but measure just 10 kilometres across Neutron stars have been described as giant atomic nuclei because their density is comparable with the density of the nucleus of an atom, a staggering million billion times that of water. This makes their gravitational field enormous - 100 thousand million times Earth surface gravity, enough to smash a person into a thin film. Neutron stars form in some supernova explosions.

Spinning neutron stars emit beams of light from their poles which swing around with the star as it rotates. if the Earth lies in the plane of the beams we see the star, called a pulsar, as a cosmic lighthouse, rapidly flicking on and off. In 1967, a pulsar became the first neutron star ever to be detected. Astronomers were slow to make the connection though and initially suggested that the signal was a message from extraterrestrials.

Over the years, neutron stars have fuelled further speculation. Physicist Frane Tipler, of Tulane University, New Orleans, has suggested that a time machine could be made from a stack of ten spinning neutron stars. And Freeman Dyson, of the Institute for Advanced Study, Princeton and Frank Drake, president of the SETI association, even proposed that superdense life based on neutrons could exist on a neutron stars surface.

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