Australian Sky & Telescope - 02.2019 - 03.2019

44 AUSTRALIAN SKY & TELESCOPE February | March 2019

T

hismonth’stopicisastar

that shines more than half

amagnitudebrighterthan

Arcturus.It’sevenwellwithina

magnitudeofmatchingthepeerlessly

bright Sirius.

Butthatonlyscratchesthesurfaceof

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

afterapossiblyhistoricfigure;theonly

object beyond our Solar System that our

classicinterplanetaryspacecraftused

fornavigationalguidance;andtheonly

star we know that hasfourseparate

runs (three in our distant past, one in

ourdistantfuture)asthebrighteststar

in Earth’s night sky.

ThestarI’mtalkingabout,ofcourse,

is Canopus.

In our skies, perhaps Canopus should

becalled‘thesecondstar’.Isaythis,

ofcourse,becauseCanopusissecond

only to Sirius in apparent brightness.

Thisisquitenoticeabletoussouthern

observersbecausethetwostarsare

fairlyclosetogetherinthesky.It’strue

The second star

Thesecondbrighteststarinourskies

isaveryintriguingobject.

thatCanopusisafull36°farthersouth

than Sirius, but it’s only 5¼°farther

west.Canopuscomestoits9:00pm

culmination(arrivalonthesky’s

central meridian) in mid-February,

justfivedaysaheadofSirius.Fromthe

rather populous 35°S line of latitude

onEarth(whichpassesnearSydney,

Buenos Aires and Cape Town) Sirius

shines about 20° north of the zenith

andCanopusabout20°southofthe

zenith in February. What a wonderful

opportunity for comparison.

Our cousins at mid-northern

latitudescan’tmakethedirect

comparisonbetweenSiriusand

Canopusbecausethelatteriseither

forever below their horizon or low in

theirsouthernskywhereatmospheric

extinction is great. From places like

southern Florida and southernmost

Texas,Canopusclimbsnohigherthan

about10°or12°—whereevenona

very clear night it’s dimmed by about

amagnitudeandthereforelooksonly

aboutasbrightasProcyon.

Canopusisbelievedtolieabout

310light-yearsfromEarthandshines

withanabsolutemagnitudeof–5.7

andaluminosityof15,000timesthat

oftheSun.It’sprobablyarareF-type

supergiant (the spectrum is usually

given asF0Ib;butnotethatsome

sources classify it asF0II andA9II,

placingitinthebrightgiantcategory).

Basedonitsluminosity,itsmassis

estimatedtobe8to10timesthatof

theSun,whichisborderlineforittogo

supernova.Itsultimatefatemaybeto

enduplikeSiriusB:amoremassive-

than-average—andpossiblyevenarare

neon-oxygen—whitedwarf.

It’s only fitting that the ‘second star’

getsasecondcolumn.Soinournext

we’lldiscussthecolourofCanopus,

theoriginofitsname,thereasonforit

being the unique navigational star for

interplanetaryspacecraft—andmore.

Andwe’llalsocovertheremnantsof

Canopus’soriginalconstellation,Argo.

„ FRED SCHAAFfirst learned about the

phenomena of atmospheric optics from

the Dover Publications edition of Marcel

Minnaert’s classicThe Nature of Light

and Color in the Open Air.

SOUTHERN STARCanopus is the bright star to the

left of the Large Magellanic Cloud. More luminous

Siriusisinthetopleft-handcorneroftheimage.

G. HÜDEPOHL (ATACAMAPHOTO.COM)/ESO

UNDER THE STARS by Fred Schaaf