Crowning the Heavens
Look overhead to Corona Borealis
With week’s end, the sun sets at 8:20 and each night after almost a minute later. But it’s still more than an hour later that the glow of dusk gives way fully to darkness. By that time Saturn shines high in the south, the only planet visible until well before dawn.
Don’t confuse Saturn’s steady golden glow for the cool-blue twinkle of Spica, equally bright but 15 degrees to the southeast. High above the two is much brighter Arcturus, easily the brightest star visible and the fourth brightest of all.
Halfway between Arcturus and the barrel-shaped keystone of Hercules’ chest, look for a near-perfect semi-circle of seven stars. Corona Borealis, the northern crown, rests almost directly overhead at midnight. The constellation’s leading star is the second-magnitude Alphecca, also known as Gemma, the Pearl of the Crown. A blue-white star about as bright as Polaris, Alphecca is actually two eclipsing binary stars 75 light-years from earth. The larger of the two is roughly three times as massive as our sun yet burns 50 times more luminous. Riding the wave of the universe’s expansion, Alphecca is receding from us at more than 20 miles a second.
Distinct from the crown are two other stars of note. Outside the semi-circle and to the lower left is R Coronae Borealis, a yellow supergiant nicknamed the Fade-Out star or Reverse Nova star. It shines at magnitude 6, just visible to the unaided eye. But every so often — sometimes in a matter of months, other times over many years — it fades from sight, gradually returning to its normal brightness.
Within the circle, again to the lower left, is T Coronae Borealis, the Blaze Star. A recurring nova, this 10th magnitude star is at the limit of most binoculars. But every so often it flares into view, reaching magnitude 2 in 1866 and magnitude 3 in 1946.