This image shows NGC 4639, a spiral galaxy located 78 million light-years away in the Virgo cluster of galaxies. The blue dots in the galaxy's outlying regions indicate the presence of young stars. Among them are young, bright stars called Cepheids, which are used as milepost markers to obtain accurate distances to nearby galaxies. Astronomers measure the brightness of Cepheids to calculate the distance to a galaxy.
One team of astronomers led by Allan Sandage have 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 Cnstant depends on Cepheids and secondary distance methods.
Camera: WFPC2
Technical Information: Color image from separate exposures taken in the visible and near-infrared regions of the spectrum.
Credit: A. Sandage(Carnegie Observatories, A. Saha (ST ScI), G.A. Tammann and L. Labhardt (Astronomical Institute, University of Basel), F.D. Macchetto and N. Panagia (ST ScI and ESA), and NASA.
