(This press release from May 9, 1996, is reproduced courtesy of the Space Telescope Science Institute.)
This Hubble Space Telescope image shows NGC 4639, a spiral galaxy
located 78 million light-years away in the Virgo cluster of galaxies.
The blue dots in the galaxys outlying regions indicate the presence of
young stars. Among them are older, bright stars called Cepheids, which
are used as reliable milepost markers to obtain accurate distances to
nearby galaxies. Astronomers measure the brightness of Cepheids to
calculate the distance to a galaxy. Allan Sandages team used Cepheids
to measure the distance to NGC 4639, the farthest galaxy to which
Cepheid distance has been calculated. After using Cepheids to
calculate the distance to NGC 4639, the team compared the results to
the peak brightness measurements of SN 1990N, a type Ia supernova
located in the galaxy. Then they compared those numbers with the peak
brightness of supernovae similarly calibrated in nearby galaxies. The
team then determined that type Ia supernovae are reliable secondary
distance markers, and can be used to determine distances to galaxies
several hundred times farther away than Cepheids. An accurate value
for the Hubble Constant depends on Cepheids and secondary distance
methods. The color image was made from separate exposures taken in
the visible and near-infrared regions of the spectrum with the Wide
Field Planetary Camera 2.
Credit: A. Sandage (CarnegieObservatories), A. Saha
(Space Telescope Science Institute), G.A. Tammann, and L. Labhardt
(Astronomical Institute, University of Basel), F.D. Macchetto and N. Panagia (Space
Telescope Science Institute and European Space Agency) and NASA