Unsurprisingly, then, our Solar System is an insignificant speck within the overall scale of the Milky Way - all the planets and other large bodies orbiting the Sun are confined to a region just a few light hours across. In our part of the galaxy, stars are spaced far enough apart that even our nearest stellar neighbours appear as mere specks of right The closest of all, the triple star Alpha Centauri, is still around 4.3 light years away.
The overall shape of our galaxy has been likened to two fried eggs placed back to back, with a broad, flat disc of scattered stars, gas and dust sunounding a bulging central hub where stars are more densely packed together. Our Solar System and its neighbours lie in a relatively sedate, outlying region of the Milky Way, about halfway between the centre and the edge With a little understanding of the Milky Way's structure, the band of right across the night sky is easy to understand: when we look across the plane of the Milky Way's disc, we see far more stars lying in any given direction and these effectively merge together into a generalised glow. On the other hand, when we look "up" or 'down', out of the galactic plane, we are staring into largely empty intergalactic space, with only relatively nearby stars in our part of the disc to get in the way.
In 1785, astronomer William Herscbel made the first attempt to map the Milky Way in detail, by exhaustively counting the number of stars he could see in different directions across the sky. He proved beyond doubt that the Milky Way was a flattened plane, but unfortunately misunderstood its shape because he believed that our own Solar System was near the centre of the galaxy.
In the late-Twenties, Jan Oort set out to study the movement of individual stars in different parts of the sky. He soon confirmed that the Milky Way is rotating, showed that its centre lies in the direction of the constellation Sagittarius, and also proved that galaxies do not rotate like solid bodies - instead the stars closer to the galactic hub move more quickly along their orbits, while stars further out circle more slowly - a phenomenon known as "differential rotation'. The Sun, for instance, takes around 200 million years to complete a single orbit Later measurements in different parts of the disc showed that orbital speeds do not change as dramatically as they should do if they are governed by the mass of the Milky Way's visible matter alone - evidence that our galaxy contains large amounts of transparent but weighty 'dark matter.
Even today, our understanding of our galaxy is constantly evolving thanks to new theories and new observing technologies. Radio, infrared, ultraviolet and X-ray observations can pierce the dense clouds of stars and dust that obscure large parts of the Milky Way in visible light, while new analytical techniques allow astronomers to learn far more from the stars we can observe. So what is our current understanding of this enormous stellar system? The Milky Way is a barred spiral galaxy, around 100,000 light years across and with a disc roughly 1,000 light years thick in most places. Its central hub is formed by a densely packed ball of stars roughly 8,000 light years in diameter, out of which a bar of stars some 27,000 light years long emerges. The bar points more or less directly towards our Solar System, and as a result its existence was uncertain until it was finally confirmed by NASA's infrared Spitzer Space Telescope in 2005.
Two major spiral arms emerge from the ends of this bar. Until recently, our galaxy was thought to have four major arms, but further observations have led to two of them being downgraded. The two survivors are known as the Scutum-Centauras Arm and the Perseus Arm. They alternate with the two recently demoted arms - the Sagittarius and Norma arms. The picture is complicated by numerous other disconnected regions that follow the general sweep of the spiral arms. For instance, our own Solar System lies on the inside edge of a 10.000 light year fork in the Sagittarius Arm. known as Orion Spur, while both the Sagittarius and Norma arms are thought to trail off into disconnected clumps that wrap their way around the galaxy's outer perimeter. The outer extension of the Norma Arm. which lies on our side of the galactic centre and is therefore more easily seen, is known as the Outer Arm.
The Milky Ways spiral arms are not permanent linked structures - otherwise the differential rotation discovered by Jan Oort would cause them to 'wind up' after just a few rotations. Instead, they perpetually renew themselves. In tact the arms seem to be celestial traffic jams, created where stars and gas in circular orbits around the galactic hub enter a spiral 'density wave' region where they are slowed down and jammed together. This triggers the creation of new star-forming nebulas that light up the spiral arms with the pinkish glow of hydrogen emission, and tftimately give birth to 'open clusters' of stars.
However, these stellar foundries are short-lived on a galactic timescale - they exhaust their supplies of star-forming gas within a few million years, and the heaviest stars squander their fuel in a few million more. As a result, the most brilliant stars don't survive long enough for their orbits to carry them out of the spiral arms - instead it's the more sedate, longer-lived stars like our own Sun that make it out of the traffic jam to continue their orbits around the galactic disc over billions of years.