Discover the secrets of the Northern Pinwheel Galaxy – Astronomy Magazine

The western spiral armIf you have at least a 12-inch scope, trace the western arm outward from where it abruptly turns south; about 3.3' southwest of the galaxys nucleus, youll find a small bright area hugging the edge of the narrow dark lane. This is NGC 5453. Continue to follow the western arm south and youll find a triangle of stars framing the area where it fans out and vanishes. Again, Ive seen more than one sketch depicting all three points of this triangle as foreground stars, but the southernmost point is not a star careful 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 associated 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 discovered (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 M101s 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.

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 M101s 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 galaxys nucleus. If you see a subtle brightening there, youve 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 youve seen NGC 5451 when you havent.

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!

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Discover the secrets of the Northern Pinwheel Galaxy - Astronomy Magazine