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The centre of the galaxy – as never before March 24, 2010

Posted by Peter Hornby in astronomy.

Here’s a remarkable movie loop, prepared by Professor Andrea Ghez and her research team at UCLA, and presented at a public lecture following her recent Aspen Center for Physics conference on “The Formation and Evolution of Black Holes”.

Let’s look at what’s going on here.  We’re looking at a tiny, tiny area, no more than a second of arc across, at the exact centre of the Milky Way galaxy, roughly 25,000 light years away.  We’re pretty certain that, like many, if not all, other galaxies, our Milky Way has a huge black hole at its centre.  One way astronomers can probe the location, size and habits of the central black hole is by watching the orbits of objects close to it.  Professor Ghez’s animation shows the movement of some of these objects over the fifteen year period from 1995 to 2010.  It’s made from real, actual images, taken using telescopes such as the Keck 10-meter monsters on Mauna Kea, and using the most sophisticated adaptive optics techniques imaginable.  It’s an astonishing feat of observational astronomy.

What we see is that these objects are orbiting something – you can see that two of them have completed, or almost completed, full orbits over the fifteen years spanned by the movie.  The object (S0-16) coming in from upper left, taking a fast turn around the black hole, and heading out again, approaches as close as 45 astrononomical units (1 AU is the distance of the earth from the Sun), which, to give you some scale, is around four billion miles, not much larger than the distance of Pluto from the Sun,  At its closest approach, it’s moving at 4% of the speed of light!  These orbits give a direct estimate of the mass of the central black hole – it comes out as roughly 3.7 million solar masses.  The current limiting factor on this estimate is that we don’t know exactly how far away the centre of the galaxy actually is!

Nowhere in the galaxy are conditions anything like as extreme as we’re seeing here.  The gravitational gradients are huge.  Professor Ghez has suggested, according to a commenter at the Cosmic Variance blog entry linked below, that it should actually be possible, within a decade or so, to directly observe the precession of the orbits of these objects which is predicted by General Relativity.

The idea that we can directly watch stars orbiting a black hole at the centre of our galaxy defies belief.

Link from Daniel Holz at Cosmic Variance.

Acknowledgement : This animation was created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes.  Other images and animations can be seen at Prof. Ghez’s group website

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