Our Galaxy is a vast system of 100 billion stars, occupying a volume of space with a diameter of 60 kpc. The youngest stars form a disc some 500 pc thick. Half the stars are contained within 1 kpc from the disc, and the oldest stars pervade a roughly spherical halo with 30 kpc radius around the centre of the disc. Between the stars, a tenuous galactic atmosphere of mainly hydrogen and helium extends throughout the plane, to a thickness of about
200pc. In the disc, spiral arms with a cross section of 400 by 2000 pc are observed to contain very young stars (100000yrs or less in age) and H+ regions. The galactic centre consists of a stellar bulge which is strongly concentrated towards the nuclear region , containing an active radio source in a flat, rapidly rotating, gaseous nuclear disc. The entire system rotates about an axis through the nucleus at right angles with the plane, and the angular velocity decreases steadily outward. The tangential velocity in the plane first increases and then, after having reached a maximum at 8kpc, slowly decreases outward. The Sun lies in the galactic plane,10kpc from the centre

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