A Side of Science: The Hubble Deep Field Images

In my latest Cosmic Game Connection, I talk about red nugget galaxies – a type of galaxy that was among the first formed after the Big Bang. 

The light from these very distant galaxies has been traveling since they formed over 10 billion years ago. Capturing images of these galaxies can be very challenging because the further an object is the less light that we can see – the light has spread out over time as it travels.

One way to get images of such galaxies is to have a telescope stare at a portion of the sky for a long time. If it stares long enough, the telescope can gather enough photons to get an image.

Credit: R. Williams (STScI), the Hubble Deep Field Team and NASA/ESA

That’s exactly what the Hubble Space Telescope did to get its “deep field” images. These remarkable images have quite a history.

  • The idea of the first Hubble Deep Field image (HDF-N) was to stare at a portion of the sky that was as empty as possible. They chose a region in the constellation Ursa Major.
  • Hubble stared at that first patch for 10 consecutive days in December 1995.  The exposure time was more than 100 hours, where a typical Hubble observation is just a few hours.
  • That first deep field image revealed nearly 3,000 galaxies….in a portion of the sky chosen for being as empty as possible.
  • Shortly after the first one, the telescope team decided to plan a second deep field image, this time choosing a patch of sky in the Southern Hemisphere in the direction of the constellation Tucana (which isn’t terribly familiar to those of us in the Norther Hemisphere!).
  • The second image was taken in September through October 1997 and had a similar exposure time. It unveiled about 2,500 galaxies – showing that the original deep field was not unique.
  • It would take 900,000 years for Hubble to take similar images of the entire sky!
  • After Hubble’s capabilities were upgraded with new cameras, they performed another Deep Field – this time seeing the furthest galaxies that can be seen with visible light. This was the image that revealed some of those red nugget galaxies.
  • If you take the number of galaxies seen in the deepest of the Hubble Deep Fields and assume it’s representative of the entire universe, then there are 100-200 billion galaxies in the visible universe.

These images have been treasure troves for researchers – unveiling new types of galaxies, showing us galaxies at different stages of our universe, and telling us about the history of our universe.

Credit: R. Williams (STScI), the HDF-S Team, and NASA/ESA

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