Where is our galaxy?

At the inner limit of the spiral arms, the bar and hub are surrounded by a structure known as the 5-kiloparsec Ring (one kiloparsec is around 3,260 light years). Although we cannot see it in visible light, the ring seems to contain huge concentrations of star-forming nebulas and young stars: it's probably the main generator of new stars in the Milky Way.

Above and below the main disc lies a relatively empty region known as the halo. Many faint, long-lived stars pass through this region on tilted orbits, but the halo's most obvious occupants are globular clusters - dense balls containing many tens of thousands of old, red and yellow stars that are generally found above and below the galactic hub. Similar red and yellow stars dominate the hub and bar - they are relatively poor in heavy elements, which allows them to shine for billions of years without evolving significantly. As a result, they are known as "Population II stars, in contrast to the younger, faster-evolving and heavy-element-enriched Population II stars in the galactic disc.

Among all these stars, the huge majority are low-mass red and orange dwarfs - stars with a fraction of the mass of the Sun, which shine so faintly that they can only be seen when they are relatively nearby. Brighter and more massive stars are much rarer, but tend to shine out over huge distances and so appear more prominently in our skies. Similarly, ageing but brilliant red and orange giants are common among the naked-eye stars seen from Earth, but in fact far rarer than they might appear at first glance.

What's more, stars in our galaxy seem to be gregarious - although they gradually drift apart from the open clusters in which they form, many stars remain together in binary or multiple star systems. Recent research also suggests that planetary systems are also common - there may be at least as many planets as there are stars in the sky.

Within the hub, stars become more densely packed towards the centre of the Milky Way - the galactic core. Only X-rays, radio waves and some infrared waves can pass through these dense star clouds unaffected, but they reveal an intriguing picture of the strange and violent conditions in the core itself.

At radio wavelengths, the core is marked by a complex radio source known as Sagittarius A - it consists of a bubble-like structure (Sagittarius A West) a few tens of light years across - probably the remnant of an enormous supernova explosion. Embedded within this is a three-armed spiral called Sagittarius A East, roughly ten light years across. The middle of the spiral coincides with the densest concentration of stars in the Milky Way, and a third, point-like source of radio waves known as Sagittarius A* that is believed to mark the Milky Way's centre.

X-ray emissions reveal huge bubbles and twisted lobes of superhot gas across the region - a mix of supernova remnants and the effects of hot stellar winds blowing out from massive stars. Infrared telescopes, meanwhile, show the distribution of stars themselves - the region is home to two of the largest open clusters in the galaxy, the Quintuplet Duster and the Arches Cluster. A third cluster surrounds the Sagittarius A* region itself, but while its individual stars have been mapped, none coincides exactly with the mysterious radio source.

Since the Seventies, astronomers have suspected that Sagittarius A* might mark the location of an invisible black hole with the mass of millions of Suns, but the evidence to back this up has only recently been found. By mapping the central star cluster over several years, astronomers have plotted the orbits of two stars that orbit the object every few years. Based on the speed of these stars, Sagittarius A* must contain the mass of at least 4 million Suns concentrated in a region roughly the size of Uranus's orbit around the Sun. With such enormous density, it can only be a supermassive black hole.

However, Sagittarius A* seems surprisingly quiet compared to similar objects elsewhere in the universe r other supermassive black holes in distant galaxies wreak havoc with their surroundings. Astronomers think the big difference is that these black holes are feeding on material that strays too close. Sagittarius A*, on the other hand, seems to have long ago cleared out its immediate surroundings, and anything that still orbits in the central region stays safely out of reach.

Recent discoveries, however, suggest that Sagittarius A* may have been active surprisingly recently. In 2007, NASA's Chandra satellite discovered X-ray 'echoes', created as radiation when an event roughly 50 years previously affected gas clouds near the central black hole. It seems likely that Sagittarius A* took a substantial snack at this time, when a large gas cloud strayed too close. And in 2010. the Fermi Gamma-ray Space Telescope found evidence for an even larger outburst thousands of years ago.

There's one final question that's worth asking about our galaxy - where does it end? Although the spiral arms seem to lose their intensity and the number of disc stars falls off dramatically over 50,000 light years from the hub, a disc of neutral hydrogen gas extends out to almost 100,000 light years and seems to retain the overall spiral structure The Milky Way's gravitational influence extends even further - the halo of stars and globular clusters reaches out to around 150,000 light years or mora However, beyond this distance the orbits of halo objects are disrupted by two interlopers - the Magellanic Clouds. Until recently, these two irregular clouds of gas were believed to be in orbit around our galaxy, but it now seems that they are merely passing by on their own paths through space.

On a larger scale, the Milky Way is one of the major components of the Local Group - a galaxy cluster 10 million light years across. The other is the Andromeda Galaxy, some 2.5 million light years away. These two galaxies exert a strong influence on everything in their vicinity, and are approaching each other at 110 kilometres per second. They will eventually collide and merge together about 4 billion years from now in a process that win signal the end of the Milky Way as an independent galaxy.