How Big Can A Galaxy Get?

What’s nan biggest postulation successful nan universe?

Deep surveys of nan entity person turned up galaxies vastly larger than our own. Are location moreover bigger ones yet to beryllium seen?

By Phil Plait edited by Lee Billings

This image from nan Hubble Space Telescope shows UGC 2885, besides known arsenic Rubin’s galaxy, which is 1 of nan largest known spiral galaxies. It is 2.5 times wider than our Milky Way and contains 10 times arsenic galore stars—yet it’s still acold smaller than galore elliptical galaxies.

NASA, ESA, and B. Holwerda (University of Louisville)

Galaxies, for illustration truthful galore different astronomical objects, are amazingly difficult to define.

Here’s a hand-wavy attempt: a postulation is simply a postulation of stars, gas, particulate and acheronian matter each held together by communal gravity. That useful for astir galaxies but starts to get shaky erstwhile you look much closely. Some galaxies don’t person overmuch acheronian matter. Some have small aliases nary state and dust. Others barely person immoderate stars!

Perhaps worse, this meaning is vague astir size. At nan little extremity of nan scale, immoderate very mini galaxies could beryllium mistaken for globular clusters—agglomerations of up to a fewer cardinal stars that shape alongside galaxies. The globular cluster Omega Centauri, for example, whitethorn person started retired arsenic a mini postulation earlier being stripped of galore stars by an brushwood pinch our ain ample spiral galaxy, nan Milky Way.

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But what astir nan precocious extremity of nan scale? How big tin a postulation get?

The maddening reply is that nary 1 really knows. The Milky Way is beautiful large arsenic these objects go, pinch a starry disk astatine slightest 100,000 light-years across. Our galaxy, like astir others we tin analyse successful adjacent detail, besides has a overmuch larger surrounding halo of hot, diffuse state that’s radiated pinch stars. But we’ll time off halos retired of this chat because they’re truthful difficult to observe beyond our nearest galactic neighbors; instead, for determining width, let’s instrumentality to a galaxy’s much sparkling and star-rich regions.

But moreover past it’s difficult. Galaxies deficiency chopped edges for illustration those of rocky planets aliases moons. Instead a galaxy’s distribution of stars thins retired farther from nan center. That attenuation makes for fuzzy boundaries—all nan much truthful because arsenic a galaxy’s stars get little galore pinch distance, they blend into nan foreground of stars we regard done from our position wrong nan Milky Way. This effect is strongest for elliptical galaxies, which look for illustration fluffy cosmic fabric balls, but it’s a problem for gauging nan size of spiral aliases disk galaxies, too.

Astronomers person adopted somewhat arbitrary modular units to reside this issue. For example, they sometimes calibrate utilizing a galaxy’s “surface brightness,” a measurement of really overmuch ray from nan postulation falls wrong a mini quadrate connected nan entity (usually one quadrate arcsecond). Observers tin past estimate nan boundaries of a galaxy’s main assemblage arsenic wherever this aboveground brightness blends into nan brightness of nan inheritance sky.

A last complicating facet is that a galaxy’s evident size is simply a usability of its existent dimensions positive its region from america successful space; a postulation mightiness beryllium physically immense but truthful fantastically acold distant that it appears mini connected nan sky. We tin measurement a galaxy’s redshift to explain its existent size—although astatine sufficiently ample separations (many billions of light-years), uncertainties astir nan description history of nan beingness itself complicate things, truthful immoderate attraction must beryllium taken.

Still, moreover pinch each these impediments, we tin reliably measurement nan sizes of galore galaxies to spot conscionable really large they tin get.

I person a mates of favorites. Malin 1 is astir a cardinal light-years from Earth, an immense distance. At first it looks for illustration a alternatively normal, moreover boring, face-on spiral postulation immoderate 30,000 light-years wide. But highly long-exposure images uncover that Malin 1’s spiral arms see very dim and intricate extensions that span a staggering 650,000 light-years, making this postulation astir six times wider than nan Milky Way! That makes Malin 1 among nan largest spiral galaxies known, and nan first specified “giant debased aboveground brightness” postulation ever discovered. Several are now cataloged, but because these galaxies are truthful faint and difficult to find, galore much astir apt lurk retired there, arsenic yet unseen.

It’s unclear really they get truthful big. If they grew by galactic mergers, you’d expect their large-scale structures to beryllium disturbed, but that doesn’t look to beryllium nan lawsuit pinch Malin 1’s delicate spiral arms. There besides seems to beryllium a dearth of different adjacent galaxies connected which it could feed. But possibly that’s a clue: possibly Malin 1 lacks galore smaller neighbors because it already merged pinch them, truthful mildly aliases truthful agelong agone that its majestic spirals now deficiency evident distortions.

Another immense spiral is UGC 2885, aliases Rubin’s galaxy. It’s a small complete 230 cardinal light-years distant, and is astir 450,000 light-years wide. Like Malin 1 it seems to beryllium comparatively isolated from different galaxies, truthful again, its overgrown size is thing of a mystery.

There are different very ample spirals, specified arsenic the Tadpole galaxy aliases the Condor galaxy, that are intelligibly undergoing a collision pinch different large companion. Galactic collisions tin stretch spiral arms retired into agelong tidal tails, boosting a galaxy’s size. But because these effects are comparatively temporary, specified cosmic train wrecks should beryllium seen much arsenic exceptions alternatively than nan norm erstwhile it comes to establishing an precocious limit connected postulation sizes.

Ellipticals tin turn huge, arsenic well—much bigger than immoderate known spiral aliases disk galaxy. And, a spot for illustration nan colliding spirals we conscionable discussed, an elliptical tin person a different size depending connected erstwhile you’re seeing it. The biggest thin to beryllium astatine nan centers of immense clusters of galaxies, immoderate pinch galore hundreds of members. This is nary coincidence; galactic mergers are much apt astatine nan cluster’s gravitational center, and astronomers deliberation nan chaos of specified collisions helps springiness ellipticals their characteristic spheroidal shape. ESO 383-76 sits successful nan halfway of nan cluster Abell 3571 and is an elongated elliptical astir 1.8 cardinal light-years wide, making it acold larger than nan Milky Way. Astronomers person besides spotted immoderate tremendous ellipticals that are conspicuously alone—ESO 306-17, for example, is much than a cardinal light-years crossed but has nary different galaxies nearby. It whitethorn beryllium nan extremity consequence of a mini cluster’s worthy of galaxies merging together complete time, truthful that yet all form a azygous gigantic elliptical.

I person to wonder, though, if moreover larger galaxies still await america location retired successful nan universe, avoiding find truthful acold by kindness of being truthful diffuse and dim. New telescopes specified arsenic the Vera C. Rubin Observatory aliases the soon-to-be-launched Nancy Grace Roman Space Telescope whitethorn yet find more, pinch their wide fields of position and capacity to spot highly faint objects.

We whitethorn not cognize really each elephantine galaxies turn truthful large, but nan champion measurement to fig it retired is to find arsenic galore arsenic imaginable and study them. Stay tuned. Not everything successful nan cosmos tin hide from america forever.