This varies greatly by discipline and tradition - sometimes even within the same research groups. For example, some people put the senior person / advisor first

This varies greatly by discipline and tradition - sometimes even within the same research groups. For example, some people put the senior person / advisor first, some put this person last, some go by alphabet. In my experience, there is even more variation in the amount of work expected to be put into a publication by said senior person. To put it mildly. ^^