My first Magazine Sky & Telescope - 02.2019 | Page 47

Under the Stars by Fred Schaaf
The Second
Star
You may not have (yet) seen
the second brightest star
in our heavens, but read
on to learn more about this
intriguing object.
his month’s topic is a star that
shines more than half a magnitude
brighter than Arcturus. It’s even well
within a magnitude of matching the
peerlessly bright Sirius.
But that only scratches the surface of
what’s unusual and even unique about
this particular object. We’re talking
about potentially the closest supergiant
to us; the only very bright star named
after a possibly historic figure; the only
object beyond our solar system that our
classic interplanetary spacecraft used
for navigational guidance; and the only
star we know that has four separate
runs (three in our distant past, one in
our distant future) as the brightest star
in Earth’s night sky.
The star I’m talking about is Canopus.
The second star. In the United
States, Chicago is fondly known as “the
Second City,” and I’m told that Lyon is
bestowed (by many) the same title in
France. In the heavens, perhaps Cano-
pus should be called “the Second Star.”
I say this, of course, because Cano-
pus is second only to Sirius in apparent
brightness. This is quite noticeable in
some parts of the world because the
two stars are fairly close together in the
heavens. It’s true that Canopus is a full
36° farther south than Sirius, but it’s
only 5¼° farther west. Canopus comes
to its midnight culmination (arrival on
the sky’s central meridian) on Decem-
ber 27th and its 9 p.m. culmination on
February 10th. Both figures are just five
days ahead of those for Sirius. Interest-
ingly, from around the rather populous
T
Like a meadow which no scythe has shaven,
           Which rain could never bend or whirl-blast shake,
       With the Antarctic constellations paven,
       Canopus and his crew, lay the Austral lake.
—Percy Bysshe Shelley,
“The Witch of Atlas”
p ON THE HORIZON From La Palma, in the Canary Islands, Canopus hangs very low on the
southern horizon, while Sirius shines higher up.
35°S line of latitude on Earth (which
passes near the cities of Sydney, Buenos
Aires, and Cape Town) Sirius shines
about 20° north of the zenith and
Canopus about 20° south of the zenith
in February. What a wonderful oppor-
tunity for comparison.
How far north is Canopus visible?
Of course, in reality, observers at mid-
northern latitudes can’t make the direct
comparison between Sirius and Cano-
pus because the latter is either forever
below their southern horizon or low in
their southern sky where atmospheric
extinction is great. From places like
southern Florida and southernmost
Texas, Canopus climbs no higher than
about 10° or 12° — where even on a
very clear night it’s dimmed by about
a magnitude and therefore looks only
about as bright as Procyon.
But how far north is Canopus visible
at all? The limit without refraction in
our atmosphere should be about 37°N.
I know observers in South Carolina who
see Canopus. But have any of you observ-
ing in the mountains of North Carolina
seen it, at least with optical aid?
The nature of Canopus. Canopus
is believed to lie about 310 light-years
from Earth and shines with an absolute
magnitude of –5.7 and a luminosity of
15,000 times that of the Sun. It’s prob-
ably a rare F-type supergiant (the spec-
trum is usually given as F0Ib; but note
that some sources classify it as F0II and
A9II, placing it in the bright giant cat-
egory). Based on its luminosity, its mass
is estimated to be 8 to 10 times that of
the Sun, which is borderline for it to go
supernova. Its ultimate fate may be to
end up like Sirius B: a more massive-
than-average — and possibly even a rare
neon-oxygen — white dwarf.
Next month — Canopus contin-
ued. It’s only fi tting that the “Second
Star” gets a second column. So next
month we’ll discuss the color of Cano-
pus, the origin of its name, the reason
for it being the unique navigational
star for interplanetary spacecraft — and
more. We’ll also discuss the remnants
of Canopus’s original constellation,
Argo — including the parts that can be
seen well from mid-northern latitudes.
¢ Contributing Editor FRED SCHAAF
fi rst learned about phenomena of
atmospheric optics from the Dover
Publications edition of Marcel Minnaert’s
classic The Nature of Light and Color in
the Open Air.
sk yandtele scope.com • FE B RUA RY 2 019
45