Sky & Telescope - April 2019 - 25

peek above the horizon from latitude 90° - 36° = 54° north.
And from my current home near Boston, Massachusetts, at
latitude 42° north, they reach 12° above the horizon when
they're highest, which happens around midnight in mid-April
or 10 p.m. in mid-May. That makes them easy to spot as long
as there aren't any major obstructions to the south.
Under dark skies you should also be able to see the handsome four-star asterism that marks the Head of Centaurus.
It's halfway between the shoulders and a bit to the north;
the stars are numbered 1, 2, 3, and 4 on some charts. If you
can't spot the asterism with your unaided eyes, binoculars
should show it easily.
Because they're two-dimensional, the stick ﬁgures used in
modern star charts do a poor job of showing many constellations' traditional ﬁgures. The ancient Greeks saw both of the
Centaur's shoulders and all four of its hooves clearly, something that's only possible if its torso is twisted as shown at
left in a plate from Johannes Hevelius's beautifully engraved
1690 star atlas. Hevelius's stars are very accurately positioned;
see how well you can correlate them with the modern chart
on page 22 and with the actual stars in the sky, noting that
the atlas is ﬂipped. Can you visualize two shoulders and a
head in the stars of northern Centaurus?

CEN TAURUS A: ESO; JE WEL BOX CLUSTER: ESO / Y. BELE TSK Y

Central Centaurus
The great globular star cluster Omega Centauri, also called
NGC 5139 and often shortened to Omega Cen, lies near the
constellation's center. It's been spotted as far north as Point
Pelee in Canada, but you need to be around latitude 35°N or
farther south to get a good view. Fortunately that includes
much of the United States.
The Omega designation comes from Johann Bayer's 1603
star atlas Uranometria, which shows a star with that label at
approximately the correct position for the globular cluster.
Likewise, the catalog in Ptolemy's Almagest includes a star
whose position and brightness match the cluster's pretty well.
But both works have enough inaccuracies that we cannot
say for sure whether these entries were meant to denote the
cluster or some faint star near it.
In any case, although Omega Cen is readily visible to the
unaided eye in a moderately dark sky, it appears as a circular
patch of light, not at all starlike to my eyes. The cluster is
amazingly prominent even when it's very low in the sky. It's
the most luminous known globular cluster in our galaxy, and
also one of the closest - just 17,000 light-years away, compared to 24,000 for the familiar northern globulars M5 and
M13. The combination of luminosity and proximity makes
Omega Cen appear ﬁve or six times brighter than M5 or M13.
It's overwhelmingly big and bright through binoculars, and
an 8-inch telescope shows countless individual stars. The
farther south you can travel, the better it looks.
The exotic galaxy Centaurus A (NGC 5128) lies 4½°
north of Omega Cen. As its name implies, it's the brightest
source of radio waves in its constellation. That's due to emissions from its active galactic nucleus - a supermassive black

pp EXOTIC GALAXY Centaurus A looks very different from any other
galaxy in the sky. It combines the circular outline of a typical elliptical
galaxy with a dust lane typical of an edge-on spiral, but much broader
and more complex. Although it lies farther north than Omega Centauri,
it's harder to observe from northerly latitudes, because atmospheric
extinction harms nebulous objects much more than star clusters.
p HOT YOUNG BEAUTY NGC 4755, the Jewel Box, is very easy to
locate 1° southeast of Beta Crucis. It's one of the brightest and most
compact open clusters in the sky. All of its brightest stars are short-lived
supergiants. This proves that the cluster is exceptionally young, with an
estimated age of around 14 million years.
sk yandtele scope.com * A PR I L 2 019

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Sky & Telescope - April 2019

Table of Contents for the Digital Edition of Sky & Telescope - April 2019