Wonders of the Universe - Brian Cox [8]
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 celestial activity, filled with all sorts of different systems that interact and influence each other. The Arches Cluster is the densest known star cluster in the galaxy. Formed from about 150 young, intensely hot stars that dwarf our sun in size, these stars burn brightly and are consequently very short-lived, exhausting their supply of hydrogen in just a couple of million years. The Quintuplet Cluster contains one of the most luminous stars in our galaxy, the Pistol Star, which is thought to be near the end of its life and on the verge of becoming a supernova (see Chapter 2). It is in central clusters like the Arches and the Quintuplet that the greatest density of stars in our galaxy can be found. As we move out from the crowded galactic centre, the number of stars drops with distance, until we reach the sparse cloud of gas in the outer reaches of the Milky Way known as the Galactic Halo.
This artist’s impression shows the Arches Cluster, the densest known cluster of young stars in the Milky Way Galaxy.
NASA
Along with the