Dark shadows

Astronomers are closing in on the dark monster at the center of our galaxy. New radio telescope images indicate that an invisible concentration of material at the Milky Way’s core is some 4 million times as heavy as the sun and squeezed into a region no larger than the distance between the sun and Earth. That’s half the distance that previous observations had indicated, making a more compelling case that the concentrated mass must be a black hole.

Zhi-Qiang Shen of the Shanghai Astronomical Observatory in China and his colleagues homed in on Sagittarius A*, a radio-emitting structure powered by the giant black hole proposed to reside at the galactic center. Using the Very Large Baseline Array of radio telescopes, the researchers obtained the highest-resolution image ever recorded of Sagittarius A*.

The observations, described in the Nov. 3 Nature, also raise the possibility that astronomers might soon see a telltale ring-shaped shadow of the black hole, comments Christopher Reynolds of the University of Maryland at College Park.

The shadow would be created by light from behind the black hole, which swallows all radiation from objects that come too close to it. Surrounding the shadow should be a bright ring, arising from light that barely escapes the black hole’s clutches and is bent by the monster’s gravity, as predicted by Einstein’s theory of gravitation.

Over the next 5 to 10 years, astronomers are likely to obtain radio telescope images of Sagittarius A* sharp enough to see the shadow, notes Reynolds. Future X-ray telescopes will provide additional data on the shadow, further testing Einstein’s theory, he adds.

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