Spotting the Milky Way's enigmatic center.

Spotting the Milky Way's enigmatic center

Two tiny spots of light may hold the key to unlocking the mystery of the Milky Way's center. These celestial objects, detected by astronomers using the European Southern Observatory's New Technology Telescope (NTT) at La Silla, Chile, lie very near the position of a source of intense radio waves in the southern constellation of Sagittarius, in the direction of the galactic center.

The discovery marks the first identification of visible objects at the radio source's position. Careful study of the light emitted by these objects, provisionally designated GZ-A and GZ-B, may enable astronomers to determine whether a black hole or a compact cluster of hot, young stars lies at the heart of the Milky Way.

To obtain a clear image of the galactic center, Michael R. Rosa of the European Southern Observatory in Garching, Germany, and his collaborators adopted a novel strategy for penetrating the dense interstellar gas and dust clouds that otherwise obscure the view. They decided to make their observations at wavelengths ranging from 850 to 1,100 nanometers. At these wavelengths, about one-millionth of the light emitted by any sources present would penetrate the surrounding clouds -- sufficient light for the NTT to obtain an image.

Using a variety of image-processing techniques, the astronomers combined five 40-minute exposures to create one sharp optical image. That allowed them to see that one of two relatively bright stars in the picture appeared elongated, clearly indicating the presence of other, fainter images nearly coincident with that of the bright star. Further processing subtracted the bright star's image, unveiling two closely spaced, previously unknown star-like objects.

GZ-A lies closer to the galactic center's radio source, about 28,000 light-years from Earth. It looks quite blue, suggesting a large energy output, and apparently shines with an intensity a few million times greater than the sun's. It also seems to produce very little infrared radiation, whereas GZ-B appears to coincide with a known source of infrared radiation.

Rosa and his colleagues speculate that GZ-A could be an extremely dense cluster of hot stars. However, it's also possible that one or both of the objects are associated with a black hole at the galactic center. Spectroscopic studies of the light emitted by the two objects may indicate which idea is correct.