DELTA HER (Delta Herculis). Located just south of the famed Keystone of Hercules, third magnitude (3.14) Delta Herculis, 79 light years away, ranks third in brightness (though practically tied with Pi Herculis), and is thus a prominent part of the classical constellation. At first it appears like one of the many white class A stars (an A3 subgiant, suggesting the beginning of stellar death) that helps make so many of the constellations. But like many other stars, it surprises, first by being a close double with three more potential outliers, and then by being not a dying subgiant at all, but made of two vibrant, fairly youthful, hydrogen-fusing dwarfs. Don't even think about tackling Delta Her's duplicity with a telescope, though. The pair, called Delta Her Aa and Ab (B, C, and D already in use: see below), was at best observed to be a mere 0.06 seconds of arc apart, and required a sophisticated interferometer to split. While there is no calculated orbit, we can still draw some healthy conclusions. Delta Her Aa, the brighter, at near-fourth magnitude (3.49), radiates the light of 18.5 Suns from its 8500 Kelvin surface, which gives a radius of 2.0 times that of the Sun. With a projected equatorial rotation velocity of 270 kilometers per second, the star really whizzes around, its rotation period under nine hours. Luminosity and temperature -- plus theory -- give a mass of 2.0 Suns and an age of 370 million years, only about a third of its hydrogen-fusing lifetime. We have to do some guessing about the companion, Delta Her Ab. From its absolute visual brightness, it is probably a class F (F0) dwarf with a temperature of 7500 Kelvin, a luminosity of 6.8 Suns, a radius of 1.5 solar, and a lower mass of 1.6 solar. A physical separation between Aa and Ab of at least 1.45 Astronomical Units, together with the sum of masses, yields a period of at least 335 days. With respective current angular separations from Delta A of 12 seconds of arc, 172 seconds, and 192 seconds lie 8th magnitude Delta Herculis B, 10th magnitude Delta C, and 11th magnitude (10.6) Delta D, making the Delta Her look quintuple. But B and C are nowhere near close to matching the motion of Delta A, showing that they are just line-of-sight coincidences. Delta D's motion does not match well either, the experts concluding that none of these share Delta A's gravitational space. Too bad, as from Delta D (were it a real companion at a separation of 4600 AU), the inner pair would present quite a sight, two brilliant stars comparable to a gibbous-to-quarter Moon barely a minute of arc apart. (Thanks to Bill Hartkopf for discussion.)
Written by Jim Kaler 5/30/08. Return to STARS.