M-81 is one of the most magnificent spirals in the heavens, remarkable for its strikingly symmetrical structure and dynamic appearance. The spiral whorls enclose a bright ovoid central mass composed of millions of faint stars. This bright central region surrounds an almost stellar nuclear condensation. Narrow dust lanes border the well defined spiral arms on their inside edges; the arms themselves are well resolved into star clouds and bright patches of nebulosity. In stellar content M81 seems to resemble the great galaxy M31 in Andromeda.
The long dimension corresponds to about 36,000 light years at the accepted distance, and the actual luminosity is equal to about 20 billion suns. E. Holmberg estimated the total mass to be about 250 billion suns, somewhat exceeding the mass of our own Milky Way system.
Click below to
George Normandin, KAS
March 2nd, 2005