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Starts With A Bang

How Big Is The Universe’s Largest Galaxy, Really?

This optical (blue) and NASA’s Chandra X-ray (red-orange) composite image shows Abell 2029, a cluster of galaxies where the galaxy at the center, IC 1101, is the largest known galaxy in the Universe. (OPTICAL: NOAO/KITT PEAK/J.USON, D.DALE; X-RAY: NASA/CXC/IOA/S.ALLEN ET AL.)

The last image puts it all in perspective.


Compared to what we find in our Solar System, galaxies are truly enormous.

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. (NASA / JPL-CALTECH)

The smallest known galaxy is Segue 2, with only about ~1000 stars inside.

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. (MARLA GEHA AND KECK OBSERVATORIES)

These stars are spread out over ~500 light-years: billions of times the physical size of any individual star.

Globular clusters, like Omega Centauri, have some of the highest stellar densities ever observed. Through a modest telescope, they appear like dense fuzzy balls of light. But if we take a very sharp, high-resolution photo, such as with Hubble, we can find that even in these densest regions, there are still only a few hundred stars, at most, within each cubic light year. (NASA, ESA, AND THE HUBBLE SM4 ERO TEAM)

Galaxies can get much larger, but many “relative sizedepictions are 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. (ASTRO BOB / BOB KING / DULUTH NEWS TRIBUTE)

Our own Milky Way, typical of modern spirals, is slightly over 100,000 light-years across.

From inside our Milky Way, we cannot get a good picture. Here, the Southern Pinwheel Galaxy, M83, shares many features with our own galaxy. It has spiral arms, new star formation, a central bulge and bar, and arms and spurs shooting off of the central structure. Unlike our Milky Way, however, the Southern Pinwheel Galaxy is only about ~27,000 light-years in radius: about half that of the Milky Way. (GÁBOR TÓTH / CC BY-NC-ND / ASTRO.I-NET.HU)

Andromeda’s diameter is roughly double ours: 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. (ADAM EVANS / CC-BY-2.0)

But interacting galaxies can 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 mail 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. (NASA, H. FORD (JHU), G. ILLINGWORTH (USCS/LO), M. CLAMPIN (STSCI), G. HARTIG (STSCI), THE ACS SCIENCE TEAM, AND ESA)

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 at approximately 800,000 light-years in diameter. It has approximately 10 times as many stars as the Milky Way inside of it. It is truly a G.O.U.S.: a galaxy of unusual size. (NASA, ESA, AND B. HOLWERDA (UNIVERSITY OF LOUISVILLE))

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

The low-surface-brightness galaxy 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. (KITT PEAK / ZAGURSKY & MCGAUGH, 2008)

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

A selection of approximately 2% of the galaxies in 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. (JOHN BOWLES OF FLICKR)

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. (ESO)

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. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA)

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. (R. WILLIAMSON ET AL., ASTROPHYSICAL JOURNAL 738(2):139 · AUGUST 2011)

But the biggest one 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. (DIGITIZED SKY SURVEY 2, NASA)

Half the light is contained within a 2 million light-year central 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. (J. MCCLEARY ET AL., APJ, 893, 1, 8 (2020))

Its full span is 5.5 million light-years across: nearly double the Local Group’s full 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 (ANDREW Z. COLVIN)

The true relative size differences highlight galactic 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. It is much, much larger than the Milky Way or Andromeda (or any spiral galaxy), but also dwarfs even other typical giant ellipticals. (E. SIEGEL)

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

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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