44 AUSTRALIAN SKY & TELESCOPE April 2019
UNDER THE STARS by Fred SchaafJOHANNES HEVELIUS /ATL AS COELESTISL
ast issue we began our discussion
of the second-brightest of all
the night’s stars, Canopus. For
somereason,thisstarseemstogetless
publicity than almost any of the sky’s
21 brightest stars. Could Canopus be
relativelylackinginfamebecausethe
only thing fascinating or distinctive about
it is its great brightness? Nothing could
be further from the truth. Let’s continue
our study of this star that all stargazers
should take the time to observe.
Thethricepastandoncefuture
king.Legend says that King Arthur will
somedayreturn,andsohehasbeen
calledthe‘onceandfutureking’.But
Canopus earns a more impressive title.
In the 10-million-year period spanning
from5millionyearsagoto5million
years in the future, Canopus is the only
star that appears brightest in Earth’s
skiesmanytimes—thriceinourpast
andonceinourfuture.Themainreason
is the star’s great intrinsic brightness.
Otherstars,likeSiriusandVega,usurp
the title of brightest in our skies when
they pass quite close to us. But Canopus
hasanabsolutemagnitudeof–5.7and
therefore can sometimes appear brighter
than any star in Earth’s night sky even
from several hundred light-years away.
TherudderofthegreatshipArgo.
Today, Canopus is Alpha Carinae,
the brightest star in the constellation
Carina, the Keel. But before the
establishment of the 88 official
constellations by the International
Astronomical Union in 1930, Canopus
was the brightest star in the biggest
constellation of all in Western classical
culture: Argo, the ship of Jason and
hiscrewinGreekmythology.Argo
wasthenwassplitintoCarina,the
Keel;Vela,theSails;andPuppis,the
Poop Deck. Oddly, only about half of
Argowaseverportrayedbystarsinthe
heavens,andtheshipaspicturedsailsSailing with the argonauts
There’s a lot more to learn about Canopus, the night sky’s second-brightest star.
backwards in its nightly journey across
the sky. So Canopus, which marks
the rudder at the back of the ship,
actually leads Argo (or today the three
constellations that were parts of Argo)
in the journey across the sky each night.
The pilot or navigator of ships and
spaceships. In Greek legend, Canopus
was the name of the man who piloted
the fleet of Menelaus when that red-
haired king and husband of Helen tried
to sail home from the Trojan War. The
story goes that when the ships landed
in Egypt, Canopus went ashore and was
killed by the bite of a venomous snake.
In his honour, Menelaus named after
his pilot not only the harbour there but
also the bright star that rose during his
dedicatory speech.
At least 3,000 years after the reputed
time of the Trojan War, the star Canopusbecame a new kind of navigator. The star
was used along with the Sun, Earth and
other planets to guide interplanetary
spacecraft. Why? For a reason I figured
out playing with my Norton’s Star Atlas
as an adolescent: Canopus is the really
bright star farthest from the ecliptic in
the heavens.
How much yellow do you see in
the heavens’ second-brightest star?
Historical claims that Canopus appears
yellow or even orange are probably based
on the reddening of the star when seen
near the horizon. But Canopus is most
likely an F-type star, possibly even a
supergiant. What do you say about its
colour when you see it high in the sky?Contributing Editor FRED SCHAAF
welcomes your letters and comments
at [email protected].SARGO NAVIS Once a single large constellation, Argo Navis was divided into three smaller
ones. Canopus, now shining in Carina, the Keel, is the large star highlighted in blue.