The radio features shown here, in orange, highlight the giant radio galaxy Alcyoneus, as well as the central black hole, its jets, and the lobes at either end. This feature is the largest known in the Universe to correspond to a single galaxy, and makes Alcyoneus the largest known galaxy in the Universe at present. (Credit: M.S.S.L. Oei et al., Astronomy & Astrophysics, 2022)

Giant radio galaxy Alcyoneus is now the largest known galaxy in the Universe

Move over, IC 1101. You may be impressively large, but you never stood a chance against the largest known galaxy: Alcyoneus.

Ever since other galaxies were discovered, we’ve wondered, “which is the largest?“

Most of the largest known galaxies in the Universe are found at the hearts of massive galaxy clusters, like the Hercules galaxy cluster shown here. Because galaxies assemble and grow over cosmic time, it’s the closest galaxies, on average, that are most likely to be the largest ones we observe today. (Credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute)

Most galaxies are smaller than ours, with under 1% of the Milky Way’s stars.

The low-mass, dusty, irregular galaxy NGC 3077 is actively forming new stars, has a very blue center, and has a hydrogen gas bridge connecting it to M81. One of 34 galaxies in the M81 Group, it’s an example of the most common type of galaxy in the Universe: much smaller and lower in mass, but far more numerous, than galaxies like our Milky Way. (Credit: ESA/Hubble and NASA)

In physical extent, the Milky Way’s stars span a diameter of ~130,000 light-years.

Often touted as a galaxy similar to the Milky Way, the Sunflower Galaxy, Messier 63, also displays stellar streams and debris that’s evidence for a recent, and perhaps even an ongoing, minor merger: an example of galactic cannibalism. Although we’d love to have a picture of our Milky Way from outside of it to know what our true galactic extent is, the sheer magnitude of cosmic distances make that an impossible task. (Credit: Tony and Daphne Hallas/Astrophoto.com)

Andromeda, just next door, is almost twice the size: ~220,000 light-years.

The Andromeda galaxy (M31), as imaged from a ground-based telescope with multiple filters and reconstructed to show a colorized portrait. Compared to the Milky Way, Andromeda is significantly larger in extent, with a diameter that’s approximately 220,000 light-years: comparable to double the Milky Way’s size. If the Milky Way were shown superimposed atop Andromeda, its stellar disk would end roughly where Andromeda’s dust lanes appear darkest. (Credit: Adam Evans/flickr)