How Many Galaxies Can You Count in This Picture?

For three years, the Hubble Space Telescope spent hundreds of hours peering at distant galaxy clusters and their surrounding areas, looking for the hazy light of the earliest stars in the universe. The mission, known as Frontier Fields, produced dozens of photos of dark backgrounds riddled with galaxies of all shapes, bright jewels so numerous you’d think they were all Photoshopped in.

After 630 hours of observation time, Frontier Fields has come to an end. The mission released its final image Thursday, and it’s no exception. The photo shows Abell 370, one of the six galaxy clusters the program targeted in its observations, and the last to be imaged in such detail. Abell 370 is located 6 billion light-years away in the constellation Cetus, and is home to hundreds of galaxies. Here’s the complete image in all its glory, a combination of optical and infrared measurements:

Not all of the galaxies pictured in this image are actually inside Abell 370. Some of the arcs and streaks of blue and gold light are actually the distorted images of galaxies behind Abell 370. The long, glowing arc in the lower left of the photo, for example, is actually two images of a single spiral galaxy that sits behind the cluster. These objects appear to us this way because of a phenomenon known as gravitational lensing. When light traveling from these distant galaxies reaches Abell 370, the giant cluster’s gravitational pull bends it. The light gets magnified and swings around the cluster, sometimes splitting and going in different directions. When it reaches Hubble, it can look warped and even appear more than once in the same picture.

The Frontier Fields program targeted Abell 370 and five other galaxy clusters because they’re good at gravitational lensing. By observing these galaxy clusters, astronomers can zoom in on distant galaxies behind the clusters that are too faint to be seen by Hubble alone. The magnifying effect of gravitational lensing allows them to detect galaxies up to 100 times fainter than those in the Hubble Ultra Deep Field, the famous image of thousands of galaxies that in 2004 was the deepest image of the universe ever captured. Frontier Fields can reveal galaxies as they were just a few hundred million years after the Big Bang.

Hubble previously imaged Abell 370 in detail in 2009, but used fewer observation hours to create the photo:

Clearly, more observation time paid off.

The Frontier Fields program's images of early galaxies allow astronomers to study how the first galaxies formed 13 billion years ago. It also allows them to measure the distribution of normal matter—like stars and dust—and dark matter, the mysterious, invisible particles that astronomers believe make up most of the universe, measurable only by perceived effects on gravity. Astronomers say Abell 370 contains two clumps of dark matter, which suggests the galaxy cluster was formed when two smaller clusters merged.

Earlier this spring, astronomers from Yale used data from Frontier Fields to study blobs of dark matter inside galaxy clusters and produce a topographical map of dark matter.   

Hubble’s gaze can only go so far. To see galaxies even fainter and younger than the ones Frontier Fields observed, astronomers need telescopes that can detect light at infrared wavelengths. For that, they’ll have to wait for Hubble’s scientific successor, the far more powerful James Webb Space Telescope, to launch into orbit in 2018. If Hubble’s latest photos look Photoshopped to you, just wait until Webb gets out there.