Credit: Hubble Space Telescope WFPC Team, NASA/ESA, Hubble Space Telescope WFPC Team, NASA, STScI

This photo mosaic, which shows a field of distant galaxies, is a computer enhanced reproduction of a picture taken 4 March 1994 with the repaired Hubble Space Telescope. It combines 16 exposures of 15 minutes each, taken through two filters (F555W and F814W) with the Wide Field Planetary Camera 2. The HST WFPC2 field is chevron-shaped, because it is a mosaic of images recorded with three Wide Field cameras and one higher resolution camera (Planetary Camera) in the upper left. The brightest object in this picture is NGC 4881, approximately centered here in the Planetary Camera (the small quadrant). It is a 13th-magnitude elliptical galaxy in the outskirts of the Coma Cluster, a great cluster of galaxies more than 5 times farther away than the Virgo Cluster. The radical velocity (redshift) of NGC 4881, based on the Doppler displacement of lines in its spectrum, is about 7000 km/sec. Except for a 16th-magnitude Coma spiral at the right and a few foreground stars of the Milky Way, nearly everything else in this field lies far beyond the Coma Cluster. There is a fascinating assortment of background galaxies, including an apparent galaxian merger in progress. Purpose: This HST-WFPC2 observation was made to explore the use the globular star clusters surrounding NGC 4881 as distance indicators for inferring the distance to the Coma Cluster. They are barely visible point sources in this reproduction. The distance to the Coma Cluster is an important cosmic yardstick for scaling the over all size of the universe, because Coma (unlike Virgo) is far enough away that regional departures from a smooth expansion of the universe should not be a major source of uncertainty if Coma is used for estimating the age and rate of expansion (the Hubble Constant). The brightness distribution of globular clusters has been studied in a number of nearer galaxies. They are most numerous between -7 and -8 absolute magnitude. In the Milky Way they peak at -7.6 absolute magnitude. We mus

Provider: Space Telescope Science Institute

Image Source: https://hubblesite.org/contents/news-releases/1995/news-1995-07

Curator: STScI, Baltimore, MD, USA

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