We are situated on the edge of a spiral arm of the Milky Way, which is some 30,000 light years from the galactic centre. There are four spiral arms - the Norma Arm from the centre, the Sagittarius Arm, the Orion Arm and the Perseus Arm. The solar system is between the Orion and the Perseus Arms. When we look along the plane of the Galaxy, either towards or away from the centre, a misty band of starlight can be observed spanning the sky on a clear, dark night. This is known as the Milky Way effect. Stars in the spiral arms are typically young and hot, surrounded by a lot of interstellar gas and dust. The central budge has less of this gas and dust, but a higher concentration of older stars. The stars near the centre are believed to have formed some 15 billion years ago.
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