My first Magazine Sky & Telescope - 03.2019 | Page 47

Under the Stars by Fred Schaaf
In the
Ship’s Keel
There’s more to learn about
Canopus, the night sky’s
second-brightest star.
ast month we began our discus-
sion of the second-brightest of all
the night’s stars, Canopus. This star is
visible considerably farther north — at
least to latitude 37°N — than any of
the other 1st-magnitude and brighter
stars that are always below the horizon
from the traditional latitude 40°N line.
Yet Canopus seems to get less publicity
than almost any of the sky’s 21 bright-
est stars, including those I call the “Far-
South Six,” save for lonely Achernar
(the other members of the Six, besides
Canopus, are Alpha and Beta Centauri
and Alpha and Beta Crucis).
Could Canopus be relatively lacking
in fame because the only thing fascinat-
ing or distinctive about it is its great
brightness? Nothing could be further
from the truth. Let’s continue our study
of this star that everyone who lives in
northern lands should strive to see on
some southward trip.
The thrice past and once future
king. Legend says that King Arthur will
someday return, and so he has been
called the “once and future king.” But
Canopus earns a more impressive title.
In the 10-million-year period spanning
from 5 million years ago to 5 million
years in the future, Canopus is the only
star that appears brightest in Earth’s
skies many times — thrice in our past
and once in our future. The main reason
is the star’s great intrinsic brightness.
Other stars, like Sirius and Vega, usurp
the title of brightest in our skies when
they pass quite close to us. But Canopus
has an absolute magnitude of –5.7 and
therefore can sometimes appear brighter
than any star in Earth’s night sky even
from several hundred light-years away.
L
p ARGO 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.
The rudder of the great ship Argo.
Today, Canopus is Alpha Carinae,
the brightest star in the constellation
Carina, the Keel. But before the estab-
lishment of the 88 offi cial constella-
tions by the International Astronomi-
cal Union in 1930, Canopus was the
brightest star in the biggest constella-
tion of all in Western classical culture:
Argo, the ship of Jason and his crew in
Greek mythology. Argo was split into
Carina, the Keel; Vela, the Sails; and
Puppis, the Poop Deck. Vela and Puppis
are rich, fascinating constellations that
are pretty easily visible from latitude
40° north, but Carina is more south-
erly. Oddly, only about half of Argo was
ever portrayed by stars in the heavens,
and the ship as pictured sails 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 fl eet 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 honor, Menelaus named after
his pilot not only the harbor 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 Cano-
pus became a new kind of navigator.
The star was used along with the Sun,
Earth, and other planets to guide inter-
planetary spacecraft. Why? For a reason
I fi gured 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 readers
who observe the star high in the sky say
about its color?
¢ Contributing Editor FRED SCHAAF
welcomes your letters and comments at
[e-mail].
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