6 - Getting started with Cepheids

Summary

Delta Cephei

To get an idea of what variable-star observing is all about, here are two active, easily found stars that we will observe informally. The first is Delta Cephei, an ideal star to begin with for several reasons. It is part of a bright and compact star group, is usually around magnitude 4, and is an active star, always offering something interesting to watch.

Delta Cephei's variation was discovered by John Goodricke in 1784 (see Chapter 32), and it is the star for which all the Cepheid variables are named. The variation in this giant star is small but extremely regular with a period of several days. It is also typical that this star enjoys a leisurely decline to minimum that is followed by a rapid rise to maximum.

At maximum, Delta Cephei is easily visible at magnitude 3.5, and at minimum it shines at 4.4. If you observe every night or two you will soon see how it falls, then rises, week after week.

Notice the two stars Zeta and Epsilon in Fig. 6.1 and in the sky. Which is brighter? Let us give Zeta Cephei an arbitrary value of “1” and Epsilon a value of “5”. Each night estimate the brightness of Delta Cephei as follows:

1. – as bright as Zeta

2. – slightly fainter than Zeta

3. – halfway between Zeta and Epsilon

4. – slightly brighter than Epsilon

5. – as faint as Epsilon

Even though we are not using actual magnitude values here, we still estimate to better than 0.2 magnitude.