Right triangle
asterismTurning-point starFar western spiral armBroad dark laneWestern spiral armEquilateral
triangleCore Narrow dark laneNucleusEastern
spiral armV-shaped
asterism54 ASTRONOMY • MARCH 2020
the core. Be aware that even experienced
deep-sky observers can find it challeng-
ing to decipher the patterns of M101’s
faint spiral arms.
The eastern spiral arm begins on the
southwest side of the core and heads due
south. It then hooks around the core,
tapering into a long projection pointing
northeast and ending in a narrow tip east
of the core. The far western arm begins
on the east side of the core and heads
directly north. It becomes fainter as it
angles sharply to the northwest, and
remains barely visible (or vanishes in
smaller scopes) as it broadly curves
southwest. It brightens again as it passes
on the east side of a group of three fore-
ground stars that form a right triangle.
The arm terminates southwest of the core
by tapering into a bright, southward-
pointing spearhead shape.
The western arm begins near a fore-
ground star on the north side of the core
and heads west toward another fore-
ground star (turning-point star) where it
abruptly turns south and then fans out.
A broad dark lane separates it from the
terminal portion of the far western arm
and a narrow dark lane separates it from
the eastern arm.
That nebula is enormous!
We’ll begin with M101’s easiest nebula,
which actually lies well outside the vis-
ible galaxy. To the east of M101, just past
the tip of the eastern arm, is a V-shaped
asterism of five “stars,” with the vertex
pointing north. Careful scrutiny will
reveal that the middle star on the east
side of the V isn’t a star at all (though
I’ve seen more than one
sketch of M101 where it is
depicted as if it is). Rather,
it’s a circular fuzzy spot with
a bright center. You’ve found
the giant nebula NGC 5471.
NGC 5471 is a massive HII
region actively forming hot,
blue stars, which explains
why a good portion of it
appears intensely blue in
images. Hubble images and
stellar photometry indicate
that it is approximately 200
times the size of the Orion
Nebula and has been forming
massive stars for at least
100 million years. Such stars live fast and
die young within a few million years as
type II supernovae. Observations by the
orbiting Chandra X-ray Observatory
show three bright X-ray sources within
NGC 5471 consistent with supernova
remnants. NGC 5471 can be spotted
with a 6-inch scope.Nebulae in the eastern
spiral arm
Scan westward from NGC 5471 to the
tapered tip of the eastern spiral arm.
Near the tip, you’ll find an elongated,
brighter region oriented from north-
east to southwest. This is NGC 5462,
which William Herschel first noted.
Astroimages show many hot, blue
giant stars mixed among
hydrogen-alpha nebulos-
ity. In September 1951, the
type II supernova SN1951H
appeared near the optical
center of NGC 5462. The
supernova reached magni-
tude 17.5. NGC 5462 is visible
through an 8-inch scope.
Traveling farther inward
along the eastern spiral arm,
you’ll come to another bright
region located southeast of
the core. This is the HII
region NGC 5461. It resem-
bles a faint, fuzzy star, like a
dimmer version of NGC- It also can be spotted with an
8-inch scope. If you have at least a
10-inch scope, then continue tracing the
eastern spiral arm inward to where it
joins the core. There you may find a
small, dimly glowing patch of light,
which is NGC 5458. Images show this
nebula contains many blue giant stars.
Yo u r
patience and
determination
will be rewarded
with the
thrill of
finding nebulae
within a
distant galaxy.
This close-up image of M101 serves as a
map to familiarize you with the galaxy’s
general layout and structure, including
spiral arms, dark lanes, and foreground
Milky Way stars. Knowledge of these
features will help you locate and identify
faint NGC objects within the Northern
Pinwheel. North is up and east is left.