NGC 5451NGC 5449NGC 5447NGC 5450NGC 5455NGC 5458 NGC 5453NGC 5461NGC 5462NGC 5471WWW.ASTRONOMY.COM 55
The western spiral arm
If you have at least a 12-inch scope, trace
the western arm outward from where it
abruptly turns south; about 3.3' south-
west of the galaxy’s nucleus, you’ll find
a small bright area hugging the edge
of the narrow dark lane. This is NGC
- Continue to follow the western
arm south and you’ll find a triangle of
“stars” framing the area where it fans out
and vanishes. Again, I’ve seen more than
one sketch depicting all three points of
this triangle as foreground stars, but the
southernmost point is not a star — care-
ful examination shows it has soft edges.
This is the nebula NGC 5455. This HII
region is where the type II supernova
1970G appeared in July 1970, reaching
magnitude 11.5. The remnant of this
supernova has since been observed by
Chandra as a bright, compact X-ray
source. Brighter than NGC 5453, NGC
5455 can be seen with an 8-inch scope.
Type II supernovae — explosions of
single, massive stars — are strongly asso-
ciated with HII regions of galaxies, where
such stars are born. By contrast, type Ia
supernovae, which occur when a white
dwarf in a binary system gravitationally
siphons enough gas off its companion to
explode, are not necessarily associated
with HII regions. On August 24, 2011,
the type Ia supernova 2011fe (originally
designated PTF11kly because it was
detected by the Palomar Transient
Factory) appeared within this spiral arm
of M101 and was visible in amateur
scopes. You can see SN2011fe as a bright
blue star within the western spiral arm in
the image of M101 I took after it was dis-
covered (page 53). But in a matching
close-up from the images taken last year
for this article, SN2011fe had vanished
into obscurity.
This type Ia supernova appeared in a
faint portion of this spiral arm, rather
than within one of M101’s numerous HII
regions. Type Ia supernovae explode
when a white dwarf reaches 1.4 solar
masses, and thus are equal in brightness.
Therefore, they serve as standard candles
for calculating cosmic distances;
SN2011fe helped refine our estimate
of the distance to M101.Nebulae in the far
western spiral arm
A few more treasures hide in the far
western arm. Trace the arm outward
from the core until you reach the bright,
southward-pointing spearhead shape at
the tip. A magnitude 14 foreground star
marks its northwest edge. This bright
shape is produced by the combined light
of two adjacent nebulae: NGC 5450 in
the southern half and NGC 5447 in the
northern. Larger apertures may allow you
to see the narrow dark gap between them.If you have a large scope, you can try
to spot M101’s two most difficult targets.
Check the far western arm at a point just
south of a line between the southern star
in the right triangle and the galaxy’s
nucleus. If you see a subtle brightening
there, you’ve found NGC 5449. Next,
check the point two-thirds of the way
along a line between the southern star in
the right triangle and the turning-point
star where the western arm abruptly turns
south. A tiny bright spot there might be
NGC 5451. However, be advised that a
faint close double star in the Milky Way
may fool you into thinking you’ve seen
NGC 5451 when you haven’t.The thrill of the challenge
I hope you will enjoy the thrill of hunting
for these challenging NGC objects within
M101. Your patience will be rewarded
with the still-greater thrill of finding
nebulae within a distant galaxy. Then,
the next time someone mentions “The
Pinwheel Galaxy,” you can both impress
and surprise them by saying “Oh, yes,
the Pinwheel, in Ursa Major!”Rod Pommier is a surgeon and longtime
deep-sky observer who has written many
articles for Astronomy.This image of M101 details the locations
of the NGC objects within the Northern
Pinwheel Galaxy. Use of this map in
conjunction with the one on page 54 will
help you determine the relative positions
of the fainter NGC objects when observing
through your telescope.