Structure of the galaxy

The globular clusters contain the galaxy’s oldest stars and consequently look red dish, because many of their stars have expanded to become giants or super giants. The clusters orbit the galaxy’s centre, and often pass through the disc, but there are no collisions among the stars, which are enormous distances apart, even within the relatively closely packed globular clusters. There is almost certainly a vast amount of undetected”dark mater” spread through the halo of every galaxy. It might consist of huge numbers of brown dwarfs – too dim to be seen – but many astronomers think this matter consists of undiscovered types of subatomic particles.

The stars near the centre of the galaxy are also old and reddish, and relatively little gas were left over here from the process of star formation. But there are gas clouds in a central region of ten light years or so in diameter that can be probed only with radio telescopes. The clouds are violently agitated by some object or objects at the centre with a mass of a few million Suns. That object may be a black hole, swallowing mass of the equivalent of ten suns a year, or the remains of super massive stars that may have exploded there within the last 100 million years or so.

The disc of the galaxy is 100,000 light years across. The sun lies about two thirds of the way out from the centre, and takes about 230 million years to orbit the galactic centre. Two or perhaps four tightly coiled spirals arms are marked out by bright bluish stars. The arms are regions where gravitational effects are believed to cause ripples of slightly increased density though this is poorly understood. Stars and gas clouds pass through the arms but their passage is slowed. Star birth is triggered here. Though all kinds of stars are born, the minority of short lived; fast burning blue hot massive stars are conspicuous, marking out the arms.