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The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope.

The image was assembled from 342 separate exposures taken with the Space Telescope's Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and 28, 1995. The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe. Three years after the HDF observations were taken, a region in the south celestial hemisphere was imaged in a similar way and named the Hubble Deep Field South. The similarities between the two regions strengthened the belief that the universe is uniform over large scales and that the Earth occupies a typical region in the Universe (the cosmological principle). A wider but shallower survey was also made as part of the Great Observatories Origins Deep Survey. In 2004 a deeper image, known as the Hubble Ultra-Deep Field (HUDF), was constructed from a few months of light exposure. The HUDF image was at the time the most sensitive astronomical image ever made at visible wavelengths, and it remained so until the Hubble eXtreme Deep Field (XDF) was released in 2012.

The Medium Deep Survey (MDS) used the Wide Field and Planetary Camera 2 (WFPC2) to take deep images of random fields while other instruments were being used for scheduled observations.

Between December 18 and 28, 1995—during which time Hubble orbited the Earth about 150 times—342 images of the target area in the chosen filters were taken.

About 3,000 distinct galaxies could be identified in the images, with both irregular and spiral galaxies clearly visible, although some galaxies in the field are only a few pixels across. In all, the HDF is thought to contain fewer than twenty galactic foreground stars; by far the majority of objects in the field are distant galaxies.

Due to redshift the most distant objects in the HDF (Lyman-break galaxies) are not actually visible in the Hubble images; they can only be detected in images of the HDF taken at longer wavelengths by ground-based telescopes. One of the first observations planned for the James Webb Space Telescope was a mid-infrared image of the Hubble Ultra-Deep Field.

The field has also been surveyed with the Westerbork Synthesis Radio Telescope and the MERLIN array of radio telescopes at 1.4 GHz; the combination of VLA and MERLIN maps made at wavelengths of 3.5 and 20 cm have located 16 radio sources in the HDF-N field, with many more in the flanking fields. Radio images of some individual sources in the field have been made with the European VLBI Network at 1.6 GHz with a higher resolution than the Hubble maps.

The HDF-S survey used the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instruments installed on the HST in 1997; the region of the original Hubble Deep Field (HDF-N) has since been re-observed several times using WFPC2, as well as by the NICMOS and STIS instruments. A wider survey, but less sensitive, was carried out as part of the Great Observatories Origins Deep Survey; a section of this was then observed for longer to create the Hubble Ultra-Deep Field, which was the most sensitive optical deep field image for years until the Hubble eXtreme Deep Field was completed in 2012. Images from the Extreme Deep Field, or XDF, were released on September 26, 2012, to a number of media agencies.

Quick Facts

• Three years after the HDF observations were taken, a region in the south celestial hemisphere was imaged in a similar way and named the Hubble Deep Field South.
• The HUDF image was at the time the most sensitive astronomical image ever made at visible wavelengths, and it remained so until the Hubble eXtreme Deep Field (XDF) was released in 2012.
• The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known.
• The image was assembled from 342 separate exposures taken with the Space Telescope's Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and 28, 1995.
• The Medium Deep Survey (MDS) used the Wide Field and Planetary Camera 2 (WFPC2) to take deep images of random fields while other instruments were being used for scheduled observations.

Source material: Wikipedia - "Hubble Deep Field". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.