The key to obtaining such a sharp image was combining the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) with some images gathered by the Spitzer Space Telescope's Infrared Astronomy Camera (IRAC). Clouds of dust obscure the center of the galaxy from us, but infrared light penetrates those clouds.

It turns out that a population of massive stars fill the hot ionized gas swirling around the inner 300 light years of the galaxy. And the stars are distributed more widely than scientists realized. Says NASA:

Astronomers now see that the massive stars are not confined to one of the three known clusters of massive stars in the Galactic Center, known as the Central cluster, the Arches cluster, and the Quintuplet cluster. These three clusters are easily seen as tight concentrations of bright, massive stars in the NICMOS image. The distributed stars may have formed in isolation, or they may have originated in clusters that have been disrupted by strong gravitational tidal forces.

[Hubble via New Scientist]