As summer arrives, the most spectacular seasonal event in our night skies occurs as the central hub of our galaxy wheels into view.
Viewed from the Earth, our galaxy appears as a glowing band, the Milky Way, arcing across the night sky.
As we orbit round the Sun, our perspective on the Milky Way changes. In winter, it appears thin and tenuous as we gaze across its outer rim. In summer, it looms large, burning fierce and bright as we gaze into its central hub.
So intense is the opalescent glow that clouds of interstellar dust are silhouetted against the background luminosity, while closer to us, the constellations of Sagittarius and Scorpio provide a lattice of familiar stars through which we gaze out at a phenomenon so vast, so distant that individual stars are lost in the broiling maelstrom of light.
The absolute centre of the hub is obscured from us by a black cloud of interstellar gas, but if we analyze the infra-red radiation that cuts through we discover the presence of an extremely dense population of stars in rapid orbit round a centre that must have a gravitational pull equivalent to that of 4 million Suns. Such a super-massive gravitational centre could only be a Black Hole. It is matched to a point source of radio emission called Sagittarius A* (say, “A star”).
Yet an even greater mystery envelopes us: if we measure the speed at which the 100 billion stars sweep round the galactic centre we would expect them to move more slowly the farther out they are, just as Pluto moves more slowly round the Sun than Mercury, but, defying all expectations, we find that stellar speeds remain fairly constant no matter how far away they are.
Such deviant behaviour can only be explained by positing the presence of a new kind of matter, Dark Matter, which has a greater mass than the aggregated mass of all the stars in the galaxy. Four time greater! We don’t know what it is, but it’s there, and it fills our galaxy, determines the motion of stars and causes light to bend as it passes by.