WHY DO
GALAXIES
I
N
A
L
I
N
A L G G ? ?
The matter in our universe forms
filaments and threads like a grand
cosmic web, tugging galaxies and
clusters into place along the way.
by Michael West
The galaxy cluster
MACS J0416.1–2403
is highly elongated.
The orientations of
its brightest member
galaxies, which appear
round and yellow in
this image, reflect this
arrangement. ESA/HUBBLE,
NASA, AND HST FRONTIER FIELDS
46
A ST R O N O M Y • MAY 2018
O
ne of the most striking features
of the distribution of matter in
the universe is its filamentary
appearance, with long, luminous
strands of galaxies woven togeth-
er into a vast cosmic web.
Nowhere is this more evident than the
Perseus-Pisces Supercluster. This colossal chain
of galaxies snakes across more than 50° of the
northern sky, fed by a network of smaller fila-
ments that resemble tributaries flowing into a
river. Embedded within these filaments are
densely populated groups and clusters of galaxies.
Between them lie immense voids.
Our own Milky Way Galaxy resides in the
outskirts of a similar structure known as the
Laniakea Supercluster. (Laniakea means “immea-
surable heaven” in Hawaiian.) Home to an esti-
mated 100,000 galaxies, it’s a tangled knot of