This is so narrow, that just a few foreground stars in our Milky Way galaxy are visible and are vastly outnumbered by the menagerie of far more distant galaxies, some nearly as faint as 30th magnitude, or nearly four billion times fainter than the limits of human vision. Though the field is a very small sample of sky area it is considered representative of the typical distribution of galaxies in space because the universe, statistically, looks the same in all directions. A galaxy is a massive system of stars, dust and gas held together by their mutual gravity. Galaxies are the basic units of mass in the universe and are visible from very great distances. Galaxies come in different sizes, shapes, colors and chemical compositions.
Galaxies also have different ages and move at different speeds.
Our planet Earth is located in a spiral galaxy named the Milky Way. The nearest galaxies seen in the Hubble Deep Field are about 2.5 billion light years away. The furthest are estimated to be about 10.5 billion light years away. These stars have a variable brightness. Their light variation period has been accurately related to their luminosities, the number of ergs per second of light they produce.