Sky & Telescope - May 2021 - 54

MAY 2021 OBSERVING
Suburban Stargazer by Ken Hewitt-White

Big Bear Pair
Exploring M81 and M82 is double the fun in more ways than one.

t was New Year's Eve, 1774, and
Johann Elert Bode certainly had
reason to celebrate. That night, the
German astronomer discovered an
attractive pair of nebulous objects in
Ursa Major, the Great Bear. Word of the
discovery was slow to circulate - French
astronomer Pierre Méchain chanced
upon " Bode's Nebulae " independently
in 1779, and comet hunter Charles
Messier credited both observers when
he added the objects to his list of " false
comets " in 1781.
Today, we usually refer to Bode's
Nebulae by their Messier numbers, M81
and M82. And we know they're galaxies,
not nebulae. M81 glows impressively
at magnitude 6.9, and M82 weighs in
at magnitude 8.4. Standing 37′ apart,
these cosmic siblings comprise arguably
the ﬁnest galaxy pairing in the heavens.

Beauty and the Beast
The Bode galaxies aren't remote - M81
is only 12.1 million light-years away
and M82 a million light-years farther.
54

M AY 2 0 2 1 * S K Y & T E L E S C O P E

Together they dominate the M81 Group,
a neighbor of the Local Group in which
our Milky Way resides.
Some 90,000 light-years across, M81
is similar in size to the Milky Way. We
see the two-armed spiral nearly face-on,
its opposing arms wound in graceful
symmetry around a broad central bulge.
The fertile arms contain vast populations of hot, blue-white stars. Scores
of pink emission nebulae incubate yet
more protosuns. Curving threads of
interstellar dust lace a galactic system
of sublime elegance.
By contrast, M82 is an enfant terrible, throwing a temper tantrum. Its
vestigial spiral arms are overwhelmed
by a long, luminous bar scarred by
jagged dust clouds. This strangely
mutilated object is disgorging immense
outﬂows of hydrogen gas from its central region. Inside the disrupted core are
vast clumps of massive, ultra-hot stars
- a sign of youth in a region where
older suns are the norm. Astronomers
think M82 suffered a close encounter

S SPIRAL BEAUTY Few galaxies match our
conception of an " island universe " as well as
M81, in Ursa Major. For backyard telescope
users, M81's attractiveness is enhanced by its
proximity to neighboring galaxy M82, located
just 37′ north.

with M81 perhaps 300 million years
ago. Mighty M81 emerged from the
sideswipe relatively unscathed, but M82
was battered and bruised. The result is a
gorgeous spiral beside a disﬁgured wreck
- a celestial Beauty and the Beast.

Locator Logic
For mid-northern skywatchers, the
big bear pair are circumpolar and well
placed for much of the year. That said,
springtime is primetime.
We can star-hop to the galaxies via
the Big Dipper. A line drawn diagonally across the Dipper's bowl, from
2.4-magnitude Gamma (γ) Ursa Majoris
to 1.8-magnitude Alpha (α) UMa, then
veering slightly to the north for a dozen
degrees reaches 4.6-magnitude 24 UMa.
Trouble is, I can't spot 24 UMa in my

N ASA / ESA / THE HUBBLE HERITAG E TE A M (STSCI / AUR A)

I
Sky & Telescope - May 2021

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