Hubble stared at a tiny empty patch of sky and found 3,000 galaxies.
In December 1995 the Hubble Space Telescope spent ten days staring at a speck of sky in Ursa Major, about the size of a tennis ball seen from 100 metres, chosen because almost nothing was known to be there. The resulting image held around 3,000 objects, nearly all of them galaxies.
Astronomers can reserve a slice of Hubble's time for discretionary projects. In 1995, once the telescope's famously flawed mirror had been corrected, Robert Williams, then director of the Space Telescope Science Institute, decided to spend much of his share on an experiment: point Hubble at a typical, apparently blank patch of sky and simply keep looking.
The target had to be carefully chosen, well away from the dusty plane of our Milky Way, free of bright stars and other known sources, and in a part of the sky Hubble could watch continuously. The winning field sits in Ursa Major and is about one 24-millionth of the whole sky. Between 18 and 28 December 1995, as Hubble circled Earth about 150 times, it took 342 exposures through four colour filters. The team then removed cosmic ray hits, satellite trails and scattered light before combining them.
The finished Hubble Deep Field showed only a few stars from our own galaxy. Almost everything else was a galaxy, about 3,000 in all, some among the most distant and youngest then known. Because light takes billions of years to cross such distances, the image is also a look back in time. Many of the galaxies had redshifts as high as six, corresponding to distances of about 12 billion light-years, and they were more often irregular and disturbed than galaxies today, fitting the idea that collisions were more common in a younger, smaller universe.
Three years later Hubble repeated the exercise in the southern sky and found a similar scene, supporting the idea that the universe looks much the same in every direction on large scales. The Ultra-Deep Field of 2004 and the eXtreme Deep Field of 2012 went deeper still. Every patch of 'empty' sky, it seems, is crowded with galaxies.
Source: Wikipedia — Hubble Deep Field · Text summarised from Wikipedia (CC BY-SA 4.0)
