Astronomy

billion light-years

8.25 8.5

T

he Milky Way sits near the middle

of an assembly of galaxies called the

Local Group. This gathering, 10 mil-

lion light-years wide, lives on the edge

of a collection of galaxy clumps called

the Local, or Virgo, Supercluster.

British astronomers and father-son duo

William and John Herschel pointed their

telescopes all over the sky in the 18th and

19th centuries, taking samples of objects

they called “nebulae.” In using the word

nebula, they didn’t always mean what we

do today. They merely meant a celestial

object that wasn’t a comet but nonetheless

appeared fuzzy. And they believed all these

nebulae lay within the Milky Way.

In their 1864 catalog, John noted that

more nebulae populated the area around

the constellation Virgo. (French astrono-

mer Charles Messier also had noted such

structure a century earlier.) The reason,

though, remained nebulous.

In 1920, the “Great Debate” attempted

to resolve the nature of nebulae. American

astronomers Harlow Shapley and Heber

Curtis argued about whether these objects

represented nearby cloudiness or whether

they were galaxies in their own right, ren-

dered fuzzy by distance. The latter inter-

pretation won out. But why did these

galaxies cluster around Virgo? Was it

chance or something more?

In the 1950s, French astronomer Gérard

de Vaucouleurs observed how these Virgo

galaxies traveled through space. Curiously,

they seemed to be moving away from us at

the same speed and lay fairly close to one

another and to us. They were, in astron-

omy parlance, “dynamically related.” In

1953, he christened this crowd the Local

Supergalaxy. Five years later, he changed it

to the Local Supercluster.

As telescopes became more powerful,

astronomers could take larger surveys, cat-

aloging more galaxies and their motions.

These pixeled together a bigger picture: a

concentration of galaxies along a plane in

space. Just as most of the Milky Way’s stars

reside in a thin equatorial disk, so most of

the galaxies in the Local Supercluster lie

along its equator. Science had spoken —

stars cluster in galaxies, galaxies cluster

in clusters, and clusters cluster in super-

clusters. And the center of our supercluster

shared a spot in the sky with Virgo.

If you placed 1,000 Milky Ways end to

end, they would span 100 million light-

years — the size of our supercluster. Two-

thirds of the bright galaxies live in a disk

near the equator, around which the remain-

ing third spread like the globular clusters

around our galaxy. In total, a mass equiva-

lent to 10 quintillion Suns fills our cosmic

supercity. Much of that mass, however, isn’t

luminous. Just as dark matter dominates

the universe, it also fills the supercluster.

Still, lots of pretty light-emitting objects

live next door, cosmically speaking.

Meet the neighbors

Closest to us, perhaps 11 million light-

years away, live the Maffei I and II groups.

Although nearby, they went unnoticed

until 1968 because they lie right behind

the Milky Way’s plane, so the material that

makes up our home drowns out the light

from Maffei’s constituents. But Italian

astronomer Paolo Maffei noticed some-

thing strange when he looked at IC 1805

(a true nebula). A nearby object shone in

infrared light. He guessed it was a galaxy,

obscured by the Milky Way. In the past

20 years, astronomers have found 17 more

associated galaxies.

A million light-years farther away, the

Sculptor Galaxy Group is easier to pick out.

From 2011 to 2013, a group of astronomers

led by Tobias Westmeier of the International

Centre for Radio Astronomy Research in

Australia studied how high-velocity clouds

(HVCs) of hydrogen enrich member galax-

ies NGC 55 and NGC 300. HVCs don’t

rotate with galaxies; instead, they come

from outside and stream in like missiles.

Astronomers believe they may provide fuel

to produce more stars as galaxies run out

of their own gas. But HVCs have to come

from somewhere.

At the time of Westmeier’s study, two

theories dominated, and he wanted to

find which was true. In one, HVCs are

clumps of gas shot out of a galaxy by super-

novae; in the other, they are dwarf galaxies

Small groups

combine with larger

clusters to form

a vast network of

interconnected

galaxies that

spans 110 million

light-years.

by Sarah Scoles

THE VIRGO SUPERCLUSTER

Our 100,000 closest

galaxies

Former Astronomy Associate Editor Sarah

Scoles is a freelance writer currently living in

the San Francisco area.