The Guide to the Universe

Scan the QR codes where you see them to watch animations Consider the stars. They may all look the same in the night sky, but looks can be deceiving. They, in fact, come in many different varieties, ranging from swollen, gargantuan red supergiants – hundreds of times larger than the Sun – to puny neutron stars, as massive as the Sun and yet, incredibly, no larger than London. How many different stars are out there, what do they look like, how do they compare, how did they come to be? The Guide to the Universe has the answers. It had been designed so that you can flick through its five sections and find descriptions – alongside full-page artworks and, occasionally, animations – of all the various kinds of astronomical phenomena…

MOST PEOPLE have at least some idea of what a planet is. After all, we live on one: Earth. And if you have ever glanced up at the sky on a clear night, the chances are you will have seen other planets without necessarily knowing it. Venus and Jupiter are particularly dazzling, but to the untrained, unaided eye, they can easily be mistaken for bright stars. Up close, though, planets and stars are totally different objects. Stars (such as the Sun) shine by their own light. But planets emit no light whatsoever and give off very little heat. We’re only able to see them by the light they reflect from the Sun. Planets vary enormously from one specimen to another. By virtue of it being our home, we are all…

THE AGE of the dinosaurs ended 65 million years ago. That’s a time already far beyond human experience, but Earth, the Sun and everything in the Solar System is 70 times older still. At 4,550 million years of age, the Solar System is unimaginably ancient. Before that time, there was no Sun and no planets. There were only raw materials, in the form of a vast nebula of gas and dust called a giant molecular cloud (see page 80). This molecular cloud was knotty, with some regions being denser than others. In these denser regions, gravity pulled inwards and the knots got tighter still. And after a couple of million years, so-called globules developed, roughly spherical shells of gas and dust with a dense central core. Nearly five billion years…

Scan to see an animation of Saturn and its moons IN THE Solar Nebula, billions of years ago, there was a very strong temperature gradient. Close to the newly forming Sun, where it was hottest, icy substances were unable to condense – to turn from a gas into a liquid or solid – while substances such as iron and rock were readily able to do so. At a certain distance from the Sun, however, at the so-called snow line, the temperature dropped to a point where ices could condense. Icy particles are much more adhesive than the rocky and metallic ones that condensed closer to the Sun. And so, at the snow line, roughly five times further from the Sun than the present orbit of Earth, the planetesimals grew to…

Scan to see an animation of Neptune and Triton THE NEXT planets to appear were Uranus and Neptune, at still greater distances from the Sun than the gas giants. It’s worth pausing for a moment to put these great expanses into perspective. Astronomers use a metric called the astronomical unit (or AU) to measure distances in the Solar System. One AU is the distance between the Sun and Earth, or 150 million kilometres. Today, Jupiter orbits the Sun at an average distance of 5.2 AU. (In fact, the orbit is not a circle but rather elliptical, as are the orbits of all the planets. But most planetary orbits are very nearly circular.) Saturn, by contrast, is found at 9.2 AU. Uranus is much further out, at 19.2 AU, and Neptune…

THE FINAL planets to emerge from the chaos of the Solar Nebula were the smaller ones, of which Earth is the largest example. These are known as terrestrial or telluric planets – from the Latin words for Earth, tellus, terrestris or terra. The other terrestrial planets, three in all, are nothing like Earth. The term is used merely to describe planets with solid surfaces made of rocky and metallic materials. The growth of the terrestrial worlds – Mercury, Venus, Earth and Mars – was slow for a couple of reasons. First, the materials from which these worlds formed – rocks and metals – were much less abundant than the gases and ices that formed the outermost giant planets. Second, fragments of colliding rock or metal are much more likely to…