Hercules (constellation)

Hercules is a massive constellation located in the northern sky. It is named after the Roman hero Hercules, who was adapted from the Greek hero Heracles. This constellation is one of the 88 modern constellations recognized today. It ranks as the fifth-largest constellation in total size. It covers an area of 1225.1 square degrees. This means it occupies 2.970% of the entire night sky. Interestingly, it is the largest constellation that contains no stars brighter than magnitude +2.5.

The boundaries of Hercules are very specific. In 1930, Eugène Delporte set the official borders using a polygon of 32 segments. These borders place Hercules near several other constellations. To the north, it is bordered by Draco. To the west, it meets Boötes, Corona Borealis, and Serpens Caput. Ophiuchus lies to the south, while Aquila is to the southwest. To the east, it borders Sagitta, Vulpecula, and Lyra. For observers in mid-northern latitudes, the constellation is best seen from mid-spring until early autumn. The best time to view it is at midnight on June 13.

While Hercules lacks very bright stars, it contains many fascinating star systems. The star Alpha Herculis, also called Rasalgethi, is a triple star system. It is located 359 light-years from Earth. The primary star is an irregular variable star. This is a bright giant that is roughly 400 solar diameters in size. The secondary star is a blue-green spectroscopic binary. This star orbits the primary every 3600 years. Another important star is Beta Herculis, or Kornephoros. It is the brightest star in the constellation. It is a yellow giant located 148 light-years away.

Many other stars in Hercules exist in pairs or groups. Delta Herculis A is a double star that can be seen in small telescopes. Gamma Herculis is also a double star with a white giant primary. Zeta Herculis is a binary system that is 35 light-years away. Its two stars have an orbital period of 34.5 years. These components are expected to widen to their peak in 2025. Other notable examples include Kappa Herculis and Rho Herculis. Both are distant systems containing giant stars. Even variable stars like 30 Herculis are present. This star is a red giant that changes brightness every three months.

Beyond individual stars, Hercules holds incredible deep-sky objects. It contains the bright globular cluster M13. This cluster is the brightest in the northern hemisphere. It sits 25,200 light-years away and contains over 300,000 stars. M13 is so large that its diameter is more than 100 light-years. Another cluster, M92, is 26,000 light-years away. It is the oldest known globular cluster at 14 billion years old. Hercules also contains the Hercules Cluster, which is a group of galaxies. Even more massive is the Hercules–Corona Borealis Great Wall. This is considered the largest structure in the entire universe.

Astronomers have also discovered many planets within this constellation. Fifteen stars in Hercules are known to host extrasolar planets. For example, 14 Herculis has two known planets. The planet 14 Herculis b has a wide orbit of 2.8 AU. Another star, HD 164922, hosts a Saturn-like planet. This was the first long-period Saturn-like planet ever discovered. The planet has a mass of 0.36 MJ. Other systems include HD 155358, which has two planets. This star is notable because it has a very low metallicity. This means it has only 21% of the Sun's metal content.

The history of this constellation is quite old. The astronomer Ptolemy listed it as one of 48 constellations in the second century. Some researchers believe the shape comes from older Babylonian star-lore. Gavin White suggests it may be a version of the "Standing Gods." Other traditions link the stars to the Sumerian hero Gilgamesh. People often visualize the stars as the hero himself. Traditionally, Rasalgethi represents the head of the kneeling hero. Kornephoros represents his shoulder. Some modern maps use the "Keystone" shape as the head instead.