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 galaxy's 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 Sandage's 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.
Credits
A. Sandage (Carnegie Observatories), 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| About The Object | |
|---|---|
| Object Name | NGC 4639 |
| R.A. Position | 12h 42m 52.51s |
| Dec. Position | 13° 15' 24.09" |
| About The Object | |
|---|---|
| Object Name | A name or catalog number that astronomers use to identify an astronomical object. |
| Object Description | The type of astronomical object. |
| R.A. Position | Right ascension – analogous to longitude – is one component of an object's position. |
| Dec. Position | Declination – analogous to latitude – is one component of an object's position. |
| Constellation | One of 88 recognized regions of the celestial sphere in which the object appears. |
| Distance | The physical distance from Earth to the astronomical object. Distances within our solar system are usually measured in Astronomical Units (AU). Distances between stars are usually measured in light-years. Interstellar distances can also be measured in parsecs. |
| Dimensions | The physical size of the object or the apparent angle it subtends on the sky. |
| About The Data | |
| Data Description |
|
| Instrument | The science instrument used to produce the data. |
| Exposure Dates | The date(s) that the telescope made its observations and the total exposure time. |
| Filters | The camera filters that were used in the science observations. |
| About The Image | |
| Image Credit | The primary individuals and institutions responsible for the content. |
| Publication Date | The date and time the release content became public. |
| Color Info | A brief description of the methods used to convert telescope data into the color image being presented. |
| Orientation | The rotation of the image on the sky with respect to the north pole of the celestial sphere. |
