October 2017 Discover

October 2017^ DISCOVER^73

this diffuse glow was the collective
light of innumerable stars that are not
gravitationally bound to any galaxy
but move freely between them. Zwicky
suggested that rogue stars like this
might be the most abundant luminous
objects in the universe.
But where did these intergalactic
vagabonds come from? While it’s
possible that some might have been
born outside of galaxies, most prob-
ably weren’t. Despite their voracious
appetites, cannibals are messy eaters.
Not all of the material torn from
victims is consumed. Some escapes into
empty space where it accumulates over
time into a sea of orphaned stars and
star clusters.
Astronomers have already found
thousands of free-roaming globular
clusters in the nearby Virgo, Fornax,
and Coma galaxy clusters, though none
yet in the Local Group. But recently,
Yale University astronomers Graziella
Di Tullio Zinn and Robert Zinn
identified a dozen possible intergalactic
globular clusters in the Local Group
after they sifted through 183,791
candidates from the Sloan Digital
Sky Survey, like miners prospecting
for gold. Further investigation will
determine whether these objects are

bona fide intergalactic globular clusters

or faint faraway galaxies that have

been misidentified.

LONG LIVE THE CANNIBAL!

Gravity’s relentless pull means that

cannibalism will continue for billions

of years to come. It’s a future of fewer

but bigger galaxies. Eventually the

carnage must end, however, as the

supply of available victims dwindles.

One hundred billion years from now,

long after the sun has expired, a fully

gorged Milkomeda will settle into quiet

old age as an elliptical galaxy, the last

survivor of the free-for-all that was

once called the Local Group.

“The universe is made of stories,

not of atoms,” wrote poet Muriel

Rukeyser. Every galaxy has its own

unique story, with harrowing tales of

cannibalism, either happening to them

or that they created, woven through-

out. Telling those stories is the goal of

astronomers — the forensic scientists

of the cosmos.^ D

Michael West is deputy director for science

at Lowell Observatory in Flagstaff, Arizona.

His most recent book is A Sky Wonderful

With Stars: 50 Years of Modern Astronomy on

Maunakea (University of Hawaii Press, 2015).

Crimes Against

Galaxies

Galaxies can suffer a variety of

fates, some more macabre than

others. Here are a few of the

colorful terms that astronomers

use to describe them:

Cannibalism The act of one

galaxy devouring another.

Harassment Coined by

astronomer Ben Moore and

colleagues in 1996, this term

refers to frequent high-speed

gravitational encounters that

occur between galaxies in dense

environments, such as galaxy

clusters. These encounters are

especially harmful to small

galaxies and can even destroy

them over time.

Shock A sudden compression of

stars or gas, the cosmic equivalent

of a gasp. Shocks can trigger the

birth of new stars or cause old

star clusters to break apart.

Strangulation When a

galaxy’s gas supply is cut off,

curtailing the birth of new stars.

(Cold gas is the fuel of star

formation.) Galaxies can die

by slow strangulation in some

environments.

Stripping The escape of stars

from one galaxy caused by the

gravitational pull of others. It

also can refer to the removal

of gas from galaxies when

they plow through the hot

plasma that permeates space in

galaxy clusters.

Threshing Similar to stripping,

this is a more gradual removal

of stars through repeated

interactions that leave a galaxy’s

innermost regions intact.

Violent relaxation Rapid

fluctuations in a galaxy’s

gravitational field during its birth

or cannibalization cause stars to

lose memory of — and stray from

— their original orbits, creating a

sort of dynamical amnesia.  M.W.

In about 4 billion

years, residents of

the Milky Way will

have a stunning view

of the approaching

Andromeda Galaxy as

it looms ever larger in

the night sky, before

eventually merging

with the Milky Way.