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Starts With A Bang

What’s the largest galaxy in the Universe?

We’ve discovered some behemoths, but one outclasses them all.

“There is always a heavy demand for fresh mediocrity. In every generation the least cultivated taste has the largest appetite.” –Paul Gauguin

Our Milky Way contains some 400 billion stars, spanning 100,000 light years in diameter.

Galaxies come in a variety of types and sizes. While the Milky Way may be impressive from our location within it, it barely registers at all on a list of the largest galaxies. Image credit: Los Cumbres Observatory.

Yet compared to other galaxies, it’s not even especially large.

The Andromeda Galaxy resides in our local group, and is perhaps twice as large in diameter as our Milky Way. Image credit: Adam Evans / flickr.

In our own local group, the Andromeda galaxy is significantly larger, reaching 220,000 light years across.

Severely disrupted galaxies, like NGC 6872, can extend for many times farther than a quiet galaxy that hasn’t had a major gravitational interaction. Image credit: ESO / VLT, Judy Schmidt.

Interacting spiral galaxies can have their arms greatly extended and disrupted, with NGC 6872 spanning 522,000 light years from tip-to-tip.

The galaxy Malin 1 is one of the largest spiral galaxies ever discovered, at 650,000 light years (199 kpc) in diameter. Image credit: Boissier/A&A/ESO/CFHT.

Ultra-low surface brightness galaxies can see their stars extend even farther, with Malin 1 reaching 650,000 light years across.

The low-surface-brightness galaxy UGC 2885 is also severely gravitationally disrupted, making it arguably the largest known spiral galaxy. Image credit: Kitt Peak / Zagursky & McGaugh, 2008.

Unsurprisingly, a disrupted low surface brightness galaxy, UGC 2885, is the largest spiral known at 832,000 light years.

The galaxy NGC 262, at the image’s center (and in detail, inset), is only about the size of the Milky Way, but its hydrogen halo extends more than 10 times as far, as shown in blue in the main image. Image credit: NRAO/AUI, with the VLA (main); SDSS (inset).

Hydrogen gas and dark matter halos can continue beyond the stars, like in NGC 262, extending over 1,000,000 light years.

The giant elliptical near the center of the Coma Cluster, NGC 4874, is typical of the largest, brightest galaxies found at the centers of the most massive galaxy clusters. Image credit: ESA/Hubble & NASA.

But the largest and most massive galaxies aren’t spirals, but supergiant ellipticals, like NGC 4874 in the Coma Cluster.

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. Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona.

Coma has two enormous core ellipticals, with the even larger NGC 4889 reaching 1.3 million light years in diameter.

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. Image credit: R. Williamson et al., Astrophysical Journal 738(2):139 · August 2011.

Even larger ones reside inside more massive galaxy clusters, like the brightest galaxy in the Phoenix cluster, at 2.2 million light years.

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. Image credit: Digitized Sky Survey 2, NASA.

Still, IC 1101 beats them all, extending for 5.5 million light years.

Early, intense star forming galaxies, like the Baby Boom galaxy shown in green/red here and imaged in the infrared, can form up to 4,000 new Sun-like stars every year. This behavior, from more than 12 billion years ago, can lead to the largest galaxies of all by present times. Image credit: NASA/JPL-Caltech/P. Capak (Spitzer Science Center).

Periodic star formation, mergers, and gravitational growth over time inevitably lead to these cosmic behemoths.

Mostly Mute Monday tells the scientific story of an astronomical object, class or phenomenon in visuals, images and no more than 200 words.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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