April 29, 2008

In Today's One Minute Astronomer...

M81 & M82: Two Galaxies For The Price of One

Members of a nearby group of galaxies, M81 and M82 are a dramatic example of galactic gravitational interaction. They're a rewarding pair for northern observers with small telescopes.

The Basics

• M81 is the brighter and larger of the two galaxies. It's an elegant spiral with clearly visible arms extending continuously around the core. Galaxies with such near-perfect shape are called "grand-design" spirals. At magnitude 6.8, it's easily visible as a bright smudge in binoculars.

• In contrast, the nearby M82 is a mangled cigar-shaped galaxy. Fainter, at magnitude 8.4, it looks like a ragged edge-on spiral in the process of blowing itself to bits.

• Find M81 in the constellation Ursa Major at RA 09h55.6m and Dec +69d04m about, 10 degrees from the brightest star in the Big Dipper's bowl. M82 lies only 37' to the northeast.

• You can see both together in a wide field eyepiece. But you'll need clear dark skies and an 8" scope to see the spiral structure in M81.

M81 and M82 in a single field of view.

A Deeper Look

• At one time, astronomers thought M82 was exploding for a reason that defied explanation. Now it's believed it was a normal galaxy racked by an intense bout of star formation in its core, likely triggered by a strong gravitational interaction with M81 about 100 million years ago.

• In the infrared light, M82 is the brightest galaxy in the sky; it's brighter at infrared wavelengths than in the visible spectrum, likely because of intense star formation

• The centers of M81 and M82 are only 150,000 light years apart, and they continue to interact strongly. M82 may eventually be stripped of gas and dust and become a tight and featureless elliptical.

A Bit of History

Only one supernova has been detected in M81. On March 28, 1993, the Spanish amateur Francisco Garcia Diaz discovered the exploding star from Lugo, Spain. He discovered the 11th magnitude supernova the old-fashioned way… visually… with his 10-inch Newtonian telescope at a magnification of 111x.

Personal View

At my northerly latitude, I see this pair for most of the year, even in light-polluted skies. And when I go to really dark sky, I never miss a chance to glimpse the gossamer spiral arms of M81... I think it's better than the more famous Andromeda galaxy.