Modern era supernovae are named by the year of their discovery and given
a capital letter from the alphabet to mark the order in which they were
discovered in that year. SN is used as the abbreviation for supernova.
So, the famous supernova in the Large Magellanic Cloud which occurred in
February 1987 was named SN1987A, since it was the first supernova
observed in the year 1987. If more than 26 occur in a year, the
lettering continues as lower-case double letters ... like aa, ab, etc.