Sky & Telescope - July 2021 - 16

are the outcome of unique conditions that only occurred
in the early universe. These scenarios take various forms.
Towards the second half of the 20th century, some of the ﬁrst
models described globular cluster formation in the hot gaseous halos surrounding massive galaxies shortly after the Big
Bang. In the subsequent decades, others developed a variety of
new models suggesting that globular clusters instead formed
during major galaxy mergers (which were more common in
the early universe); that each formed within its own dark
matter halo; or that they represent the former nuclei of dwarf
galaxies, shredded by a larger galaxy's gravitational forces.
In the years since, it has become clear that none of these
ideas can explain the origin of most globular clusters: Not all
10°

Declination

5°

0°

-5°

-10°

15.5

Right ascension (hours)
S DISRUPTED CLUSTER This composite image reveals the core and
long tidal tails of the globular cluster Palomar 5, an 11.8-magnitude
target in the constellation Serpens Caput.

Pal 5

galaxies are massive enough to have had hot gaseous halos or
major mergers, there's no evidence the clusters host dark matter, and there are not enough dwarf galaxies in the universe to
account for the star clusters' observed ubiquity. In short, the
existence of globular clusters requires another explanation.
Over the last decade or so, another idea has steadily
gained traction, inspired by cluster formation we've observed
in nearby galaxies. Even at the present day, there exist rare
galaxies with global properties that were common in the early
universe, such as extremely high gas pressures and rates of
star formation - the latter up to 1,000 times higher than in
the current Milky Way. In the few galaxies where these conditions can be found today, astronomers made an astonishing
discovery: Young globular clusters are still forming. Not only
does this disprove the idea that the conditions needed for
their formation were unique to the early universe, it also provides an unprecedented opportunity to witness the conditions
that might have governed their formation.
The discovery prompted a major rethink. Could globular
clusters be the products of " normal " cluster formation in the
early universe, which then survived until the present day?
This is a simple question, but it is extremely challenging
to answer. We can only do so by constructing a complete
model for star cluster formation and evolution, linked to the
formation and evolution of the galaxy these clusters reside
in. And such a model itself also needs to overcome a difﬁcult
problem: It must naturally explain why today's population of
globular clusters differs considerably from the population of
open clusters in so many ways, from ages and number of stars
to location. Could open and globular clusters really be two
extremes of the same types of object, formed through similar
physical processes?
A series of pen-and-paper models soon demonstrated that
the idea could work in principle. These models predicted that
galaxies with higher gas pressures and star formation rates
would form a larger fraction of their stars in compact clusters, and also allow the formation of more massive clusters.
Observations of nearby gas-rich, cluster-forming galaxies
conﬁrmed these predictions. This was good news. At least
it seemed possible for globular clusters to form under earlyuniverse conditions, and quite commonly so. But what would
happen to them over the billions of years of evolution since?

Destined for Destruction

Sun

S PALOMAR 5 Based on the globular cluster's tidal tails, location, and
motion, astronomers have reconstructed Palomar 5's orbit around the
Milky Way.

J U LY 2 0 2 1 * S K Y & T E L E S C O P E

Star clusters do not have everlasting life. Over time, clusters
gradually dissolve due to three main processes, each of which
we must account for if we are to understand how globular
clusters formed and evolved into the objects that we see
around galaxies today.
First, as stars in the cluster reach the end of their lives,
they either explode as supernovae or, if they're less massive, gently shed their outer layers. In response to the loss of
material, the cluster expands and the gravitational attraction
between its stars weakens, enabling the most loosely bound
stars to escape.

PA LO M A R 5: A N A BON ACA A ND THE DESI LEG ACY IM AGING SURV E YS
(LEG ACYSURV E YS.ORG); ORBIT DIAG R A M: SDSS A ND STSCI / N ASA

Globular Cluster Revolution

http://www.LEGACYSURVEYS.ORG

Sky & Telescope - July 2021

Table of Contents for the Digital Edition of Sky & Telescope - July 2021