Wonders of the Universe - Brian Cox [7]
The Universe is an awe-inspiring place, full of wonder and demanding the answers to so many questions. We have so much to learn and so many places to explore.
OUR PLACE IN THE UNIVERSE
The scale of the Universe is almost impossible to comprehend and yet that’s exactly what we’ve been able to do from the vantage point of the small rock we call Earth. As we have discovered the grand cycles that play out above our heads we have come to realise that we are part of a structure that extends way beyond our solar system and the 200 billion stars that make up our galaxy.
Nathalie Lees © HarperCollins
OUR GALACTIC NEIGHBOURHOOD
From our small rock, we have a grandstand seat to explore our local galactic neighbourhood. Our nearest star, the Sun, is 150 million kilometres (93 million miles) away, but each night when this star disappears from view, thousands more fill the night sky. In the most privileged places on Earth, up to 10,000 stars can be seen with the naked eye, and all of them are part of the galaxy we call home.
A galaxy is a massive collection of stars, gas and dust bound together by gravity. It is a place where stars live and die, where the life cycles of our universe are played out on a gargantuan scale. We think there are around 100 billion galaxies in the observable universe, each containing many millions of stars. The smallest galaxies, known as dwarf galaxies, have as few as ten million stars. The biggest, the giants, have been estimated to contain in the region of 100 trillion. It is now widely accepted that galaxies also contain much more than just the matter we can see using our telescopes. They are thought to have giant halos of dark matter, a new form of matter unlike anything we have discovered on Earth and which interacts only weakly with normal matter. Despite this, its gravitational effect dominates the behaviour of galaxies today and most likely dominated the formation of the galaxies in the early Universe. This is because we now think that around 95 per cent of the mass of galaxies such as our own Milky Way is made up of dark matter. In some sense this makes the luminous stars, planets, gas and dust an after-thought, although because it is highly unlikely that dark matter can form into complex and beautiful structures like stars, planets and people, one might legitimately claim that it’s rather less interesting. The search for the nature of dark matter is one of the great challenges for twenty-first-century physics. We shall return to the fascinating subject of dark matter later in the book.
The word ‘galaxy’ comes from the Greek word galaxias, meaning milky circle. It was first used to describe the galaxy that dominates our night skies, even though the Greeks could have had no concept of its true scale. Watching the core of our galaxy rise in the night sky is one of nature’s greatest spectacles, although regrettably the light of our cities has robbed us of this majestic nightly display. For many people it looks like the rising of storm clouds on the horizon, but as the Earth turns nightly towards the centre of our galaxy, the hazy band of light reveals itself as clouds of stars – billions of them stretching thousands of light years inwards towards the galactic centre. In Greek mythology this ethereal light was described as the spilt milk from the breast of Zeus’s wife, Hera, creating a faint band across the night sky. This story is the origin of the modern name for our galaxy – the Milky Way. The name entered the English language not from a scientist, but from the pen of the Medieval poet, Geoffrey Chaucer: ‘See yonder, lo, the Galaxyë, Which men clepeth the Milky Wey, For hit is whyt.’
M87, also known as Virgo A and Messier 87, is a giant elliptical galaxy located 54 million light years away from Earth in the Virgo Cluster. In this image the central jet is visible, which is a powerful beam of hot gas produced by a massive black hole in the core of the galaxy.
NASA
ABOVE: Taken in December 2010, this is the most detailed picture of the Andromeda Galaxy, or M31, taken so far.