Measuring the Black Hole

Using Radio Telescopes


A Chandra X-Ray Observatory view of the black hole at the center of the Milky Way, called Sagittarius A*. Courtesy Chandra X-Ray Observatory.

Astronomers have known for a long time that there’s a black hole at the heart of the Milky Way Galaxy. It gives off radiation in x-rays, radio emissions, and heats up surrounding clouds of gas and dust, causing them to glow in infrared wavelengths.  So, it would be cool to know how big this black hole is, or rather — how big its event horizon is.  But, that’s something very difficult to do, since the immense gravity of the black hole is so strong that it intercepts and distorts the light and other emissions that come from the regions around the event horizon. This causes the size of the event horizon to appear larger — or smaller — than it really is.

Radio astronomers have managed to get pretty close to measuring the event horizon of this black hole — call Sagittarius A* (or Sag A star, for short). How did they do this?  By linking together a series of radio telescopes to make one big “virtual” telescope, through a technique called Very Large Baseline Inteferometry (VLBI). Essentially, they used telescopes in California, Arizona, and Hawai’i to make one ” big” scope the virtual size of the distance between California/Arizona and Hawai’i.  The result is a closer look at the region around the black hole and the discovery that the event horizon region is smaller than it should be. This could be because of the gravitational lensing that is bending and distorting the signals from the region.

There really are two stories here — one is about the latest measurement of the black hole region, but the other is equally important — the use of VLBI to do that measurement. It requires very precise atomic clocks in tandem with the telescopes (to keep the signals correlated to each other in the right time frame), and a sophisticated computer array that correlates the data after it comes from the telescopes.  The system that correlated the Sag A* data is located at MIT’s Haystack Observatory in Massachusetts, which is where Dr. Shep Doeleman, who led the team of astronomers in the work, is headquartered. The story has been making headlines since the team’s paper describing this work came out in late 2008.  Articles in Nature, Physics Today, and NewScientist have been followed by crews of filmmakers interested in capturing the story for such outlets as the BBC. This is a great testament to the fascination that black holes have for scientists as well as the general public. They just (pardon the expression) suck your attention right in!

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