In the document "Summary of Key Findings from the Hubble Deep Field" http://oposite.stsci.edu/pubinfo/pr/97/hdf-key-findings.html, you will find information under these headings:

• Small Galaxies in the Early Universe
  
• Open versus Closed Universe
  
• Disturbed Galaxies
  
• Stellar Baby Boom
  
• In Search of Hidden Stars
  
• Missing Mass — Still Missing
  

They use the colors of the galaxies. Different types of galaxies tend to be different colors. For example, elliptical galaxies have reddish colors because they are mostly composed of old red stars. Astronomers study the colors of nearby elliptical, spiral, and irregular galaxies and compare these colors to those of the galaxies in the Hubble Deep Fields. Comparing the colors allows them to classify the galaxies.

Because of the way astronomers' instruments work, they can be reasonably sure that they have detected all galaxies with a certain range of brightnesses in the Hubble Deep Fields. Astronomers may be able to identify fainter objects, but they cannot be sure that they have detected all of the fainter objects that exist.

When studying populations of objects, astronomers need to make sure that the sample they choose is representative. The very faintest objects do not form a representative sample since astronomers do not know if they have detected all of the faintest objects. Therefore, they limit their sample to objects in a certain brightness range. The sample is then said to be "statistically complete" to that brightness level. For the HDF-N, the statistically complete sample consists of 1067 galaxies.

In 2003, the Hubble Space Telescope looked even farther back in time and found as many as 10,000 galaxies, using the recently installed Advanced Camera for Surveys (ACS) and the recently-revived Near Infrared Camera and Multi-object Spectrometer (NICMOS). The million-second-long exposure (11.3 days) captured galaxies that are too faint to be seen by ground-based telescopes, or even by Hubble in its previous faraway looks, called the Hubble Deep Fields.

The ACS image, known as the Hubble Ultra Deep Field (HUDF), shows a wide range of galaxies of various sizes, shapes, colors and ages. Some were formed just a short time after the universe was created, about 13 billion years ago. The infrared NICMOS image complements the visible-light ACS image and reveals some of the most-distant galaxies ever seen.

The ACS picture required a series of exposures taken over the course of 400 Hubble orbits around Earth. This is such a big chunk of the telescope's annual observing time that Space Telescope Science Institute Director, Steven Beckwith, used his own Director's Discretionary Time to provide the needed resources. Just like the previous HDFs, the new data are expected to galvanize the astronomical community and lead to dozens of research papers that will offer new insights into the birth and evolution of galaxies.

The Hubble Deep Fields represented the faintest, deepest visible light images ever taken using the technology available at that time — the Wide Field and Planetary Camera 2. The Hubble Ultra Deep Field was produced using an instrument with newer technology — the Advanced Camera for Surveys. The differences in the HDF and HUDF are due to the quality of the instruments being used. Concerning the variety of galaxies visible in the fields, it has been said that the HDF captured images of galaxies when they were youngsters but the HUDF captured images of galaxies as toddlers.