JOHN CHUMACKMADHUP RATHI60 ASTRONOMY • JANUARY 2022
92 M81
M81 is one of the spiral wonders of the night
sky. It joins neighboring galaxy M82 (see #70) to
make one of the most dynamic pairings of galax-
ies in the high northern heavens. At 12 million
light-years distant, M81 is a primary member of
the M81 Group of galaxies — one of the nearest
groups of galaxies to our own Local Group. It
is also one of the most distant objects in the
universe accessible without optical aid.
German astronomer Johann Elert Bode
discovered this “nebulous patch” Dec. 31, 1774,
calling it “more or less round, with a dense
nucleus in the middle.” Most observers following
him hailed the brightness of the galaxy’s nucleus,91 M41
The dazzling 4.5-magnitude open star cluster M41 dan-
gles in Canis Major some 4° south of Sirius, the brightest
star in the night sky. From a dark site, the cluster shines
to the unaided eyes as a misty glow, like a metal tag on
the collar of the celestial Dog.
M41 may have been identified by Aristotle, who
described a star with a tail in the Dog — not surprising
coming from a man who took a shining interest in com-
ets. Today, the cluster appears to unaided eyes exactly
as he described: a circular form with a dim extension to
the north.
The cluster lies about 2,300 light-years distant and
is 240 million years old. There is little intervening dust
to affect its appearance, so its 100-odd stars shine
with uncommon purity. At least three of them are 7th
magnitude and should be within range for sharp-eyed,
naked-eye observers. Handheld binoculars will resolve
about a dozen members. Roughly 50 of its stars shine
between 7th and 13th magnitude, putting them within
reach of small- to moderate-sized telescopes.
M41’s stars will fill a telescopic field of view 30 percent
larger than the Full Moon. Look for a striking reddish
star at the heart of the sprawling cluster, with streams of
TERRY HANCOCKirregularly bright suns curling outward. The view is best at
low power. Larger-aperture telescopes will reveal the pres-
ence of many red (or orange) giant stars; the hottest star
is of spectral type A. Up to 80 percent of its stars may be
binaries. While we cannot see most of these, many of the
cluster’s visible stars do form pairings. — S.J.O.including Isaac Roberts, who photographed
it and described it as a “spiral with a nucleus.”
Indeed, M81 was the first known spiral galaxy to
reveal evidence of rotation.
M81 is similar to the Milky Way, resembling
a grand-design spiral with two arms. But these
arms consist of spiral fragments that branch
into many secondary filaments. The arms also
host a population of stars formed in an episode
of star formation that started about 600 million
years ago. This is evidence that M81 may be
undergoing a surge of star formation along
the spiral arms due to a close encounter with
M82; the pair reached their closest points about
300 million years ago.
To find M81, look about 2° east of 24 UrsaeMajoris. If you live under a dark sky, use bin-
oculars first to locate the wonder, which will be
paired with M82 just 38' away. The telescopic
view is splendid: A vast oblique glow, about the
apparent size of the Full Moon, shines forth with
a pale-yellow light punctuated by a vivid stellar-
like nucleus. Two 11th-magnitude stars burn just
south of the core and can easily be mistaken
for supernovae. Ironically, in 1993, a supernova
in M81 blazed to prominence just west of these
stars. At magnifications of 70–150x, the galaxy
core transforms into a misty spring of light
caressed by dark vapors. Delicate wisps of spiral
arms surround the core, and together they look
like a still photograph of the grandest rotating
lawn sprinkler in the cosmos. — S.J.O.