  The last tidal encounter between M82 and M81, some 500 Myr ago, had a major
impact on what was probably an otherwise normal, quiescent disc galaxy. It
caused a concentrated burst of star formation in the form of massive star
clusters, which decreased rapidly, within a few 100 Myr. The current starburst
in the centre of the galaxy is likely either due to large-scale propagating
star formation or possibly related to late infall of tidally disrupted debris
from M82 itself. It may, in fact, be a combination of these two mechanisms, in
the sense that the star formation in the active core is actually propagating,
while the overall evolution of the starburst is due to tidal debris raining
back onto the disc of the galaxy, causing the present-day starburst.
