JUNE 2017 OBSERVING Celestial Calendar by Alan MacRobert The Fast Pulse of the RR Lyraes Above: The globular cluster 11 M5, located 25,000 lightyears away in Serpens, is 12 nearly 13 billion years old - 13 a little less massive than the Sun to have become fast-pulsing RR Lyrae variables. These are the green dots in the color-magnitude 14 15 17 18 cluster's stars. Some free- 19 closer to us and in easy reach of amateur scopes. J U N E 2 0 1 7 * SK Y & TELESCOPE Color-magnitude diagram of stars in M5 16 diagram of 15,000 of the floating RR Lyraes are much 48 V magnitude plenty old enough for stars RR Lyrae stars Red Giant Branch Horizontal branch Asymptotic Giant Branch Other stars 20 -0.5 0.0 0.5 1.0 B-V color index 1.5 2.0 ADAM BLOCK / MT. LEMMON SK Y CENTER, LITHOPSIAN / WIKIMEDIA COMMONS An amateur program tracks the quirks of the oldest standard-candle stars in the universe.