44 AUSTRALIAN SKY & TELESCOPE July 2019
UNDER THE STARS by Fred SchaafAKIRA FUJII“Everythingthatlivesstrivesforcolor.”
— Johann Wolfgang von Goethe, as
translatedintheDoverPublications
editionofMinnaert’sThe Nature of
Light and Colour in the Open AirW
inter is always just a few
kilometres from us. By that
I mean a few kilometres
upward, in the cirrus clouds where ice
crystals can project the circles, arcs,
pillars and patches of often vividly and
variedly coloured halo phenomena.
Winter is also the season for rain. But
sometimes that rain is mixed with
sunlight to give us the multi-hued
pleasure of the rainbow.The colours of winter
Look up into July’s night sky to seestarstingedwithorange,
yellow, blue and possibly even green.Tintsofthebrightestwinterstars.
Let’sstartwithsomeofthebrightest
singlestarsofJulyeveningsandwhat
theirmostlysubtlehuesmaybe.
Arcturushasbeendescribedas
beingthecolourofgingerbeer.The
star’scombinationofspectraltypeand
apparentbrightnessmakeitshuetruly
uniqueamongstarsseenwiththenaked
eye.Whatcolourwouldyoucall it?
A sensitive artist friend of mine
portrayed Altair as shining an “intense
saffron yellow,” but most of us will
detect at best a touch of pale yellow
in its glow. As for Antares — most of
us agree that it glows with a warm
campfire orange-gold. Can you see the
contrast between the hue of Antares
and the white of the brightest star
clouds of the winter Milky Way?
Of course, in the deep south we have
Alpha Centauri, which to the naked eye
appears to be one star but is in fact two
(or three, if you count the extremely
faint Proxima Centauri, thought to be
the third member of the system). Alpha
Cen A has an almost identical colour
to that of our Sun; slightly yellowish.
Alpha Cen B has more of an orange tint.
Also deep in the south is Achernar, a
double system whose primary is spectral
type B, making it blue. Achernar is also
the 10th brightest star in the sky (in
apparent magnitude), so its blue blaze is
very prominent.
An oft-overlooked wonder of colour
is the elusive green companion of
Antares. A magnitude-5.4 star, it is 3′′
from the 1st-magnitude blaze of Antares
and requires only a medium-size
telescope to detect. Is the companion’s
colour really green or just an illusion
from contrast with the primary’s
orange-yellow? Some skilled observers
have seen the companion emerge green,
all alone before the primary appeared
from behind the edge of the Moon.
Next issue. In Part 2 of this column
we’ll cover the colours of winter
nebulae, supernova remnants, star
clusters and atmospheric optics effects.¢ FRED SCHAAF welcomes your letters
and comments at [email protected].At night, however, beginning
amateur astronomers soon learn
that the colours seen in visual
astronomy are far less intense than
those that appear in the images of
astrophotography. Even so, there’s great
beauty in the subtle tints observed in
many celestial objects — and a great
satisfaction felt by those who learn
how to perceive them. This is especially
so in winter, when we can see many
of our most colourful stars and traces
of colour in classic planetary nebulae
and supernova remnants — and even
globular star clusters. We’ll begin to
explore them in this, the first of a two-
part column.Orange-gold
Antares gleams
as the Scorpion’s
heart.