his colleagues, for example, have proposed
that the snail shell pattern is an echo of the
Milky Way’s central bar buckling long ago.
“I wouldn’t be surprised if somebody comes
up with a new model tomorrow that says this
Sagittarius thing is junk and there’s another
explanation,” says Chervin Laporte at the
University of Barcelona in Spain.
Astronomers are eagerly awaiting Gaia’s
next data release in the first half of 2022 for
more clues. Pushing deeper inside the Milky
Way, the data will offer a much bigger and
more representative sample of our galaxy’s
stars, offering new information about their
temperature, size and chemical composition,
alongside their precise positions and
movements. The phase-space data – in
which the snail shell was found – will also
increase five-fold, revealing patterns in star
movements in finer detail. Helmi expects
there will be many more patterns hidden in
the ebb and flow of the Milky Way.
No one expects definitive answers any time
soon. Theoreticians are only beginning to
grapple with the mind-boggling complexity
of our shape-shifting galaxy. “We’ll need
20 years to understand the ESA Gaia data,”
says Schoenrich. But at least now we can
begin to piece together the finer details
of how our home galaxy came to be. ❚“ Data from the
Gaia satellite
is ushering
in a new era
of ‘galactic
seismology’ ”
ES
A/GAIA/DPACThomas Lewton is
watching this spacesuspects that Sagittarius may even be
responsible for giving birth to the star that
warms Earth to create just the right conditions
for liquid water – and life – today.
Stars in a galaxy are a bit like people in a
city, says Ruiz-Lara. From year to year, the
number of people born fluctuates, and so
does the city’s population. “This is what
we are trying to characterise with the star
formation history,” he says. “How many
stars were born 10 billion years ago? How
many stars are being formed right now?”
Ruiz-Lara and his colleagues were searching
Gaia data for answers when another
unexpected pattern emerged: three distinct
bursts of star formation that coincided with
the three passages of Sagittarius. Tantalisingly,
one of these explosions of star births was
calculated to happen between 4 and 6 billion
years ago. “The age of the sun is 4.6 billion
years,” says Ruiz-Lara. “Can we say for sure the
formation of the sun was triggered by this
event? We cannot, but it’s compatible. It could
be that we are talking now because of this
collision with Sagittarius.”
When galaxies clash, shock waves emanate
outwards, creating high-pressure belts of gas
and dust that collapse into stars. Although we
know that some groups of stars are formed
in these violent collisions, it is much harder to
say why specific stars appear where they do.
The way to figure out the unique life stories of
stars is to look for their families, says Ruiz-Lara.
“We might find that our sun has a lot in
common with a particular family of stars
that was caused by a certain event.”
Joining up the dots in this way requires
huge data sets, which is why Gaia
measurements are now being combined
with data from Earth-based telescopes that
have been collecting detailed information
on the chemical fingerprints and brightnesses
of tens of millions of individual stars.
Similar techniques are revealing previously
obscure primordial galaxies such as Gaia
Enceladus, which is a million times more
massive than the sun and is thought to have
merged with the Milky Way in its youth about
10 billion years ago. We are just beginning to
untangle these mergers, like archaeologists
uncovering cultural artefacts from the Roman
and Viking invasions of Britain, says Ruiz-Lara.
A map of 1.8 billion
stars drawn from data
from the Gaia satelliteYet the trove of intriguing patterns that the
Gaia satellite is unearthing is also raising
difficult questions. Few thought that the
Milky Way would still be recoiling so blatantly
from its collisions with minor galaxies billions
of years ago. “We have to proceed more
cautiously,” says Binney, because we don’t yet
have the tools to model this responsive and
dynamic galactic ecosystem. “You have to
think holistically in terms of the whole beast.”
Indeed, Gaia is a stark reminder of how hazy
our underlying picture of the Milky Way is.
Until this develops, it is hard to be sure what
the new data glut is telling us, and these
notions of primordial mergers, spiral arms and
the birth of the sun are by no means certain.
Other ideas are certainly conceivable.
Sergey Khoperskov at the Leibniz Institute
for Astrophysics Potsdam in Germany and18/25 December 2021 | New Scientist | 55