SCIENCE

The sky is brighter than astronomers imagined | by Ethan Siegel | Starts With A Bang! | Apr, 2025

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Around galaxy UGC 00180, a great stellar halo consisting of approximately four billion stars’ worth of matter can be discovered by optimizing this telescopic image for low surface-brightness features. It turns out that a great many galaxies, maybe even all galaxies, have such a stellar halo, but wanton oversubtraction in calibrating telescope images can render this feature otherwise undetectable. (Credit: Gran Telescopio Canarias (GTC) / Gabriel Pérez (IAC))

The most famous Hubble images show glittering stars and galaxies amidst the black backdrop of space. But more was captured than we realized.

Although we’ve now firmly entered the JWST era in astronomy, our deepest views of the faintest objects of all still come courtesy of the Hubble Space Telescope. Now working in its 35th year since launch, Hubble has spent more time viewing certain specific, dedicated regions of sky than any observatory ever, achieving unprecedented depths in the process. The deepest views of the Universe ever remain the Hubble Ultra Deep Field, with a cumulative total of 11 days of observing time (across all wavelengths of light), and a portion of the Ultra Deep Field that was cumulatively imaged for roughly twice as long: the Hubble eXtreme Deep Field.

Even though JWST has also viewed this region, the shorter observing times mean that, in many ways, Hubble’s views are more sensitive to fainter signals, particularly at optical (visible light) wavelengths. However, there’s something that’s a little bit peculiar about the bright, massive galaxies that appear in these Hubble images: it’s as though their light abruptly stops at a particular edge for each galaxy, rather than diffusely continuing the way we see light continuing within most nearby, extended galactic objects. A 2018 reanalysis of the raw data from these fields showed that the extended, diffuse light at the outskirts of these galaxies is real, and that the methods use to produce the more-famous images commonly shown are artificially removing starlight that’s very, very real. Here’s what everyone should know about it.

The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal, but when we extrapolate over the entire observable Universe, we expect to obtain a total of ~2 trillion galaxies. (Credit: HUDF09 and HUDF12 teams; Processing: E. Siegel)

Above, you can see the Hubble Ultra Deep Field, with the Hubble eXtreme Deep Field inlayed within it, in all their glory. These views represent some of the deepest images of the ultra-distant Universe ever taken, where many of the galaxies shown here are billions or even tens of billions of light-years away, with the most distant of all being around ~32 billion light-years distant. This is due to the combinations of facts that:

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