Astronomy

50 ASTRONOMY • SEPTEMBER 2017

hen most

Northern

Hemisphere

observers

think of globu-

lar clusters, they

imagine a concentration low

in the southern summer sky.

The dense nebulae and stellar

clouds in that area are bright

enough to cast a subtle shadow

as they point to the core of our

Milky Way.

Earth’s placement halfway

from our galaxy’s center gov-

erns our perspective much as a

suburban view toward a city

contrasts sharply with farms

and fields in the opposite direc-

tion. Ref lecting this skewed

dispersal is the placement of

globular clusters, enormous

stellar spheres f loating against

a seemingly infinite firmament.

They contain, on average,

several hundred thousand

closely packed stars, and their

density is a few hundred to a

few thousand times greater

than that in our Sun’s

neighborhood.

Sagittarius joins its promi-

nent neighboring constellations

Scorpius and Ophiuchus in

hosting roughly half of these

great stellar metropolises.

Analyses over the years have

shown that the globular clus-

ters form a large halo extend-

ing past the main body of

our galaxy. While the disk

of the Milky Way is

dense, dynamic, and

rife with star forma-

tion, the halo is rela-

tively calm. The

globulars are placid,

stable, and nearly as

old as the universe

itself.

Among Milky Way objects

they are distant, and despite

their great numbers of stars,

they are not conspicuous

naked-eye objects. Only two

are plainly visible without opti-

cal aid (both in the southern

sky), and a handful of others

hover at the threshold of vis-

ibility. Views of other galaxies

clearly

Target this list of two dozen beautiful star cities, none of which you’ ll find in the old standbys

Scorpius, Ophiuchus, or Sagittarius! by F. Michael Witkoski

off-season glo

The secrets of

show their halos of globular

clusters, something we cannot

observe from within the con-

fines our own system.

Our nearby galactic neighbor,

the Andromeda Galaxy (M31),

houses about 500 globulars, a

pittance compared with the

thousands for giant elliptical

galaxies such as Virgo’s M87,

which contains some 13,000.

Our Milky Way is home

to only about 150,

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