M101 was
discovered by Pierre Méchain on March 27, 1781, and added as one
of the last entries in Charles Messier's catalog. It was the first
"spiral nebula" identified as such by William Parsons, the third Earl
of Rosse.
At the
new distance from the HST and Hipparcos, it has a linear diameter of
over 170,000 light years and is thus among the biggest disk galaxies,
and its total apparent visual brightness of 7.9 mag corresponds to an
absolute brightness of -21.6 magnitudes, or a luminosity of about 30
billion (3*10^10) times that of our sun.
