Hercules is one of the 88 recognized modern constellations. Covering over 1,225 square degrees, it ranks as the fifth-largest constellation, though it is not particularly bright. It was included in the list of 48 constellations cataloged by the astronomer Ptolemy in the second century, known then as the “Kneeler.” The constellation represents the Roman hero Hercules, adapted from the Greek hero Heracles, often depicted in a kneeling posture.
Locating the Constellation
Finding Hercules is best done during the summer months, as it sits high in the Northern Celestial Hemisphere. It lacks first-magnitude stars, so observers typically scan the sky between two bright, easily identifiable stars: Vega in Lyra and Arcturus in Boötes. The constellation is bordered by several others, including Lyra and Corona Borealis, the Northern Crown.
Once the general area is found, the constellation’s most recognizable feature is the “Keystone” asterism. This prominent trapezoid shape of four stars forms the body or torso of the mythological hero.
The Primary Stars of Hercules
Kornephoros (Beta Herculis) is the brightest star in the constellation, shining at an apparent magnitude of 2.78. This evolved yellow giant star is approximately 139 light-years from Earth. Kornephoros is part of a binary system, and the main star has swelled to nearly 20 times the Sun’s radius.
Rasalgethi (Alpha Herculis) is designated the “Alpha” star, though it is often not the brightest because it is an irregular variable star. Located about 360 light-years away, this massive red giant varies in apparent magnitude between 2.7 and 4.0 over periods of months to years. Rasalgethi is a multiple-star system, with the primary star estimated to be about 400 times the size of the Sun.
Sarin (Delta Herculis) is a blue-white star with an apparent magnitude of 3.12, located about 75 light-years away. This star forms the northeast corner of the Keystone asterism. Sarin is a spectroscopic binary system, meaning its companion star is detected through gravitational effects rather than direct visual observation.
Marfik (Kappa Herculis) is visible as an optical double star. The primary component is a yellow giant with an apparent magnitude of 5.0, and its companion is a dimmer star of magnitude 6.25. The two components of Marfik are not gravitationally bound but appear close together from Earth’s perspective.
Notable Deep-Sky Features
Hercules is home to two dense globular clusters of stars, the most famous being Messier 13 (M13), or the Great Globular Cluster in Hercules. M13 is a spherical collection of hundreds of thousands of stars, located about 25,000 light-years away. Shining at an apparent magnitude of 5.8, it is one of the brightest globular clusters in the Northern Hemisphere and is faintly visible to the unaided eye under dark conditions.
M13 spans approximately 165 light-years across, and the stellar density near its core is much higher than in our local stellar neighborhood. The constellation also hosts Messier 92 (M92), a second, smaller globular cluster. M92 has an apparent magnitude of 6.3 and is located about 27,000 light-years away, containing an estimated 330,000 stars. This cluster is considered one of the oldest in the Milky Way galaxy.