Look up at a dark night sky, especially in the summer months, and you can clearly see the ribbon of the Milky Way stretching right across the heavens. From our position within our Galaxy it can be difficult to visualise its shape, but it's clear from a simple naked eye observation that there are some areas with much higher density of stars, and the densest area of all is around the constellation of Sagittarius.
Not surprisingly, this is because the centre of our Galaxy lies in that direction, so our line of sight includes all the stars between us and the centre, plus all those on the opposite side. And somewhere in amongst all those stars is the Galactic Centre, the point around which the whole Galaxy rotates.
A compact, massive radio source
So what is actually at the centre of the Galaxy? It's actually not easy to see, especially with visible light telescopes, because as well as all the intervening stars there are vast tracts of interstellar dust blocking our view. But radio telescopes are able to 'see' through the dust and several decades ago detected a very strong radio source in that region, which has been named 'Sagittarius A*' (SgrA* for short). The evidence points to the existence there of something compact but very massive, about the mass of several million suns, and with a diameter perhaps comparable to the Earth's orbit around the Sun.
Careful observations of the region over several decades have revealed several stars that are rotating very near to the galactic centre, and in particular, two with very tight and incredibly fast orbits – a circuit takes just 16.5 years for star S0-2 and 11.5 years for star SO-102.
This is a blink of an eye compared with the estimated 225 to 250 million years taken by our Sun to complete one circuit. So these stars must be extremely close to the object at the centre of the galaxy, and the actual paths and timings of the orbits of these and other nearby stars can be used to determine the mass and dimensions of that object.
Astronomers believe it to be a 'supermassive' Black Hole, because nothing else of that mass could exist in such a small area.
What does a black hole look like?
Black holes exist in many other large galaxies similar to ours, so it is perhaps not a surprise that our galaxy has one too. In some galaxies they are very active, meaning that they are rapidly consuming matter, giving rise to a highly luminous accretion disk around the black hole, caused by friction from the matter accelerating as it is pulled inwards, and powerful jets of electromagnetic radiation.
In contrast the black hole at the centre of our own Milky Way appears to be relatively quiet – a jet has been observed but it is quite weak, there is no obvious accretion disc, and the black hole generally emits just two or three bright X-ray flares a month. However, last year the frequency of these flares increased to around one a day, and astronomers believe this could be linked to the movement of a mysterious dusty object (gas cloud? star swathed in dust?), dubbed G2, that passed very near by SgrA* in late 2013, a few months before the increase in flare activity. Could our nearest Black Hole be on the verge of waking up?
The jury is still out on this one as the behaviour of black holes is still not well understood and by their very nature they are difficult to observe. But astronomers continue to keep an eye on our backyard black hole, in case the sleeping giant shows more signs of stirring.
http://www.astro.ucla.edu/~ghezgroup/gc/journey/smbh.html: Evidence for black hole at centre of Milky Way
http://physicsworld.com/cws/article/news/2012/oct/04/star-seen-whizzing-around-supermassive-black-hole Two fast-orbiting stars at centre of galaxy
http://www.einstein-online.info/spotlights/milkyway_bh
http://www.nasa.gov/mission_pages/chandra/milky-way-s-black-hole-shows-signs-of-increase
d-chatter.html
http://www.nasa.gov/mission_pages/chandra/news/high-energy-particles-in-milky-way.html Evidence for jet in galactic centre
1. NASA/JPL-Caltech/ESA/CXC/STScI
2. See image
3. NASA/CXC/M. Weiss