Hercules
“The Hero (Hercules)” · Northern · best around July evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
The strongman kneels upside down in northern summer skies, marked by the 'Keystone' of four stars. On the Keystone's edge blazes M13, the northern sky's grandest globular cluster — 300,000 ancient suns, target of the 1974 Arecibo message to any listening civilizations.
How to find it
Between Arcturus and Vega; find the Keystone quadrilateral.
✦ What lives inside it
- The Great Cluster M13
- M92 globular
The deep dive
Researched for the Atlas from Wikipedia — Hercules (constellation) (15,695 characters read) · updated Sep 20, 2026
01 The sky's largest dim constellation
Hercules sprawls across 1,225.1 square degrees — about 2.970% of the entire night sky — making it the fifth-largest of the 88 modern constellations. That sheer size comes with a humbling twist: it holds the record as the largest constellation with no star brighter than apparent magnitude +2.5. Every other constellation of comparable area boasts at least one beacon that catches the eye immediately; Hercules offers none. Its brightest star, Beta Herculis (Kornephoros), shines at only magnitude 2.8, just barely below that threshold. The result is a vast region of sky that rewards patience and dark skies rather than a quick glance. Six neighboring constellations hem it in — Draco to the north, Boötes and Corona Borealis to the west, Ophiuchus and Aquila to the south, and Lyra to the east — making Hercules a useful landmark for hopping to many showpiece objects in the summer Milky Way.
02 A kneeler without a name for centuries
Long before anyone called this figure Hercules, Greek astronomers simply called it the Kneeler — Ἐγγόνασιν in Greek — because no one could agree on who he was or what he was doing. The poet Aratus, writing around the 3rd century BCE, described a mysterious "Phantom form, like to a man that strives at a task," whose right foot rests on the head of Draco. He is careful to note that "that sign no man knows how to read clearly." The identification with Hercules appears later, in the work attributed to Hyginus, De astronomia, likely written in the 1st century BCE or CE. Even then, ancient authors could not settle on a single myth: Eratosthenes favored Hercules battling the dragon of the Hesperides, while others proposed Theseus, Thamyris, Orpheus, Ixion, Prometheus, Sisyphus, Tantalus, Salmoneus, Atlas, and even a figure named Ceteus. The constellation accumulated mythological candidates for centuries before the Herculean interpretation stuck.
03 Babylonian roots older than Greece Deeper
Historian Gavin White argues that the Greek Kneeler may be a distorted descendant of a Babylonian constellation called the "Standing Gods" (MUL.DINGIR.GUB.BA.MESH). In that older tradition, the figure was imagined with a serpent's body rather than legs — a detail White believes survives vestigially in the Greek star map, where Draco now writhes beneath the Kneeler's foot. He further suggests that the original Greek name "Kneeler" is actually a conflation of two Babylonian constellations: the Sitting Gods and the Standing Gods. An alternative Babylonian reading ties a large portion of what became Hercules to a constellation called the Sitting Dog (MUL.UR.GI). Beyond Babylon, Phoenician tradition associated the figure with a sun god who slew a dragon, and Sumerian myth linked the stars to Gilgamesh. Arab translators of Ptolemy rendered the constellation's name as al-raqis, meaning "the player," which passed into Swahili as Rakisi. Few constellations carry such a layered international ancestry.
04 Rasalgethi: a giant within a giant
Alpha Herculis, known as Rasalgethi — "the kneeler's head" — is one of the most physically imposing stars visible to amateur astronomers, even though it ranks only fifth in brightness among Hercules's stars. Located 359 light-years from Earth, its primary component is an irregular variable bright giant with a diameter of roughly 400 solar diameters. If it replaced our Sun, it would engulf the orbits of Mercury, Venus, Earth, and Mars with room to spare. Its brightness wanders between magnitude 4 at minimum and magnitude 3 at maximum with no predictable rhythm. Orbiting this giant every 3,600 years is a secondary component that is itself a spectroscopic binary — meaning what looks like one star is actually two — glowing with a blue-green tint at magnitude 5.6. Small amateur telescopes can partially resolve the system. Five degrees to the east-southeast lies Rasalhague, the alpha star of Ophiuchus, and the pair may be among the closest-lying alpha stars of neighboring constellations anywhere in the sky.
05 Zeta Herculis: a binary hitting its peak Deeper
Among the constellation's many double and binary stars, Zeta Herculis stands out for a reason rooted in timing. The system sits just 35 light-years from Earth and consists of a magnitude 2.9 yellow-tinged primary and a magnitude 5.7 orange secondary locked in a mutual orbit with a period of 34.5 years. As the two stars swing around each other, their angular separation as seen from Earth widens and narrows over the decades. The article notes that the components are widening toward their peak in 2025, meaning observers with medium-aperture amateur telescopes have a limited window to catch the pair at their most easily split. Once that moment passes, the stars will begin drawing closer together again in apparent terms, gradually becoming harder to separate. This kind of time-sensitive observing opportunity — where orbital mechanics create a narrow viewing window spanning just a few years — is exactly the sort of detail that turns a routine double star into a must-visit target.
06 M92: the globular cluster time forgot
M92 tends to be overshadowed by its more famous neighbor M13, but it earns its own superlatives. At a measured age of 14 billion years, it is described as the oldest globular cluster known — older than many estimates for the age of the universe itself, which makes its precise age a subject of ongoing scientific scrutiny. It lies 26,000 light-years from Earth and shines at magnitude 6.4. Classified as Shapley class IV, M92 is quite concentrated toward the center, sporting a very clear nucleus that gives it a denser, tighter appearance than M13. In binoculars it appears as a fuzzy star; individual stars resolve only in a medium-aperture amateur telescope. The cluster's relative obscurity is largely an accident of celestial geography — it simply has the misfortune of sharing a constellation with M13, the brightest globular cluster in the entire northern hemisphere. On its own merits, M92 would be celebrated as one of the finest deep-sky objects in the sky.
07 The solar apex: Earth is moving here Deeper
Hidden inside Hercules is a point of profound personal relevance: the solar apex, the direction in which our entire solar system is currently traveling through space relative to the Local Standard of Rest. This point sits near right ascension 18h 00m and declination +30° 00′, tucked between Hercules's left elbow near Omicron Herculis and the bright star Vega in neighboring Lyra. Every planet, asteroid, and comet in our solar system is streaming toward this invisible coordinate at this very moment. A separate but related landmark also falls within Hercules: the north pole of the supergalactic coordinate system, located at right ascension 18h 55m 01s and declination +15° 42′ 32″. This coordinate anchors the reference frame used to map the distribution of galaxy clusters across the local universe. A single constellation hosting both the direction of the Sun's galactic motion and a pole of the supergalactic system is a remarkable concentration of cosmological reference points.
08 Hercules A and its monster jets
The brightest radio source in the constellation is not a star — it is an elliptical galaxy called Hercules A, located 2.1 billion light-years away. At its core lurks a supermassive black hole with a mass of 2.5 billion solar masses. To put that number in perspective, the black hole at the center of our own Milky Way has a mass of roughly 4 million solar masses; the one in Hercules A outweighs it by a factor of more than 600. The black hole powers two enormous radio jets that extend for one-and-a-half million light-years — a span that dwarfs the diameter of the Milky Way many times over. The constellation also contains the quasar 3C 345, whose jet produces an optical illusion known as superluminal motion, appearing to move faster than the speed of light. This effect arises from the geometry of the jet's orientation relative to our line of sight, not a genuine violation of physics.
09 The largest known structure in the universe
If the deep-sky objects within Hercules do not provide enough scale, consider what lies beyond them. The Hercules–Corona Borealis Great Wall, described as the largest known structure in the universe, is located in the direction of this constellation. Closer at hand, the Hercules Cluster (catalogued as Abell 2151) is a cluster of galaxies also residing within the constellation's boundaries. These large-scale structures sit far beyond anything visible in an amateur telescope, existing as statistical aggregates of matter detected through surveys of galaxy positions and redshifts. Still, the fact that Hercules is the direction toward both a massive galaxy cluster and the largest known cosmic structure adds a dimension to the constellation that transcends its modest stellar brightness. Looking toward the Keystone asterism on a summer evening, an observer is pointing simultaneously at globular clusters a few thousand light-years away and at superclusters billions of light-years distant, all stacked along the same line of sight.
10 A constellation full of planetary systems Deeper
Fifteen stars in Hercules are known to host extrasolar planets, making it one of the more productive patches of sky for planetary science. The star 14 Herculis holds two planets; at the time of its discovery, 14 Herculis b had the longest period of any known exoplanet at 4.9 years, orbiting at 2.8 AU from its star. Its companion, 14 Herculis c, was detected in 2005 but only confirmed in 2021 after years of additional observation. HD 147506 hosts HAT-P-2b, which was the most massive transiting planet known at the time of its discovery, weighing in at 8.65 times Jupiter's mass. At the other extreme, HD 156668 harbors a planet with a minimum mass of just four Earth masses. HD 155358 stands out because both its planets orbit a star with only 21% of the Sun's metallicity — the lowest metallicity of any planet-hosting star known at the time. The system TOI-561 contains four or possibly five planets, including an ultra-short-period world that completes an orbit in a matter of hours.
11 AT2018cow: a mystery from Hercules Deeper
On June 16, 2018, astronomers detected a large astronomical explosion in the direction of Hercules, catalogued as AT2018cow and informally nicknamed "the Cow." Within days it had attracted extraordinary attention across the global astronomy community. As of June 22, 2018 — just six days after detection — the event was being tentatively classified as a supernova and provisionally named Supernova 2018cow, though the article notes that the classification remained under active discussion. The explosion's rapid brightening and unusual spectral properties set it apart from typical supernovae, generating widespread debate about its true nature. This kind of event, where an entire field mobilizes within days of a detection, illustrates how modern transient surveys have transformed astronomy: instead of discovering objects weeks or months after peak brightness, real-time alerts now allow follow-up observations within hours, turning single events into global collaborative efforts.
12 Chinese and Arab celestial traditions
In Chinese astronomy, the stars that modern charts assign to Hercules are divided between two administrative sky regions: the Purple Forbidden Enclosure (紫微垣, Zǐ Wēi Yuán) and the Heavenly Market Enclosure (天市垣, Tiān Shì Yuán). These enclosures reflect a Chinese sky-mapping tradition organized around the imperial court and marketplace rather than around mythological heroes, so the same stars belonged to entirely different conceptual frameworks. When Arab scholars translated Ptolemy's works into Arabic, they rendered the constellation's name as al-raqis (الرقيس), meaning "the player" — carefully distinguished in the article from the similar-sounding al-rāqiṣ, meaning "the trotting camel" or "the dancing one," which designated a different star entirely. This Arabic name passed into Swahili as Rakisi, giving Hercules a linguistic footprint along the East African coast that traces a clear path from Hellenistic astronomy through Islamic scholarship and across trade routes to the Indian Ocean world.
13 NGC 6210 and Abell 39: nebulae in detail Deeper
Hercules contains two planetary nebulae worth seeking out with different equipment. NGC 6210 glows at 9th magnitude and lies 4,000 light-years from Earth; in amateur telescopes larger than 75 mm aperture it reveals itself as a blue-green elliptical disk, its color arising from doubly ionized oxygen and hydrogen emissions that are characteristic of planetary nebulae. The article also mentions Abell 39, described as nearly spherical — a geometry that makes it scientifically valuable for studying the symmetry of stellar mass loss, since its round shape suggests the progenitor star shed its outer layers uniformly in all directions rather than preferentially along an axis. While NGC 6210 is accessible to moderate amateur instruments, Abell 39's low surface brightness places it firmly in the category of a challenge object requiring dark skies and patience. Together they represent the two ends of the planetary nebula observing spectrum within a single constellation boundary.