The Hubble Deep Field (HDF, top image, below), was assembled from 342 separate exposures of a single tiny patch of sky taken during ten days in December 1995. (The image was released in 1996.) These observations by the Hubble Space Telescope resulted in the deepest image of the sky, revealing galaxies fainter than had ever been seen before. Most of the nearly 3,000 galaxies are so faint they have never before been seen by even the largest telescopes.
In the Hubble Ultra Deep Field (HUDF, bottom image, below), taken in 2004, Hubble uncovered 10,000 galaxies.
The light from these distant galaxies must travel for a long time before arriving at Earth. It takes billions of years for that light to arrive, so we are seeing the galaxies as they appeared when the light left them billions of years ago, long before the Sun and Earth were born.
These images are to the same scale. Notice the larger area of the HUDF and the greater number of small, red galaxies visible in the HUDF compared to the HDF. Many of these tiny, red galaxies are extremely distant from Earth.
Hubble Deep Field (top image): R. Williams (STScI), the Hubble Deep Field Team and NASA;
Hubble Ultra Deep Field (bottom image):
NASA, ESA and R. Thompson (Univ. Arizona)