Sky and Telescope - August 2016 - 21

Multiple Populations:
The Next Challenge
With the gravitational million-body problem solved, you
would be forgiven for thinking that GCs will soon return
to being "a backwater of astrophysics." The truth is quite
the opposite. Numerous crucial questions remain. For
instance, Wang and the Dragon team have yet to ﬁnd
a way to include stars of diﬀerent ages in their simulations. Astronomers only discovered near the turn of the
21st century that stars in GCs can have diﬀerent ages
and initial chemical compositions. Before then, scientists
thought all stars in a GC were created at about the same
time in protogalaxy halos. But more precise observations
have exposed multiple generations of stars in GCs. As the
assumption that all stars formed at the same time underpins most theoretical models of these clusters, multiple
generations present a serious challenge.
Two groups in Europe - one led by Mapelli and
Mario Spera in the Formation and Dynamics of Stars
(ForDyS) team, and the other by Charbonnel and an
International Space Science Institute team - want to
shine a light on this problem, aiming to resolve why
observations suggest not all GC stars were formed over
10 billion years ago. "We need to understand if the
mechanisms that lead to the formation of multiple stellar
populations in massive star clusters are universal, both in
time and space, in order to uncover the link between the
two types of clusters, young and old," explains Charbonnel. "This is crucial to probe the formation of galaxies
and their evolution over cosmic times."
Charbonnel is working on this problem by devising
new models of all types of stars in GCs, incorporating a
number of complex internal processes that should help
illuminate how they came to be. Meanwhile, ForDyS
focuses on star birth near supermassive black holes and
why stars prefer to form close to other stars in clusters.
"We are sure that there is a tiny but very important ﬁl
rouge which connects all star clusters in the universe,"
argue Mapelli and Spera. "We are searching for this
guiding thread!"

The Galactic Trillion-body Problem?
With so much eﬀort being made to understand the origin and evolution of multiple stellar populations, there
is little doubt that soon these too will be incorporated

in Wang's code or one of its successors. And it is this
constant cross-pollination of new ideas and techniques
- through collaboration, open-source code projects, and
healthy competition - that is allowing the small but
highly industrious star cluster community to make rapid
progress in simulating and understanding these fascinating and venerable components of the universe.
Wang and the Dragon team's achievement will
only accelerate this progress, not only opening up new
scientiﬁc realms to explore but also opening the minds
of star cluster scientists to just what might be possible
if they dream big. "What I would like to do in the future
is simulate the entire Milky Way, including all the stars
and planets - there may be a trillion planets in the
galaxy," Portegies Zwart envisions. "It sounds completely
ridiculous, but if you had said that you can do a millionbody simulation of a globular cluster 10 years ago it
would have sounded ridiculous, too."
If someone does solve the trillion-body problem, one
wonders what Heggie might oﬀer as a prize to celebrate
the achievement. Perhaps a barrel of whisky? ✦
Benjamin Skuse is a science communicator and writer based
in Bristol, UK. He has a PhD in mathematics from the University of Edinburgh, where he also had the privilege of enjoying undergraduate astronomy lectures from Douglas Heggie.

COURTESY LONG WANG

step in our ability to simulate globular clusters and star
clusters in general," Portegies Zwart remarks. "I would
have loved to have done this calculation, in all honesty,
but I am very happy for him that he did it. It's fantastic."
He's not overegging the accomplishment: million-body
clusters have only been modeled approximately up to
now, so the Dragon team's ability to realistically simulate
them on a star-by-star basis represents a completely new
phase in GC science.

WHISKY PRIZE Douglas Heggie (right) presents Long
Wang with a bottle of single-malt Scotch whisky at a stellar
science conference in December 2015.

F U R THE R R E AD IN G
Long Wang et al. "The DRAGON simulations: globular
cluster evolution with a million stars." Monthly Notices of
the Royal Astronomical Society. (May 11, 2016: 1450-65)
Douglas Heggie and Piet Hut. The Gravitational MillionBody Problem: A Multidisciplinary Approach to Star Cluster
Dynamics. Cambridge: Cambridge University Press, 2003.

Sk yandTelescope.com August 2016

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