Sky & Telescope - January 2024 - 45

Stories in the Stars by Stephen James O'Meara
 SCINTILLATING SPECTACLE This longexposure
photo captured in November 2018
with a 400-mm telephoto lens reveals the coruscations
of Sirius as reflected on the surface
waters of the Okavango Delta in Botswana.
brilliance or expands into a coruscating
spectrum.
Indeed, early Arab skywatchers
Scintillating Sirius
Watch the brightest star in the night sky perform
kaleidoscopic feats on dark winter nights.
H
ow many of us, I wonder, can recall
a cold winter's night when Sirius
rose above a snowy landscape sparkling
like fire-fused crystal? The star's original
Greek name (Seirios) has a plethora
of radiant meanings: glowing, sparkling,
flaming, scorching, and flamboyant, to
list but a few. Watching Sirius scintillate
is arguably one of the most animated,
colorful sights in the night sky.
Sirius resides in the constellation
Canis Major, the Great Dog, and is a
white-hot star with almost twice the
mass of our Sun. It's only 8.6 light-years
distant, so its status as the brightest star
in the entire heavens is largely linked to
its proximity. When seen on high from
the Southern Hemisphere, this brilliant
beacon shines with a steady, pure light.
But from mid-northern latitudes, Sirius
sails much lower in the sky, where it
can twinkle with rapid flashes of kaleidoscopic
color - especially as it rises or
sets. Aware that Sirius was not alone in
its colorful tantrums, Greek polymath
Eratosthenes grouped all such stars as
seirioi (the plural of seirios) " due to the
quivering motions of their light. "
Early astronomers wondered if these
rapid changes arose from internal
paroxysms of the star. Of course, we
now know they're related to turbulence
in Earth's atmosphere, but that doesn't
diminish the visual magic performed
by Sirius, whose colorful coruscations
are unrivaled. American astronomer
Garrett P. Serviss captured the essence
of scintillating Sirius in his 1908 book,
Astronomy with the Naked Eye:
[Sirius] has a hundred moods,
according to the state of the atmosphere.
Sometimes, when the air is still, the
star burns with a steady white light,
unflickering, like a core of electric fire;
then, as invisible atmospheric waves
flow over it, its rays spread and leap
and flutter, breaking into keen prismatic
darts that almost cause the eye to
wince. By turns it flames, it sparkles,
it glows, it blazes, it flares, it flashes,
it contracts to a point of intensest
knew Sirius as the star " of a thousand
colors. " And while that interpretation
leans toward the fantastic, mid-19thcentury
experiments by French physicist
Charles Montigny using a scintillometer
revealed that Sirius can have nearly
70 color changes in a second - even
when the star is about 30° above the
horizon. But the show is so much more
intense when Sirius is lower in the sky,
as it appears on this month's Northern
Hemisphere sky chart, where it's plotted
with an altitude of only about 10°.
The drama begins when the star's
light enters our atmosphere. Moving
air cells provoke the incoming light
to pulse into and out of focus (causing
light rays to rapidly converge and
diverge, respectively) or to disperse (so
that each ray of light follows a slightly
different path according to its color).
This creates an ever-shifting kaleidoscopic
effect of spectral wonder.
The twinkling of starlight is not the
same for all stars. White suns like Sirius
show all possible color variations, while
stars of warmer hues have scintillations
that lack cooler hues. You can see this
for yourself by watching Orion's red
supergiant Betelgeuse as it rises.
These colorful effects can be
enhanced anytime with the use of
binoculars, especially when the star
is defocused into a disk, across which
dances an eternal ballet of fleeting hues.
No matter how you view it, twinkling
adds a dimension of life to an otherwise
static celestial landscape. And it's
always there for you to enjoy - all you
have to do is look up.
¢ Contributing Editor STEPHEN
JAMES O'MEARA still vividly recalls
seeing Sirius scintillate over a snowy
Cambridge landscape on his way to
Midnight Mass in 1962.
sk yand tele scope .o r g * JANUARY 2024 45
STEPHEN JAMES O'MEARA
http://skyandtelescope.org
Sky & Telescope - January 2024

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