Sky & Telescope - April 2019 - 27

but not into the Cross. But whatever you call it, the formation
is shapely, compact, and very bright.
It's too bad that the Southern Cross wasn't recognized as
an outstanding asterism yet also left as part of Centaurus,
just as the Northern Cross is part of Cygnus and the Big Dipper is part of Ursa Major.
All of Crux and Centaurus are visible from the southern
tips of Florida and Texas, though very low in the sky. In
some ways that's appropriate, since the Greeks themselves
always saw Centaurus hugging the horizon. But traveling
even farther south will greatly enhance the view through
binoculars and telescopes.
The Milky Way passes through Crux and southern Centaurus, and so the region is swarming with nebulae and
open star clusters. The ﬁnest of these clusters is NGC 4755,
also known as the Jewel Box because of the contrast between
its red and blue supergiant stars. It's obvious to the unaided
eye under dark skies, and breathtaking through optical
instruments of all sizes.
Through a telescope at 100×, Alpha Crucis and Alpha
Centauri are spectacular double stars. The components of
Alpha Crucis are very similar in brightness, almost identical
in their blue-white color, and separated by 4.2″. There's also
an inconspicuous third component 90″ from the bright pair
that may or may not be physically related.
Alpha Centauri's fainter star is just 1/3 as bright as the
primary, and distinctly redder. And with an orbital period of
79.9 years, the relative positions of Alpha Centauri A and B

change perceptibly from one year to the next (see page 34 for
more on this intriguing binary star). They're 5.2″ apart in
mid-2019, widening to 10.4″ in 2030.
The Alpha Centauri system has a third component, Proxima Centauri, which is currently 0.12 light-years closer than
the AB pair. According to a 2017 paper in Astronomy & Astrophysics, Proxima orbits the AB pair about once every 550,000
years. Proxima lies a whopping 2.2° away from AB in the sky,
surely a record among double stars. With traditional doubles,
the problem is separating the stars at high power. Finding
Proxima, by contrast, requires a substantial star-hop from
the main pair; the charts at left will help. Proxima isn't much
to look at, a feeble red dwarf shining at magnitude 11.0. But
tracking it down was one of the most exciting moments in
my entire observing career, because this is truly the closest
star to our Sun, the bridge between our immediate neighborhood and the galaxy at large. To add to the thrill, Proxima has
a planet that appears to orbit in the "habitable zone," where
water can exist in liquid form.
Centaurus and Crux are challenging to observe for most
people in mid-northern latitudes. But they are sure to reward
your effort because of their great beauty, uniquely signiﬁcant
stars and deep-sky objects, and historical importance.

¢ Contributing Editor TONY FLANDERS first viewed southern
Centaurus from Chile in October 2013, one month before he
became an associate editor at Sky & Telescope, a position he
held until retirement.

TON Y FL A NDERS

LAST CHANCE VIEW
The period the author
set aside for astronomy
in Hawai'i was plagued
by uncharacteristically
cloudy weather. His first
completely unobstructed
view of Centaurus and
Crux occurred on the last
night of the trip, while
camped on a beach on
Kaua'i. The Moon was full
that night, hiding the faintest stars and reducing
the constellations to their
simplest form.

sk yandtele scope.com * A PR I L 2 019

http://www.skyandtelescope.com

Sky & Telescope - April 2019

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