The fifth-largest constellation, lying in the northern sky, representing the strong man of Greek mythology. Its brightest star is Beta Herculis, magnitude 2.8. Zeta Herculis is a close binary, magnitudes 2.9 and 5.5, with an orbital period of 34.5 years. Alpha Herculis (Rasalgethi) is both a variable star and a double. M13 is a 6th-magnitude globular cluster 25 300 l.y. away, rated as the finest globular in northern skies. M92 is a globular cluster of magnitude 6.5.