Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations today. It is the fifth largest of the modern constellations. It is bordered by Draco to the north; Boötes, Corona Borealis, and Serpens Caput to the east; Ophiuchus to the south; Aquila to the southwest; and Sagitta, Vulpecula, and Lyra to the west. Covering 1225.1 square degrees and 2.970% of the night sky, it ranks 5th among the 88 constellations in size. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 32 segments. In the equatorial coordinate system, epoch 2000, the right ascension coordinates of these borders lie between 16h 00m 26.64s and 18h 57m 49.50s, while the declination coordinates are between +3.67° and +51.32°. In mid-northern latitudes, Hercules is best observed from mid-Spring until the early part of the Fall season, culminating at midnight on June 13.
Hercules has no first or second magnitude stars. However, it does have several stars above magnitude 4. Alpha Herculis, traditionally called Rasalgethi, is a triple star system 359 light-years from Earth. The primary is an irregular variable star; it is a bright giant with a minimum magnitude of 4 and a maximum magnitude of 3. It has a diameter of roughly 400 solar diameters. The secondary, a spectroscopic binary that orbits the primary every 3600 years, is a blue-green hued star of magnitude 5.6. Its common name means “the kneeler’s head”. Beta Herculis, also called Kornephoros, is the brightest star in Hercules. It is a yellow giant of magnitude 2.8, 148 light-years from Earth. Its traditional name means “club-bearer.” Deltoide 5512 is a double star. The primary is a blue-white star of magnitude 3.1 and is 78 light-years from Earth. The optical companion is of magnitude 8.2. Gamma Herculis is also a double star. The primary is a white giant of magnitude 3.8, 195 light-years from Earth. The optical companion, widely separated, is 10th magnitude. Zeta Herculis is a binary star, as the components widen to their peak in 2025. The system, 35 light-years from Earth, has a period of 34.5 years. The primary is a yellow-tinged star of magnitude 2.9, and the secondary is an orange star of magnitude 5.7. Hercules is also home to many double stars and binary stars. Kappa Herculis is a double star. The primary is a yellow giant of magnitude 5.0, 388 light-years from Earth; the secondary is an orange giant of magnitude 6.3, 470 light-years from Earth. Rho Herculis is a binary star 402 light-years from Earth. Both components are blue-green giant stars; the primary is magnitude 4.5 and the secondary is magnitude 5.5. 95 Herculis is a binary star, 470 light-years from Earth. The primary is a silvery giant of magnitude 4.9, and the secondary is an old, reddish giant star of magnitude 5.2. The star HD164669 near the primary may be an optical double.
Fifteen stars in Hercules are known to be orbited by extrasolar planets.
- 14 Hercules has two planets. The planet 14 Herculis b had the longest period (4.9 years) and widest orbit (2.8 AU) at the time of discovery. The planet 14 Herculis c orbits much further out with very low eccentricity.
- HD 149026 has a transiting hot Jupiter planet and is one of the most prominent and studied.
- HD 154345 has the planet HD 154345 b, a long period (9.095 years) and wide orbit (4.18 AU).
- HD 164922 has the first long period Saturn-like planet The mass is 0.36 MJ and semimajor axis of 2.11 AU.
- HD 147506 has the most massive transiting planet HAT-P-2b at the time of discovery. The mass is 8.65 MJ.
- HD 155358 has two planets around the lowest metallicity planet-harbouring star (21% Sun). Both planets orbit in mild eccentricities.
- GSC 03089-00929 has a short transiting planet TrES-3. The period was 31 hours and undergoing orbital decay.
- Gliese 649 has a Saturnian planet around the red dwarf star.
- HD 156668 has a 4.15 Earth-mass planet.
- HD 164595, one known planet, HD 164595 b.
AT2018cow, a large astronomical explosion detected on 16 June 2018. As of 22 June 2018, this astronomical event has generated a very large amount of interest among astronomers throughout the world and maybe, as of 22 June 2018, considered a supernova tentatively named Supernova 2018cow.
Hercules contains two bright globular clusters: M13, the brightest globular cluster in the northern hemisphere, and M92. It also contains the nearly spherical planetary nebula Abell 39. M13 lies between the stars η Her and ζ Her; it is dim, but may be detected by the unaided eye on a very clear night. M13 is a globular cluster of the 6th magnitude that contains more than 300,000 stars and is 25,200 light-years from Earth. It is also very large, with an apparent diameter of over 0.25 degrees, half the size of the full moon; its physical diameter is more than 100 light-years. Individual stars in M13 are resolvable in a small amateur telescope. M92 is a globular cluster of magnitude 6.4, 26,000 light-years from earth. It is a Shapley class IV cluster, indicating that it is quite concentrated at the centre; it has a very clear nucleus. M92, like M13, is denser and smaller than the more celebrated cluster. The oldest globular cluster known at 14 billion years, its stars are resolvable in a medium-aperture amateur telescope. NGC 6229 is a dimmer globular cluster, with a magnitude of 9.4, it is the third-brightest globular in the constellation. 100,000 light-years from Earth, it is a Shapley class IV cluster, meaning that it is fairly rich in the centre and quite concentrated at the nucleus. NGC 6210 is a planetary nebula of the 9th magnitude, 4000 light-years from Earth visible as a blue-green elliptical disk.
The Hercules Cluster (Abell 2151) is a cluster of galaxies in Hercules.
The Hercules–Corona Borealis Great Wall, the largest structure in the universe, is in Hercules. Credit: Wikipedia.