In late December 1995, the HST spent 10 days looking almost continuously at a single small speck of sky in the constellation Ursa Major. The image seen here - called the Hubble Deep Field - was assembled by adding together 342 separate exposures with the WFPC2. The bewildering array of at least 1,500 galaxies includes the faintest ever recorded. At 30th magnitude, these faint objects are about four billion times dimmer than the human eye is capable of seeing. Almost certainly, some of these galaxies are also the most distant ever to be observed.
Astronomers use the term `deep' for images that record faint objects. The dimmest galaxies include the most distant ones (as well as nearer ones that do not shine brightly). Because their light has taken billions of years to reach the HST, recording the Hubble Deep Field is like using a `time machine' to peer into the remote past of the universe. Some of the galaxies are being viewed as they were more than ten billion years ago, when in the process of formation.
The area of sky for the Hubble Deep Field was chosen because it is not cluttered with nearby objects, such as stars in our own Galaxy. It is a `peephole' out of the Galaxy that allows a clear view all the way to the horizon of the universe. Staring at this one spot for ten days, the HST took one picture after another. Each exposure was typically 15 to 40 minutes long. Separate images were taken in ultraviolet, blue, red and infrared light. This enables astronomers to infer, at least statistically, the distances, ages and compositions of the galaxies. Combined, the different colored images give a reasonably `true color' view.
Camera: WFPC2
Credit: R. Williams and the Hubble Deep Field Team/STScI, and NASA
