A newly-released picture taken by the Hubble Telescope is adding more color to the Hubble Ultra Deep Field (HUDF) image by detecting thousands of galaxies in the ultraviolet spectrum. The study, called the Ultraviolet Coverage of the Hubble Ultra Deep Field (UVUDF), directly imaged stars and other celestial bodies that would have been impossible to observe on the ground, and gives astronomers critical information that will prove useful as the launch of the more powerful James Webb Space Telescope approaches.

Before Hubble was launched into orbit in 1990, astronomers could only observe celestial bodies up to seven billion light-years away. Over the last two decades, we've made plenty of progress: the infrared cameras in Hubble's Ultra Deep Field survey have detected the very earliest galaxies, formed as early as 13 billion years ago; and in the meantime, ultraviolet telescope facilities on Earth have also allowed us to investigate star formation taking place in nearby galaxies.

But there's still a big gap in our knowledge. Stars that are newly formed, very massive or very hot all emit light in the ultraviolet spectrum, which is mostly filtered out by the Earth's atmosphere. As a consequence, we know relatively little about the period of time between five and ten billion years ago, when most of the stars in our universe were formed.

The Ultra Deep Field infrared and visible light image, on the left, and the newly released ultraviolet image, on the right (Image: NASA/ESA)

Luckily, though, Hubble has been equipped with the right tools to fill in the blanks. Using its Wild Field Camera 3, Hubble focused on the same patch of sky that had previously been imaged in the visible and infrared light spectra by the Ultra Deep Field survey, but this time gathering data over the ultraviolet spectrum. After collecting data over 841 orbits, the space telescope gave us a direct and unprecedented look at galaxies that would have otherwise been impossible to detect, in what is one of the most colorful images it has produced to this date.

As astronomy and physics professor Rogier Windhorst told Gizmag, the picture was taken by combining no less than 3,185 exposures, each with an average duration of 669 seconds, for a total exposure time of over 24 days per fully-exposed pixel.

Observing the skies at these wavelengths is allowing astronomers to understand which galaxies still appear to be forming stars, and where in the galaxy those stars are forming. Scientists can then use this information to better understand how galaxies form and evolve over time.

Hubble is currently the only telescope equipped to obtain this type of data over the UV spectrum. Combining its images with the infrared images that will come from the James Webb Space Telescope (JWST) will allow us to understand much more about the history and composition of our universe, from the Big Bang up to the most recent days.

"Because its mirrors are thinly gold-coated for maximum infrared performance, JWST will not be able to image bluewards of about 600 nm, the wavelength of yellow" Prof. Windhorst told Gizmag. "Therefore, high-resolution diffraction-limited UV and blue imaging is something unique to Hubble, and something Hubble can and must do before JWST flies and before we no longer have Hubble with us."

The James Webb Space Telescope) is scheduled for launch in 2018. With an aperture of 6.5 meters (21 ft 4 in) to Hubble's 2.4 meters (7 ft 11 in), this extremely powerful instrument will use its mirrors to, among other things, look for faraway exoplanets and investigate the epoch of First Light, or the first 500 million years of star-formation after the Big Bang.

"If JWST were really to spend 600 hours on a single 'Hyperdeep' field, it would go at least six times deeper than Hubble," Windhorst told us. "For astronomers, that would be a depth fainter than 32 mag, or the brightness of 0.1 firefly if it were seen from the distance of the Moon (with the Moon assumed not there for this comparison)."

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