Sky and Telescope - February 2017 - 45

Under The Stars by Fred Schaaf

Between the Dogs
Canis Major and Canis Minor frame a dim but intriguing patch of sky.

ORION

CANIS
MINOR
Procyon

MONOCEROS

Sirius

CANIS
MAJOR

A K IR A FUJII

L

ast winter, I was driving home late
on some December evenings, then
early on some January evenings. On
these occasions, when skies were clear,
I noticed a wonderful thing: to either
side of the dark country highway there
shined, low over the treeline, two brilliant stars. To the left was Procyon,
brightest light of Canis Minor, the Little
Dog. To the right was Sirius, chief luminary of Canis Major, the Big Dog. I was
driving to a home between the Dogs.
Now that February has arrived,
Sirius and Procyon have moved to the
southeast and south by mid-evening.
They're high, making this a good time
to observe the stars and clusters in the
sky that lies "between the Dogs."
The Milky Way and Messier.
Monoceros, the Unicorn, is the only
constellation traversed by the line that
connects Sirius to Procyon. This line
passes through the band of the winter
Milky Way that divides those two Dog
Stars from each other. Under moderately dark skies when the Milky Way

reaches the meridian, its long dim band
can be detected running from upper
right to lower left between the dogs and
onward south and east. A moonless February evening under dark skies is also a
prime time to look for the tilted pyramid of gentle glow called the zodiacal
light. Can you trace it from low in the
west to as far upper left as the Pleiades,
or even farther?
In contrast to these somewhat elusive
and extended glows, there's a rich open
cluster of Monoceros about one-third
of the way along the line from Sirius to
Procyon. This is Messier 50, whose magnitude-5.9 fuzzy spot can be glimpsed
with the naked eye in very dark skies.
Roughly 100 stars sparkle within about
16 arcminutes, with one ruddy star
prominent among them. More than
one deep-sky writer has noted that the
cluster as a whole is heart-shaped.
Winter Triangling the Unicorn. Let's
seek even more sights by extending our
"between the Dogs" criterion to include
not just the sky between Sirius and

Procyon, but also the expanse between
each pair of stars that forms the Winter
Triangle, the corners of which are
pinned down by Sirius, Procyon, and
Betelgeuse.
I've noted that M50 lies about onethird of the way between Sirius and
Procyon. The splendid triple star Beta
(β) Monocerotis shines a little more
than one-third of the way between
Sirius and Betelgeuse. English-German
astronomer William Herschel discovered the triplicity of this 3.7-magnitude
brightest star of the dim Unicorn in
1781, the same year he discovered the
planet Uranus.
The third side of the Winter Triangle
also has a key object one-third of the
way between Betelgeuse and Procyon,
just about 1° north of the connecting
line. 13 Monocerotis is the middle star
of what I like to consider the horn of
the Unicorn. The celestial wonders in
and near this horn are so complex and
marvelous that we'll focus on them in
next month's column.
One dog from another. It's interesting to be between the two dog stars
in the sky but there's little point in
being between them in space. What,
however, do they look like as seen from
each other? Sirius and Procyon would
each appear about a magnitude brighter
than they do in our own sky. Each star's
white dwarf companion would burn
much brighter in the star's own sky
than our Full Moon does for us. But
perhaps most interesting is how bright
Procyon would look from Luyten's Star,
the red dwarf star of extremely low
luminosity just 1.2 light-years from
Procyon. From Luyten's, Procyon would
blaze at an amazing magnitude -7.
¢ Contributing Editor FRED SCHAAF
has been writing about the skies above
us for more than 40 years.

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* FEBRUARY 2017

45
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