The largest galaxy ever discovered is so wide it would take light 1.7 million years to cross it. Now, astronomers have finally measured its true edge. The galaxy, called IC 1101, is a cosmic monster sitting at the heart of a cluster called Abell 2029, about a billion light-years away. A team led by Carlos Marrero-de la Rosa at the Instituto de Astrofísica de Canarias used the Isaac Newton Telescope in the Canary Islands to take the deepest images ever captured of the galaxy. What they found confirms IC 1101 as the biggest single galaxy known to exist, stretching about 520 kiloparsecs across. One kiloparsec equals roughly 3,260 light-years, which is why the number translates to an almost unimaginable size.
Galaxy size is tricky to measure. The biggest galaxies sit at the center of dense clusters, surrounded by a fog of diffuse stars and gas that makes their edges hard to see. The starlight at the far ends of IC 1101 is so faint it blends into the background, like trying to see the last wisps of fog in morning sunlight. To solve this, Marrero-de la Rosa's team built a careful model of the telescope's behavior using dozens of calibration stars, then carefully subtracted the scattered light from more than 250 individual stars in their images. Only then could they see the galaxy's true outline. The work was published on the arXiv preprint server in July.
For scale, our own Milky Way spans only about 30 kiloparsecs. IC 1101 stretches roughly 17 times wider. Its stars alone weigh about 3.4 trillion times as much as our Sun. That is more than double the total mass of the entire Milky Way, including the invisible dark matter that holds our galaxy together.
Beyond the main edge, the team spotted eight large, faint structures stretching outward. Two of them line up with known disturbances in the hot gas surrounding the galaxy, suggesting IC 1101 is still actively pulling in smaller galaxies rather than sitting quietly. The researchers wrote that the galaxy's outskirts "show clear signatures of ongoing mass assembly." In other words, this cosmic giant is still growing. A previous X-ray study found evidence that IC 1101 may have collided with another galaxy group roughly 2 to 3 billion years ago, leaving ripples in the gas around it.
The finding reminds us that even the most extreme objects in the universe are still in motion, still changing. IC 1101 has been smaller galaxies for billions of years, and according to these observations, the feast is not over yet.