Delphinus
“The Dolphin” · Northern · best around September evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
A tiny, charming dolphin leaping beside the Summer Triangle — the rescuer of the musician Arion. Its two named stars, Sualocin and Rotanev, are a hidden joke: read backwards they spell Nicolaus Venator, an Italian astronomer who quietly named them after himself.
How to find it
The compact diamond-with-tail east of Altair.
✦ What lives inside it
- The 'Job's Coffin' asterism
The deep dive
Researched for the Atlas from Wikipedia — Delphinus (7,866 characters read) · updated Sep 20, 2026
01 A name hidden backwards in the stars
The two brightest stars of Delphinus carry names that look exotic but hide a very human joke. Alpha Delphini is called Sualocin and Beta Delphini is called Rotanev — and if you reverse both names you get Nicolaus Venator, the Latinized form of Niccolò Cacciatore, a director of the Palermo Observatory who died in 1841. Cacciatore apparently slipped his own name into a star catalogue, and because catalogue names tend to stick, the tribute has endured for nearly two centuries. It is one of the few cases in astronomical history where a working observatory employee managed to have stars named after himself, not through any official process but through a quiet act of editorial cleverness. The reversal trick went unnoticed long enough that by the time anyone caught on, the names were already in wide circulation and proved impossible to dislodge. Today both Sualocin and Rotanev are the officially recognized names, ratified by the International Astronomical Union, making Cacciatore's self-promotion permanent.
02 Two novas that rewrote the textbooks Deeper
Delphinus has hosted two naked-eye novae in living memory, and both turned out to be scientifically unusual. HR Delphini blazed to magnitude 3.5 in December 1967, bright enough to spot without optical aid, but what made it remarkable was the pace of the event: it took an unusually long time to reach peak brightness, which astronomers interpret as evidence that it barely satisfied the conditions needed for a thermonuclear runaway on the white dwarf's surface. Most novae race to maximum; HR Delphini dawdled, sitting right at the edge of the physical threshold. The second nova, V339 Delphini, appeared in 2013 and peaked at magnitude 4.3. It earned a place in textbooks for a different reason: it was the first nova ever observed to produce lithium. Lithium is one of the lightest elements and its cosmic origin has long been debated; V339 Delphini provided direct observational evidence that novae can manufacture it, helping close a gap in our understanding of where the universe's lithium inventory actually comes from.
03 Gamma Delphini and its ghostly neighbour
Among the binary stars that amateur astronomers prize, Gamma Delphini stands out for the colour contrast between its two components. The primary shines as an orange-gold star of magnitude 4.3, while the secondary — sometimes described as light yellow and sometimes as green, depending on the observer — glows at magnitude 5.1. A small amateur telescope is all that is needed to split them; the two stars sit 10 arcseconds apart. At 125 light-years away they form a genuine gravitationally bound pair, and their estimated orbital period exceeds 3,000 years, meaning no human lifetime will witness a perceptible change in their configuration. Nearby, only 15 arcminutes from Gamma Delphini, lies Struve 2725, nicknamed the Ghost Double because it mimics the arrangement of the main pair but at dimmer magnitudes of 7.6 and 8.4, separated by 6 arcseconds. Adding further intrigue, an unconfirmed exoplanet with a minimum mass of 0.7 Jupiter masses may orbit one of the stars in the Gamma Delphini system, though as of the article's writing this detection remained unverified.
04 A star that wandered into the constellation Deeper
Constellation boundaries are fixed lines on the sky, set officially by Eugène Delporte in 1930, and stars do not usually move enough in a human lifetime to cross them. Rho Aquilae is an exception worth noting. This magnitude 4.94 A-type main-sequence star, located about 150 light-years away and distinguished by a lower metallicity than the Sun, formally resided in the neighbouring constellation Aquila for all of recorded history — until its proper motion carried it across the boundary into Delphinus in 1992. That crossing is recent enough that many older star atlases still list it under Aquila. Proper motion, the slow drift of a star's position against the background sky caused by its real movement through space, is usually imperceptible on human timescales, but Rho Aquilae's motion happens to be directed almost precisely toward the Delphinus border. It serves as a neat reminder that constellation membership, while administratively fixed, is not eternally fixed from the star's own perspective.
05 Deep-sky treasures in a rich Milky Way field
Because Delphinus sits against a dense Milky Way star field, the background alone rewards a slow sweep with binoculars. Among the constellation's more structured deep-sky objects, NGC 6934 is a globular cluster shining at magnitude 9.75, located roughly 52,000 light-years from the Solar System and ranked Shapley-Sawyer Concentration Class VIII, meaning it is moderately concentrated toward its core. Researchers believe it may share a common origin with a globular cluster in the neighbouring constellation Boötes, pointing to a possible ancient accretion event in the Milky Way's history. NGC 6934 has an intermediate metallicity for a globular cluster but was described as poorly studied as of 2018. At the other extreme of distance, NGC 7006 sits roughly 137,000 light-years away, placing it at the outermost reaches of the galaxy's halo. It is a faint magnitude 11.5 object and ranks Class I, indicating a tight, highly concentrated structure that requires a telescope with decent aperture to resolve individual stars.
06 Planetary nebulae and an oxygen-rich stellar corpse Deeper
Delphinus contains two planetary nebulae of note. NGC 6891 is the brighter of the two at magnitude 10.5, a modest target for a backyard telescope. The second, NGC 6905, is known as the Blue Flash Nebula and displays broad emission lines in its spectrum — a signature of high-velocity gas expanding outward from the central star. What makes NGC 6905 particularly striking to spectroscopists is the nature of that central star: it carries a spectral classification of WO2, denoting a Wolf-Rayet-type object extraordinarily rich in oxygen. WO2 stars are among the rarest spectral types in the galaxy; they represent a very late stage of stellar evolution in which the outer layers have been shed so thoroughly that the products of helium burning — principally carbon and oxygen — are exposed at the surface. The presence of such an object at the heart of a planetary nebula gives astronomers a rare close-up view of the material that will eventually be recycled into the next generation of stars and planets.
07 Exoplanets old and new in a small constellation
Despite its modest size, Delphinus hosts a growing catalogue of confirmed exoplanets. The star 18 Delphini, officially named Musica, is a G6 III giant and one of five planet-bearing stars in the constellation. Its planet Arion carries a mass at least 10.3 times greater than Jupiter, making it an exceptionally dense and massive world. Musica and Arion were notable participants in the first NameExoWorlds contest, which allowed the public to propose official names for exoplanets and their host stars. More recently, in 2024, two planets were confirmed around the host star TOI-6883. The larger, TOI-6883 b, has an orbital period of 16.249 days, a radius 1.08 times Jupiter's, and a mass 4.34 times Jupiter's; it was initially spotted from a single transit in data from the TESS spacecraft and confirmed by a network of citizen scientists. Its companion, TOI-6883 c, is a Neptune-sized world with an orbital period of 7.8458 days, a radius 0.7 times Jupiter's, and roughly one-third of Jupiter's mass.
08 How three star cultures saw this small group
Greek mythology supplied two distinct stories to explain Delphinus, but other astronomical traditions treated the same stars in entirely different ways. Chinese astronomers placed the stars of Delphinus within the vast system of the Black Tortoise of the North, one of the four symbolic animals that organized Chinese sky knowledge, rather than imagining a dolphin at all. In Polynesia, two geographically separate cultures independently identified Delphinus as a distinct grouping. The people of Pukapuka called it Te Toloa, while the Tuamotu archipelago knew it as Te Uru-o-tiki, different names that suggest each culture arrived at the grouping on its own terms. Hindu astronomical tradition mapped the stars of Delphinus to the Nakshatra called Dhanishta, one of the twenty-seven or twenty-eight lunar mansions that divide the sky in Vedic astrology. Across these four traditions — Greek, Chinese, Polynesian, and Hindu — the same small knot of stars was noticed, named, and woven into entirely separate systems of meaning, a reminder that the human impulse to pattern the sky is universal even when the patterns differ completely.
09 The stars themselves: a pocket guide to the main cast
Alpha Delphini (Sualocin) is a blue-white main-sequence star of magnitude 3.8 and absolute magnitude −0.4, sitting 241 light-years from Earth. It is a spectroscopic binary, meaning its companion is revealed only through Doppler shifts in its spectrum rather than visual separation. Beta Delphini (Rotanev) looks to the naked eye like a single white star of magnitude 3.6 at a distance of 97 light-years, but in 1873 it was found to be a binary; the two components orbit each other with a period of 27 years, and the gap between them is just wide enough to split with a large amateur telescope. Delta Delphini is an A-type star of magnitude 4.43 that is itself a spectroscopic binary, and uniquely, both stars in the pair are Delta Scuti variables — pulsating stars that oscillate in brightness over periods of hours. Epsilon Delphini, named Deneb Dulfim meaning 'tail of the Dolphin,' is a class B6 III giant whose magnitude varies around 4.03. Zeta Delphini, an A3Va main-sequence star of magnitude 4.6, was discovered in 2014 to host a brown dwarf companion, designated Zeta Delphini B, with a mass of 50 ± 15 times that of Jupiter.
10 Size, boundaries, and position on the sky
Delphinus covers 188.5 square degrees of sky, which amounts to 0.457 percent of the entire celestial sphere — a small patch, ranking 69th out of 88 modern constellations. Its official three-letter abbreviation, adopted by the International Astronomical Union in 1922, is Del. The formal boundaries were drawn by the Belgian astronomer Eugène Delporte in 1930 as a 14-sided polygon. In right ascension the constellation spans from roughly 20 hours 14 minutes to 21 hours 09 minutes, and in declination from just over +2.4 degrees to just under +21 degrees, placing it close to but slightly north of the celestial equator. Any observer south of latitude 69° South can see at least part of Delphinus, which effectively means the constellation is accessible from virtually every populated location on Earth. It is neighboured clockwise from north by Vulpecula, Sagitta, Aquila, Aquarius, Equuleus, and Pegasus, tucked into a crowded corner of the late-summer northern sky.