Sizing up a black hole

Astronomers are closing in on the dimensions of the supermassive black hole at our galaxy’s center. By observing a strong source of radio waves emanating from the Milky Way’s core, researchers have calculated that the black hole occupies a volume that would fit inside Earth’s orbit around the sun.

POWER-HOUSE. Core of our galaxy with Sagittarius A* shown as central white dot. AUI, NSF, J.-H. Zhao and W.M. Goss/NRAO

Material spiraling into the central black hole emits intense radiation, including radio waves. Unlike visible light, radio waves can penetrate dust at the Milky Way’s core, providing a window on the black hole’s maelstrom of activity. That window, however, is not entirely transparent. Radio waves scatter off of electrons in the gas that lies between the black hole and Earth. The scattering, which is weaker at shorter radio wavelengths, blurs the image of the radio-wave source, known as Sagittarius A*, that surrounds the black hole.

By modeling the effect of that blurring and by observing Sagittarius A* at a range of wavelengths, Geoffrey Bower of the University of California, Berkeley and his colleagues pinned down an upper limit on the size of the radio-emitting source. In so doing, the team has placed the “tightest constraint [ever obtained] on the size of the central black hole” powering Sagittarius A*, says Bower. His group describes its findings in an upcoming Science.

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