OF THE ROGER N. CLARK

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VISUAL ASTRONOMY OF THE DEEP SKY
MS l (NGC 3031), SPIRAL- GALAXY IN
URSA MAJOR
R.A. 09h 55.6m, Dec. 69° 04' (2 000.0)
Technical. M81 is sometimes known as
Bode's Nebula, after its discovery by ]. E.
Bode in 1774. It is a spiral galaxy tilted 32°
from edge-on. Relatively close to us at about
8 or 10 million light-years, it has a diameter
of 40 000 to 50 000 light-years. M81 is the
brightest member of a group of about a dozen
galaxies; its nerest neighbor and closest rivl
in brightness IS M82 (NGC 3034). RadIO
observations show hot gas extending twice
the length of the spiral arms seen on photographs.
This hot gas has been traced all the
way across a "bridge" to M82. Presumably
the bridge is the result of a close encounter
between the two galaxies about 200 million
years ago.
A VISUAL ATLAS OF DEEP-SKY OBJECTS
Visual. M81 appears as a diffuse oval with a
brigh t cen ter. I t has a size of 18 by 10 arcminutes,
and its total visual magnitude is 8.0.
The mean surface brightness is 22.3 magnitudes
per square arc-second. Through the
8-inch under good skies, only the slightest
hint of the spiral arms was seen. The outer
spiral arms are of very low Contrast. On
short-exposure photographs through large
telescopes, spiral arms with thin dust lanes
can be traced to within half an arc-minute of
the nucleus, but none could be seen through
the 8-inch.
The easiest part of the outer arms to detect
is west and slightly north of the nucleus. The
arm here consists of several bright knots,
which in large telescopes look like a string of
faint stars.
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