>A galaxy passes
through other
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Stephan’s quintetH
UMANS have always been captivated
by the Milky Way. To the ancient
Greeks, it was a squirt of breast milk,
while the Mayan farmers of Mesoamerica saw
it as a growing maize stalk. Today, astronomers
understand that what we see when we gaze up
at the hazy streak of light stretching across the
night sky is actually just one part of our host
galaxy – one of four spiral arms emanating
from an enormous disc of dust and gas that
together contain at least 100 billion stars.
But even as we have traced our cosmic
origins on the grandest scales, from the
big bang to the growth of trillions of other
galaxies entangled in vast webs, the history
of our galactic backyard has remained opaque.
It isn’t for want of trying. The problem is that
when you are inside something, it can be
hard to see what is really going on.smaller star clusters pulled together under
gravity into a thin disc. We know that this
disc is warped, with a thick, bar-shaped bulge
at its centre. We also know that all of this
is encapsulated in a spherical halo of stars
and an ill-defined, even bigger halo of dark
matter – the mysterious stuff that seems
to hold galaxies together.
We know that we aren’t alone, too. There
are dozens of smaller dwarf galaxies orbiting
the Milky Way, entangled in intricate and
occasionally violent cosmic dances. Our best
computer simulations suggest that mergers
with bigger galaxies, several of them on a par
with the Milky Way, were formative in our
galaxy’s youth. “But we don’t know when
or how or precisely how many,” says Amina
Helmi, an astronomer at the University
of Groningen in the Netherlands.Galactic ghosts
Intricate patterns in the movements of stars
are revealing the epic collisions that shaped
the Milky Way, finds Thomas Lewton
Now, a clearer picture of how our galaxy,
the Milky Way, came to look the way it does is
beginning to emerge thanks to data gathered
by the European Space Agency’s Gaia satellite.
Hidden patterns in the movements of stars
have taken astronomers by surprise, unveiling
in rich detail a turbulent history of cataclysmic
collisions and aftershocks.
Some say that soon we will even be able to
pinpoint the specific events that led to the
formation of our sun – the reason we are here
to ponder such things. “It’s our heritage, a way
of knowing ourselves,” says Tomás Ruiz-Lara,
an astronomer at the Institute of Astrophysics
of the Canary Islands, Spain.
Over the past century, astronomers have
sketched an outline of the Milky Way. We
now know that it formed some 14 billion years
ago, probably from clouds of gas and dust andX-
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18/25 December 2021 | New Scientist | 53