15 - Not too regular

Summary

In the field of the exquisite Double Cluster in Perseus is a special type of red supergiant, a representative of a family whose periods of variation are far less certain than those of the Mir as. S Persei is a fine specimen of a semiregular variable star. The 5000 semiregular variables are a highly independent group of stars, a little too independent to predict conveniently.

Compared to other variables we've looked at, S Per can be glacially slow. If it happens to be quiescent, you may not see much change over several months. While it does have a stated period and range, these figures are somewhat decorative in that they are based on many years of observations and do not necessarily show the star's current behavior. From one month to another, it may not change at all, or it may surprise you. Observe this star at least once each 30 days; S Persei's monthly salutation yields a bonus – a simultaneous look at one of nature's finest sights, the Double Cluster, of which S is a member.

Find your way, using the chart in Figure 15.1, by going north from the cluster NGC 869 (No. 869 in Dreyer's 1888 New General Catalog), until you pass the 6.2 magnitude star 7 Persei, then continuing north to 8 Persei, of 6.1 magnitude. Take care with the next step, which is a jump to the northeast by about one degree.