Recently, M82 has undergone at least one tidal encounter with M81. This resulted in a large amount of gas being funnelled into the galaxy's core over the last 200 million years (myr). The most recent such encounter is thought to have happened around 2–5 years ago and resulted in a concentrated starburst together with a marked peak in the cluster age distribution. This starburst ran for up to ~50 myr at a rate of ~10 "M" per year. Two subsequent starbursts followed, the last of which (~4–6 myr ago) may have formed the core star clusters.