Taken with the Hubble telescope, the Ultra Deep Field observations represent a narrow, deep view of the cosmos. Peering into the Ultra Deep Field is like looking at the night sky through an eight-foot-long soda straw. In ground-based photographs, the patch of sky in which the galaxies reside (just one-tenth the diameter of the full Moon) is largely empty. Located in the constellation Fornax, the region is so empty that only a handful of stars within the Milky Way galaxy can be seen in the image.
This composite image was assembled from exposures taken by the near-infrared camera and the advanced camera. Astronomers incorporated the advanced camera’s visible-light observations into the image to better discriminate the colors of the distant galaxies. The near-infrared camera observations were taken from Sept. 3 to Nov. 27, 2003.
To illustrate the narrow, deep view of the image, the animation below shows what the image looks like in relation to the night sky. For further perspective, if you were to hold up a single grain of sand, the patch of sky that it covers contains roughly 10,000 galaxies.
According to astronomers, there are at least one hundred billion galaxies in the observable universe (though some estimates put it as high as 200 billion).
How did they get that number? They took a portion of sky imaged by the telescope (in this case, Hubble). Then — using the ratio of the portion of the sky to the entire universe and taking into account the Cosmological Principle — they multiplied it out and determined the total number of galaxies in the observable universe.