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Four times a year, Ghez focuses the telescope on the stars of the very heart of our Milky Way. She is looking for the telltale high speeds that reveal the presence of a black hole. The center of the Milky Way is so near and the Keck telescope so powerful Ghez is able to see closer into the center of the galaxy than anyone has ever done before.

Here's an example of one of the images we got just last night. The seeing was, this was kind of a typical night, not the best night, not the worst night. Each one of these blobs here is a star, and what you see is each star is distorted, that's what the atmosphere does. It's like looking through a pond, like if you want to look at a penny at the bottom of a pond and the water's moving. It looks all distorted. It looks different every time you look, so this is one exposure and the next exposure looks like this.

By superimposing thousands of these pictures taken overnight, the computer can compensate for the atmosphere's distortion producing a detailed picture of the centre of the galaxy.

You can see the positions of the stars very accurately. If we go in to the centre here, rescale it, we actually see that there are fainter stars towards the center of our field of view. And these stars are extremely important. It's the motion of these stars that reveal(s) the presence of the black hole.

Ghez has been following the motions of these stars for the last five years. If there was no black hole, they'd be moving very slowly, but she's discovered they are circling at speeds of over 1,000 kilometers a second.

These stars that we've been watching are two light weeks from this, from the center of our galaxy. So their motion, the fact they are going a thousand kilometers per second tells us that within two light weeks there is two million times the mass of the sun of matter there.

There's only one thing in the universe this dense. Lying at the center of this necklace of spinning stars is a supermassive black hole. You can't see it, but it's there. The most destructive force in the universe is lurking at the heart of our very own galaxy, the Milky Way. The puzzle for cosmologists now is what effect it has on the galaxy around it.

seeing: Astronomy the quality of observed images as determined by atmospheric conditions

blob: an indeterminate roundish mass or shape

superimpose: place or lay (one thing) over another, typically so that both are evident