Sky and Telescope - November 2018 - 34

OUTSIDE THE BOUNDS

by Steve Gottlieb

In Search of

Extragalactic Globulars
What's the farthest star ball you can see with an amateur telescope?
Start your hunt outside the Milky Way.

N OV E M B E R 2 018 * SK Y & TELESCOPE

N ASA / ESA / H U B B LE LEG ACY A RC HI V E

Scottish-born astronomer James Dunlop spied
he Milky Way is home to roughly 160 globular clusters,
NGC 1835 in 1826 using his homemade 9-inch speculum
spread around the galactic halo and toward the cenmetal reﬂector at Parramatta, near Sydney, Australia. He
tral bulge. These densely packed stellar spheres formed
called it "a small round pretty well-deﬁ ned nebula, bright at
roughly 10 billion years ago and contain tens to hundreds of
the centre." Dunlop's copper-tin alloy mirror was equivathousands of gravitationally bound stars. As ancient relics of
lent in light-gathering to a modern 6.5-inch telescope, but
the Milky Way, they offer important astrophysical clues to
his description applies to most of the LMC globulars - they
stellar evolution and dynamics, as well as to galactic forappear compact and unresolved, with strongly concentrated
mation and later accretion events. And, of course, they're
cores and faint smooth envelopes. NGC 1828 and NGC
among the most breathtaking sights in the sky.
1830, both smaller and fainter open clusters, lie in the
The number of globular clusters in a host galaxy corsame ﬁeld of view, a common occurrence in the LMC.
relates strongly with the galaxy's luminosity. The supergiEight years later John Herschel began a monumental surant elliptical M87 boasts an estimated 12,000 clusters
vey of the southern sky from Cape Town, South Africa, and
while ubiquitous low-luminosity dwarf galaxies contain
discovered NGC 1916 along the central bar of the LMC.
at most a few. Within the more than 100 nearby galaxies
This compressed globular sports a prominent central hub
that comprise our Local Group, M31 commands the largest
and a 40″ halo.
collection, with more than 500 conﬁrmed globulars. But
Although relatively bright, NGC 1916 is outclassed by
how many of these can you see with an amateur scope?
NGC 1903, a gorgeous open cluster just 8′ to its northwest.
Let's head outside the Milky Way and explore some of the
NGC 1903 masquerades as a globular cluster, but studies
globulars in our galactic neighbors.
yield a relatively youthful age of less than 100 million years.
A few words of caution: Extragalactic globulars lack the
A 24-inch scope revealed a 20″ blazing core encased in a 1′
optical impact of familiar ones in the Milky Way so are best
halo studded with nearly two dozen glittering stars. Northappreciated with your mind as well as your eyes. Consider
west of NGC 1903 is NGC 1910, a large star cloud containM13, arguably the Northern Hemisphere's ﬁnest globular. If
ing S Doradus, the prototype of a class of extremely masit were located at the distance of the Andromeda Galaxy, it
sive, highly evolved stars called luminous
would appear as a 16th-magnitude speck!
blue variables. With a mean magnitude of
So as you observe, take time to contemapproximately 9.5, S Doradus is the single
plate the astrophysical importance and
brightest star in the LMC.
true grandeur of these globulars - it will
Argentinean astronomer José Luis
make the hunt much more enjoyable.
Sérsic discovered the Reticulum Cluster
Our ﬁrst stop is the well-studied
in 1973 on plates taken with the 0.7-m
Large Magellanic Cloud (LMC), which
Maksutov camera of the Cerro El Roble
lies at a distance of 165,000 light-years.
Observatory in Chile. He described it
It boasts 16 ancient metal-poor globuas a "Dwarf in Reticulum, probably a
lars along with 100 "intermediate-age"
member of the Local Group." Later pho(1-3 billion years) massive clusters.
tometric investigations demonstrated it
You'll need to observe these from the
was a highly extended, low-luminosity
Southern Hemisphere, but if you have
p SMALL SPARKLER In amateur
scopes, extragalactic globular clusters
globular. The LMC holds a tenuous grip
the opportunity, exploring the clusterlike NGC 1835 remain unresolved. This
on the Reticulum Cluster as it lies at
and nebula-rich ﬁelds of the LMC is an
Hubble Space Telescope image reveals
the extreme limits of its halo. One day
unforgettable experience. My observathe 11th-magnitude cluster's core. NGC
it may be snatched by the Milky Way's
tions were made under dark transparent
1835 is the brightest globular in the Large
gravitational pull.
skies in rural Australia.
Magellanic Cloud.

Sky and Telescope - November 2018

Table of Contents for the Digital Edition of Sky and Telescope - November 2018