NGC 6273

Messier 19 (NGC 6273) is a rich globular cluster in the constellation Ophiuchus, located approximately 28,000 light-years from Earth but only about 5,000 light-years from the Milky Way's galactic center — making it one of the Messier globulars that sits closest to the heart of our galaxy. It was discovered by Charles Messier on June 5, 1764. The gravitational influence of the galactic core has made NGC 6273 one of the most noticeably flattened globular clusters in the sky: its outline is distinctly elongated along its major axis, spanning roughly 65 light-years, while its minor axis is noticeably shorter. This oblateness is believed to result from the tidal forces exerted by the galactic bulge over billions of years.

NGC 6273 is a dense, fairly concentrated cluster containing hundreds of thousands of stars with an estimated age of about 13 billion years. Its apparent brightness is magnitude 6.8 — just at the limit of naked-eye visibility — though its position near the galactic plane means that dust between us and the cluster dims and reddens its light somewhat. The cluster is receding from us at about 146 kilometers per second. Only four RR Lyrae variable stars have been identified in M19, a modest number compared with richer globulars. Because of its location nine degrees above the galactic plane and near the center, M19 is seen against a dense background of Milky Way stars and traces of dust.

Binoculars show M19 as a round, moderately bright haze; a 150 mm telescope begins to resolve a granular texture at the edges. This image was assembled from six exposures taken in July 1997 at the Kitt Peak National Observatory 0.9-meter telescope during the Research Experiences for Undergraduates (REU) program supported by the National Science Foundation.