Gravitational Lens and Galaxy Cluster, MACSJ 1206

Four and a half billion light years away in the constellation Virgo, scores of galaxies have been drawn together by the mutual pull of their gravity. NASA's Hubble Space Telescope peers through the depths of space and eons of light travel-time separating us and the galaxy cluster, known as MACS 1206, allowing scientists to study the distribution of mass in our universe at truly astronomical scales.

The eye is drawn toward the image's center: a glinting, glowering red pupil at the cluster's core. This central object is a giant elliptical galaxy plump with billions of old, reddish suns, surrounded by a thinner halo of stars. Moving outward, we find disk-shaped spiral galaxies in the image. Above and to the left of the center, two blue-white spiral galaxies pose face-on toward Hubble, both showing off a defined structure of arms encircling their central bulges.

These two main galaxy types, seen at different orientations and distances, occur again and again in this stunning Hubble image. Except for the lone diamond-spiked star in the bottom left, each and every extended bright object is a galaxy made up of roughly one hundred billion stars. The bluer galaxies have stars actively forming within them and, consequently, host groups of young blue stars that contribute to their overall hue. In contrast, red galaxies – especially those elliptical galaxies like the center one – are more stable in their behavior, with very few little recent star formation.

Though studded with striking objects, this isn't merely a hodgepodge of galaxies from cluster MACS 1206. This image has a radial symmetry around its center, creating a mesmeric effect that draws in the viewer. The circular pattern is evidence of an effect called gravitational lensing. Gravity from the cluster's immense mass bends the space around it, causing the images of more distant galaxies directly behind the cluster in our line of sight to be warped and cast into double images and arc-like smears of light. Perhaps the most obvious example of these optical distortions is the orange streak to the right of the image center.

Astronomers know that invisible "dark matter" vastly outweighs regular matter in clusters like MACS 1206, meaning that the luminous objects we see – all of the galaxies here – are surrounded by a much bigger cloud of dark matter. The bright galaxy at the heart of the lens lurks like a spider in the center of its web, a visible reminder of the unseen net of dark matter it rests in. Unwittingly, light from faraway galaxies flies through the cluster, only to become twisted up in the web of the cluster's lensing gravity.

Credit: NASA, ESA, M. Postman (STScI), the CLASH Team, and the Hubble Heritage Team (STScI/AURA)