M81 - Spiral Galaxy

Designations: M81, NGC 3031            Visual Magnitude: 6.9
Object Type: Spiral Galaxy                    Size: 26.9 min X 14.1 min
Constellation: Ursa Major                      Distance: 4.5 million light years
RA: 99hrs 55.6 min                                   Discoverer: Johan Elert Bode, 1774
Dec: +69 deg 4 min

Designations: M82, NGC 3034             Visual Magnitude: 8.4 SB: 12.8
Object Type: Irregular Galaxy                 Size: 11.2m X 4.2m
Constellation: Ursa Major                       Distance: 17 million light years
RA: 9h 55.8m                                             Discoverer: Johan Elert Bode, 1774
Dec: +69deg 41m

Visual Description: Unquestionably, this is the most popular close pair of galaxies in the heavens.

M81 and M82 lie about 2° east of 24 Ursae Majoris - a 4.6-magnitude star about a fist-width northwest of Alpha (a) Ursae Majoris. If you live under a dark sky, use binoculars first to locate the galaxy pair. The telescopic view is splendid: M81's egg-shaped, 6.8-magnitude oval, about the apparent size of the full moon, sailing past the smaller, fainter (8.4-magnitude), cigar-shaped ellipse of M82. These two galaxies, separated by mere 38', voyaged past one another about 200 million years ago with ominous consequences (the encounter is described a little later); now the galaxies are moving apart. Indeed, although they appear to be cosmic neighbors, M82 is 12.5 million light years more distant than M81.

At 23 x in the 4-inch M81 looks enormously larger and more robust than M82. Immediately noticeable is M8l's needle-sharp nucleus, which shines with a pale yellow light. Two 11th-magnitude stars burn just south of the core and can easily be mistaken for supernovae. Ironically, in 1993, a supernova in M81 blazed to prominence just west of these stars. Low power also reveals a mysterious "bar" of light crossing the nucleus, but this feature is probably nothing more than an enhancement of the spiral arms seen obliquely along the galaxy's major axis. Indeed, with a prolonged look the bar seems to point toward M82, which can force you to look at that neighbor galaxy. When I do that my averted vision suddenly picks up swooping spiral arms on either side of M81! This is the magic of peripheral vision

Something odd happens when I change to 72 X. M82 increases in grandeur, while M81 loses some of its luster (because its faint outer arms have been over magnified). But the biggest surprise comes at 130 x, when M81's core is transformed into a misty spring of light caressed by dark vapors, especially to the southeast where a prominent lash of darkness abuts this region. Is this a paler version of M64's "black eye"? The bright bar slicing through the nucleus is now contained in a tiny inner ring of nebulosity, the southwestern half of which appears brighter than the northeastern half. Delicate wisps of spiral arms surround the core, and together they look like a still photograph of a rotating lawn sprinkler. The most difficult detail to coax out (but the most rewarding once you see it) is the feathered texture of the spiral arm that lies midway between the outer, northeast arm and the burning core. This texture shows best with moderate power. Thus, to pick out the finest details of M81, you really have to study the outer arms at low power, the core at high power, and the middle arms at moderate magnification.

The highly disheveled appearance of M82 stands in marked contrast with its properly groomed spiral neighbor. Here is an extragalactic radical with spiked "hairs" bristling off a cigar-shaped body tattooed with dark matter. A very unusual galaxy, M82 appears in red-sensitive photographs to have a midsection that is bursting at the seams. Long filaments stream out at right angles from the galaxy's central region; the filaments can be traced out about 34,000 light years; the galaxy itself is only 55,000 light years long! Although M82 was once believed to be experiencing a violent explosion, astronomers now believe its central region is the site of intense starburst activity - containing perhaps 40 or so supernovae in the early stages of expansion. The first starburst episodes probably began 40 million years ago, after M81, which is 10 times more massive than M82 ploughed past M82 like a speeding truck whizzing past a bicyclist. During that encounter, a strong wake of gravity would have smashed into M82 causing interstellar clouds to collapse and trigger new star formation M82's disrupted appearance probably results from interstellar material being gravitationally dragged away from its disk during that encounter Newly formed supernovae, which usually flare up during episodes of star formation, could have also blasted material out of the galaxy's plane. As M81 sailed farther away from M82, the jostled interstellar medium of M82 would have gradually fallen back onto its parent galaxy, triggering yet more starburst episodes, which we may still be witnessing today.

Telescope: Takahashi TSA 102
Focal Length: 816mm (F8)
Mount: Takahashi NJP160
Camera: SBIG ST10XME
Exposure: Approximetly 3 hours of luminance and 1 hour of Hydrogen Alpha combined
Other: SBIG ST402XME autoguider

Image Processing: combined in CCDStack and processed with Photoshop CS2

The Visual description of the M81 Galaxy was writen by Steven James O'Meara in the book "The Messier Objects" by Stephen James O'Meara. Page. 227. ISBN number 0-521-55332-6.

doug@dougsastro.net