Ultraviolet Image of NGC 6752 Highlights Evolving Stars

Release date: 12 June 1995

The globular cluster NGC 6752, in the southern constellation of Pavo, as seen at in ultraviolet light a wavelength of 1620 Angstroms with the Ultraviolet Imaging Telescope (UIT). The photograph was obtained on March 13, 1995, during the Astro-2 mission of the Space Shuttle Endeavour.

Although NGC 6752 contains more than one hundred thousand stars, nearly all of these are too cool to emit any ultraviolet light. The 355 stars seen in the UIT image are in a short-lived stage near the end of their lives (known as the horizontal branch phase) when they can reach a surface temperature of up to 30,000 degrees. The UIT image is able to resolve the hot stars into the cluster core, while the UIT 40 arcminute field of view is big enough to encompass the entire cluster. Thus, the UIT image provides the first complete census of the hot horizontal branch stars in NGC6752.

The overexposed bright star about 4 arcminutes southwest of the cluster center is the foreground star HD 177999 (with a visual brightness of 7.5 magnitudes and spectral type of A0) and is unrelated to the globular cluster.

UIT is a 15-inch (0.38-m) telescope which was designed and built at the Goddard Space Flight Center, Greenbelt, MD. NASA's UIT instrument flew in space during the Astro-2 mission of Space Shuttle Endeavour, March 2-18, 1995. This UIT photo was presented to the American Astronomical Society meeting in Pittsburgh, Pa., June 12, 1995.

Technical Data: The exposure time was 781 seconds and the photograph was made at an effective wavelength of 1620 angstroms (162 nanometers), with a bandwidth of 220 angstroms (22 nanometers). The photograph was obtained during the daytime portion of Endeavour's orbit on March 13, 1995. The region shown here is about the apparent diameter of the full moon; objects appearing to be 1,000 times smaller than this can be resolved in this image. The original UIT image was recorded on black and white film; the image is displayed here with color coding indicating intensity of the ultraviolet light.

Credit: UIT Science Team, Theodore P. Stecher.