Thursday, November 03, 2005

A smaller black hole?

I saw on the news today that the Milky Way's black hole is smaller than previously thought. This made me perk up, because the word "smaller" can mean physical size (like a foot is smaller than a mile), or it can mean mass (like a pound of brick is smaller than a ton of brick).

My first reaction was that the word "smaller" meant "less massive," especially since the physical size of a black hole is kind of a fuzzy comment. This is why I perked up. Astronomers have been able to make some very accurate measurements of the mass of our galaxy's black hole: about 2 million times the mass of the sun. These measurements are based on the movement of stars around the black hole, and physics from Isaac Newton in the 17th century is sufficient to figure out the mass of the black hole. So if its mass were smaller, this would be quite surprising.

But it is not the mass that is smaller, it is the physical size. Astronomers using radio telescopes on Earth were able to make the sharpest picture of the gas around the black hole yet made. They were able to resolve details only 93 million miles across, or the size of Earth's orbit around the sun. Not bad for taking this picture from 24,000 light-years away!

We are still fairly far outside the edge of the black hole, though. The black hole's event horizon is still at least 12 times smaller than this picture. So we still haven't "seen" the black hole, but we are pretty close!

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