How large is the biggest galaxy in the Universe?

Compared to our Solar System, galaxies simply outclass us.

A logarithmic chart of distances, showing the planets, the Voyager spacecraft, the Oort Cloud, and our nearest star: Proxima Centauri. The Sun may be 109 times the diameter of Earth, but the Earth-Sun distance is over 100 times larger than the Sun’s diameter; the distance to Voyager 1 or 2 is ~100 times larger than the Earth-Sun distance; the Oort Cloud’s density peaks ~100 times farther away than Voyager 2, and the distance to the nearest stars are ~100 times farther away than even that.

The smallest known galaxy, Segue 2, only contains ~1000 stars total.

Only approximately 1000 stars are present in the entirety of the smallest dwarf galaxies such as Segue 1, 2, and 3. Gravitationally, the masses of these galaxies can be estimated at around 550,000–600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. These galaxies have the largest dark matter-to-normal matter ratios known.

Even these dwarfs extend for hundreds of light-years: billions of times larger than even the largest stars.

This illustration shows some of the largest stars in the Universe, along with the orbits of Saturn (brown ellipse) and Neptune (blue ellipse) for comparison. The stars, from left to right, are the largest blue hypergiant, yellow hypergiant, orange hypergiant, and then the largest two stars of all: the red hypergiants UY Scuti and Stephenson 2-18. The largest stars are approximately 2,000 times the diameter of our Sun, but the temperatures at the surfaces of these stars range from only a few thousand K all the way up to Wolf-Rayet stars, with temperatures of ~200,000 K.

Galaxies can obtain tremendous sizes, but illustrations are often woefully inaccurate.

A common image showing relative sizes (incorrectly) for a number of galaxies. Andromeda is too large for the Milky Way; M87 is too small for Andromeda; IC 1101 is way too small compared to M87. When it comes to comprehending distance scales, it’s vital to not share misleading images.

Our relatively typical Milky Way exceeds 100,000 light-years in diameter.

The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star formation and hint of a central bulge, may be the single most analogous galaxy for our Milky Way yet discovered. It is neither gravitationally interacting nor merging with any nearby neighbor galaxies, and so the star-formation occurring inside is driven primarily by the density waves occurring within the spiral arms in the galactic disk.

Andromeda’s diameter roughly doubles our own: 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.

Interacting galaxies, however, become tidally disrupted, vastly increasing their extent.

The Tadpole Galaxy, shown here, has an enormous tail to it: evidence of tidal interactions. The gas that’s stripped out of one galaxy gets stretched into a long, thin strand, which contracts under its own gravity to form stars. The galactic element itself is comparable to the scale of the Milky Way, but the tidal stream alone is some ~280,000 light-years long: more than twice as large as our Milky Way’s estimated size.

The Tadpole galaxy’s tail alone is 280,000 light-years long.

This galaxy, UGC 2885, also known as Rubin’s galaxy, is the largest spiral galaxy ever discovered, and possesses about 10 times as many stars as the Milky Way. UGC 2885 is severely gravitationally disrupted. At an estimated 832,000 light-years across, it is arguably the largest known spiral galaxy, although its tidal arms and distorted shape are likely temporary on cosmic timescales.

Severely disrupted, UGC 2885 is our largest spiral: 832,000 light-years in extent.

Giant elliptical galaxy NGC 584, shown here, was discovered and recorded in 1785, and is located approximately 62 million light-years away. Although it was not known to be an extragalactic object until the 1920s, it was briefly the most distant object known and recorded until NGC 1 was identified a few months later.

Elliptical galaxies, however, are the largest galaxies of all.

Markarian’s chain, shown here, represents an alignment of large, massive galaxies found within the Virgo cluster. There are approximately 1,000 large galaxies in the Virgo cluster, a large fraction of which were discovered way back in the 18th century. The Virgo cluster is located some 50–60 million light-years away from our Milky Way and is the largest concentration of galaxies in the extremely nearby Universe, containing many giant ellipticals.

Messier 87, the Virgo supercluster’s largest galaxy, is 980,000 light-years across.

Located approximately 55 million light-years from Earth, the galaxy M87 contains an enormous relativistic jet, as well as outflows that show up in both the radio and X-ray. This optical image showcases a jet; we now know, from the Event Horizon Telescope, that the rotation axis of the black hole points away from Earth, tilted at about 17 degrees.

The Coma Cluster’s biggest, NGC 4889, spans 1,300,000 light-years in diameter.

The two bright, large galaxies at the center of the Coma Cluster, NGC 4889 (left) and the slightly smaller NGC 4874 (right), each exceed a million light years in size. But the galaxies on the outskirts, zipping around so rapidly, point to the existence of a large halo of dark matter throughout the entire cluster. The mass of the normal matter alone is insufficient to explain this bound structure.

Meanwhile, the Phoenix Cluster’s brightest central galaxy measures 2.2 million light-years across.

The brightest cluster galaxy of the Phoenix cluster, shown at left from the South Pole Telescope and at right from Blanco/MOSAIC-II optical/infrared imagery, is one of the largest galaxies of all, still rapidly forming stars at hundreds of times the rate of our own Milky Way.

But the biggest galaxy of all? That’s IC 1101.

The giant galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5 million light years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all. As massive and impressive as this galaxy cluster is, it’s unfortunately difficult for the Universe to make something significantly larger owing to its finite age and the presence of dark energy.

Half its light is contained within a central, 2 million light-year radius.

This image shows a gravitational lensing map overlaid atop cluster Abell 2029. At the center of Abell 2029, the largest known galaxy in the Universe, IC 1101, can be seen. Although its half-light radius, or the radius within which half of the arriving light comes from, is ~2 million light-years, the full visible diameter of the galaxy ranges from 5.5 to 6 million light-years.

It spans a full 5.5 million light-years across: greater than the Local Group’s extent.

Our Local Group of galaxies is dominated by Andromeda and the Milky Way, there is debate over which one dominates in terms of gravitation. While Andromeda appears to be larger in physical extent and have more stars, it may yet be less massive than we are. If the galaxy IC 1101 were shown next to our Local Group, it would be comparable to the size of this image in its full extent.

Showing the true relative sizes of galaxies highlights our cosmos’s diversity.

Composite of galaxies from the smallest to the largest, shown (approximately) actual size. The giant elliptical galaxy at the heart of cluster Abell 2029, IC 1101, is the largest known galaxy in the Universe, at least in terms of stellar extent. It is much, much larger than the Milky Way or Andromeda (or any spiral galaxy), but also towers over even other typical giant ellipticals.

Only large-scale galactic jets,

The radio data from LOFAR and GMRT clearly shows the features of a coherent, bipolar, linear black hole pair of jets that extend for 23-24 million light-years in extent. This feature, named Porphyrion, is the largest black hole jet ever seen.

enormous galaxy clusters,

This map shows the El Gordo galaxy cluster, as imaged by Hubble, with a mass map overlaid atop it. The mass was inferred from a combination of weak and strong gravitational lensing effects, while other, complementary studies have shown that this cluster is a merger between two smaller clusters. All told, there are between 2.1 and 3.0 quadrillion (10^15) solar masses worth of matter in the El Gordo cluster.

and large-scale cosmic features surpass them.

The Sloan Great Wall is one of the largest apparent, though likely transient, structures in the Universe, at some 1.37 billion light-years across. It may just be a chance alignment of multiple superclusters, but it’s definitely not a single, gravitationally bound structure, as dark energy is in the process of driving it apart. The galaxies of the Sloan Great Wall are depicted at right.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.