Thanks to its ability to detect ultraviolet (UV) light, Hubble was able to put together this panoramic image of our old, star-bursting universe. The composite picture spans vast periods of time and space and has about 15,000 galaxies, of which there are 12,000 active, young stars. The UV light reveals stars that formed in the energetic years after the Big Bang, allowing astronomers to follow stellar evolution for 11 billion years.
Incorporating light from galaxies not only paints a complex picture of our universe, but it also gives us a glimpse of our heavenly past in the first hand. The amount of time that a galaxy needs to reach us depends on how far it is. The light of the Andromeda galaxy, for example, takes 2.5 million years to reach us since the 2.5 million light-years have been removed from the earth. This means that we always see our galactic neighbor as it was 2.5 million years ago. For more distant galaxies, however, it can take billions of years for their light to reach us. When it finally comes, we see historical galaxies as they appeared billions of years ago.
With galaxies in the neighborhood, we can pick up a wide range of light, for example visible, infrared and UV light, which gives us a good picture of their formation, evolution and star population. But the farther away the galaxy, the more difficult it is to see at several wavelengths.
Galaxies emit a high percentage of UV light as they form new stars. But due to redshift – where the expansion of the universe causes the light of distant objects to be stretched and redder on the light spectrum – UV light from extremely distant galaxies is stretched to the infra-red part of the spectrum.
And only the infrared light of the brightest of those distant galaxies is easy to detect. In order to better understand the vague, distant galaxies that we can not see, astronomers try to study their analogs instead: small, vague nearby galaxies. But these are also difficult to see – especially from the ground, because the earth's atmosphere blocks UV light.
Fortunately, the Hubble Deep UV (HDUV) Legacy Survey uses the ability of the space telescope to easily capture UV light from above the atmosphere. The initiative, which started in 2014, searches the air for nearby UV-emitting galaxies that are active population stars.
By identifying these galaxies, astronomers can study how their former counterparts evolved from the young, star-forming areas that existed shortly after the Big Bang, to the adult galaxies we see today.