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48 ASTRONOMY • MAY 2018
filaments stretching half a billion light-years from
end to end. University of Hawaii astronomer Brent
Tully, whose team discovered Laniakea in 2014, lik-
ened it to “finding out for the first time that your
hometown is actually part of a much larger country
that borders other nations.”
Everywhere we look, galaxies trace out the paths
of these filaments. But it turns out galaxies don’t just
illuminate the cosmic web — they’re also shaped by it.
When the stars align
In 1874, less than a decade after the Civil War
ended and long before anybody knew for certain
what galaxies were, astronomer Cleveland Abbe
wondered how “nebulae,” as galaxies were known
in those days, are oriented in space.
To answer this question, Abbe chose 59 of the
most extended nebulae in Sir John Herschel’s
famous Catalogue of Nebulae and Clusters of Stars
and measured their direction of elongation. His sur-
prising conclusion was that the nebulae appeared to
favor certain orientations with respect to the Milky
Way. But his study drew little attention and was
soon forgotten; Abbe moved on to a more successful
career in meteorology.
Forty years later, American astronomer Edward
Fath revisited Abbe’s question. After measuring the
orientations of hundreds of galaxies on photo-
graphic plates taken at Mount Wilson Observatory,
he reported in 1914 that they “appear to be oriented
at random.”
Decades of lively debate followed. English ama-
teur astronomer Francis Brown spent more than
30 years investigating galaxy alignments in his
spare time. In a series of papers published between
1938 and 1968, he presented evidence that galaxy
orientations in certain regions of the sky were far
from random. But many astronomers remained
skeptical, suggesting that the results might be a
consequence of measurement errors, selection
effects, or even psychological biases.
Then in 1968, Gummuluru Sastry of Wesleyan
University showed beyond any doubt that the ori-
entations of some galaxies are clearly not haphaz-
ard. Sastry discovered that giant elliptical galaxies
that populate the centers of clusters — the biggest
and brightest galaxies in the universe — have a
remarkable tendency to be elongated in the same
direction as their host cluster. For example, if a
cluster is elongated north-south, then more often
than not, its brightest member galaxy is, too. If gal-
axies were human, psychologists would call this a
textbook example of mirroring behavior.
Although Sastry’s conclusion was based on only
five galaxies, other astronomers have subsequently
confirmed his results with much larger samples.
Recent studies with the Hubble Space Telescope
— whose sharp vision allows us to see the remote
past by looking far into space — reveal that these
alignments even existed billions of years ago.
And there’s more.
In 1981, Bruno Binggeli of the University of
Basel in Switzerland showed that clusters of galax-
ies aren’t oriented at random, either. Instead, they
exhibit a remarkable tendency to “point” toward
neighboring clusters. Binggeli’s discovery was
anticipated a few years earlier by Estonian astrono-
mers Jaan Einasto, Mihkel Jõeveer, and Enn Saar.Most galaxies are elon-
gated in shape; round
ones are rare. But why?
A galaxy’s image is a
snapshot of its stars’
motions, a moment fro-
zen in time. Spiral galax-
ies like the Milky Way
owe their flattened
shapes to rotation. Just
as a ball of pizza dough
flattens when spun, a
spiral galaxy’s stars
spread into a thin disk as
it rotates. Traveling at
half a million miles per
hour, our Sun has made
nearly two dozen trips
around the Milky Way
since its birth.
Elliptical galaxies, on
the other hand, have lit-
tle or no rotation. Their
stars swarm around the
galaxy’s center like bees
around a hive, each fol-
lowing its own seem-
ingly random path.
However, these orbits
are often elongated in
one direction more than
others, stretching the
galaxy into a shape
resembling a lumines-
cent football. — M.W.WHY ARE
GALAXIES
ELONGATED?A slice from the all-sky 2MASS XSCz infrared survey
shows galaxies within 140 million to 280 million light-
years of Earth. Long chains of galaxies — filaments
— are seen stretching across vast expanses of space,
linking together to create an intricate network known
as the cosmic web. TOM JARRETT (IPAC/CALTECH)