A remarkable giant ring of hot young stars appears in the ultraviolet image (top) of the spiral galaxy Messier 94 (M94, also called NGC 4736), as photographed by the Ultraviolet Imaging Telescope on March 12, 1995 during the Astro-2 mission of the Space Shuttle Endeavour.

M94, called a starburst galaxy because of the large number of new hot stars that are present, is located about 15 million light years from Earth and has an apparent diameter equal to about 1/3 that of the full moon.

In red light (bottom), M94 shows three distinct zones: an extremely bright central bulge, composed mostly of old, cool stars; the main disk showing many short spiral arms; and an extensive, faint outer ring.

In striking contrast to its visual appearance, the UIT image (top) shows a completely different structure. Gone is the familiar spiral pattern, replaced by a well-defined ring of massive stars bracketing the nucleus. The central bulge is not observed. The stars in this ring formed within the past 10 million years, marking the ring as a site of intense, recent (in astronomical terms) star formation. A mystery, which UIT astronomers will explore, is why almost all massive star formation in M94 is concentrated in a very narrow zone and has not occurred in other parts of the galaxy. Study of the young stars in this ring will add to astronomer's understanding of galaxy evolution.

UIT is a 15-inch (0.38-m) telescope which was designed and built at the Goddard Space Flight Center, Greenbelt, MD. This UIT photo was presented to the American Astronomical Society meeting in Pittsburgh, Pa., June 12, 1995.

Technical Data: The exposure time for the UIT image was 1040 seconds and the photograph was made at an effective wavelength of 1520 angstroms (152 nanometers), with a bandwidth of 354 angstroms (35.4 nanometers). The photograph was obtained during the nighttime portion of Endeavour's orbit on March 12, 1995. The region of the galaxy M94 shown here is about one-quarter of the apparent diameter of the full moon. 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.

BOTTOM - Image of the same galaxy in red light, made with a 36-inch (0.9 meter) telescope at Kitt Peak National Observatory, near Tuscon Arizona. The image is displayed here with color coding indicating intensity of the red light. Red light image courtesy of W. H. Waller (Hughes STX, UIT Science Team)

Credit: UIT Science Team, Theodore P. Stecher

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Hartmut Frommert
Christine Kronberg
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Last Modification: 12 Nov 2002