Wonders of the Universe. Andrew Cohen

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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. It is our largest and closest spiral galaxy, and in this picture we can clearly see rings of new star formations developing.

      TOP: This image of the galaxy M51 clearly shows how it got its other name: the Whirlpool Galaxy. The spiral shape of the galaxy is immediately obvious, with curving arms of pinky-red, star-forming regions and blue star clusters.

       NASA

      ABOVE: Zwicky 18 was once thought to be the youngest galaxy, as its bright stars suggested it was only 500 million years old. However, recent Hubble Space Telescope images have identified older stars within it, making the galaxy as old as others but with new star formations.

      TOP: M33, also known as the Triangulum, or Pinwheel, Galaxy is the third-largest in the Local Group of galaxies after the Milky Way and Andromeda Galaxies, of which it is thought to be a satellite.

MAPPING THE MILKY WAY GALAXY

      Our galaxy, the Milky Way, contains somewhere between 200 and 400 billion stars, depending on the number of faint dwarf stars that are difficult for us to detect. The majority of stars lie in a disc around 100,000 light years in diameter and, on average, around 1,000 light years thick. These vast distances are very difficult to visualise. A distance of 100,000 light years means that light itself, travelling at 300,000 kilometres (186,000 miles) per second, would take 100,000 years to make a journey across our galaxy. Or, to put it another way, the distance between the Sun and the outermost planet of our solar system, Neptune, is around four light hours – that’s one-sixth of a light day. You would have to lay around 220 million solar systems end to end to cross our galaxy.

      At the centre of our galaxy, and possibly every galaxy in the Universe, there is believed to be a super-massive black hole. Astronomers believe this because of precise measurements of the orbit of a star known as S2. This star orbits around the intense source of radio waves known as Sagittarius A^* (pronounced ‘Sagittarius A-star’) that sits at the galactic centre. S2’s orbital period is just over fifteen years, which makes it the fastest-known orbiting object, reaching speeds of up to 2 per cent of the speed of light. If the precise orbital path of an object is known, the mass of the thing it is orbiting around can be calculated, and the mass of Sagittarius A^* is enormous, at 4.1 million times the mass of our sun. Since the star S2 has a closest approach to the object of only seventeen light hours, it is known that Saggitarus A^* must be smaller than this, otherwise S2 would literally bump into it. The only known way of cramming 4.1 million times the mass of the Sun into a space less than 17 light hours across is as a black hole, which is why astronomers are so confident that a giant black hole sits at the centre of the Milky Way. These observations have recently been confirmed and refined by studying a further twenty-seven stars, known as the S-stars, all with orbits taking them very close to Sagittarius A^*.

      Beyond the S-stars, the galactic centre is a melting pot of СКАЧАТЬ