Delta Andromedae
A triple star system with a long-period spectroscopic binary.
Delta Andromedae is a triple star system located in the northern constellation Andromeda. Its Bayer designation, derived from the Latinized form δ Andromedae, is also abbreviated as Delta And or δ And. To the naked eye, the system appears as a single point of light with a combined apparent visual magnitude of 3.28. Parallax measurements place it roughly 105.5 light-years from the Sun, and it is moving closer at a radial velocity of −10 km/s.
In Chinese astronomy, the asterism known as Legs includes δ Andromedae along with η Andromedae, 65 Piscium, ζ Andromedae, ι Piscium, ε Andromedae, π Andromedae, ν Andromedae, μ Andromedae, β Andromedae, σ Piscium, τ Piscium, 91 Piscium, υ Piscium, φ Piscium, χ Piscium, and ψ1 Piscium. The Chinese name for δ Andromedae itself derives from this asterism. Apart from its Bayer designation, Elijah H. Burritt also assigned it the title Delta in his star atlas.
The system contains a long-period spectroscopic binary with an orbital period of about 15,000 days (roughly 41 years). The primary star of this binary, component Aa, is classified as K3 III, an aging giant that likely evolved from an F-type main-sequence star after exhausting its core hydrogen. Its companion, component Ab, is a faint K-type dwarf that has been imaged using a stellar coronagraph. The physical separation between these two stars is known.
Two additional companions orbit the system. The closer one is a red dwarf of spectral class M3, separated by at least 940 astronomical units. An excess of infrared emission once suggested a surrounding shell of dust, but by 2003 it was determined to be more likely a circumstellar debris disk.
Quick Facts
- Bayer designation
- δ Andromedae
- Apparent visual magnitude
- 3.28
- Distance
- approximately 105.5 ly
- Radial velocity
- −10 km/s
- Orbital period (spectroscopic binary)
- approximately 15,000 days (41 years)
- Primary classification
- K3 III (aging giant star)
- Secondary classification
- K-type dwarf
Facts from the source article.
Lore & Background
Delta Andromedae is a long-period spectroscopic binary with an orbital period of approximately 15,000 days (41 years). The primary of the spectroscopic binary, component Aa, has a stellar classification of K3 III, indicating that it is an aging giant star. It most likely evolved from an F-type main sequence star after consuming the hydrogen at its core. The secondary, component Ab, is a relatively faint K-type dwarf, which has been imaged using a stellar coronagraph. The system has two companions, the closest of which is an orbiting red dwarf of class M3 with a separation of at least 940 astronomical units. An excess of infrared emission from δ Andromedae suggested that it may be surrounded by a shell of dust; in 2003 it was determined that this is more likely a circumstellar debris disk.
Reader's Guide
Delta Andromedae holds significance as a triple star system that can be observed with the naked eye, offering a combined magnitude of 3.28. Its long-period spectroscopic binary, with an orbital period of about 41 years, provides a case study in stellar evolution, as the primary is an aging giant that likely evolved from an F-type main sequence star. The detection of a faint K-type dwarf companion via coronagraphy and the presence of a red dwarf at a wide separation illustrate the system's complex architecture. The infrared excess, reinterpreted as a circumstellar debris disk rather than a dust shell, adds to its interest as a system with potential planetary or disk-related processes. In Chinese astronomy, it is part of the Legs asterism, and it was also given the title Delta by Elijah H. Burritt in his star atlas. Its legacy includes being a reference point in the constellation Andromeda and a subject of study for stellar multiplicity and debris disk formation.
A Link in the Chained Lady's Form
In the classical star pattern that Ptolemy fixed in his second-century Almagest, delta Andromedae occupies a place among the stars that trace the body and legs of the celestial figure. That figure is Andromeda herself—the daughter of Cassiopeia, queen of Aethiopia, who in Greek myth was chained to a coastal rock as a sacrifice demanded by the Oracle of Ammon to appease the sea monster Cetus. The constellation earned English names like 'the Chained Lady' and 'the Chained Woman,' while Latin tradition called her Mulier Catenata and Arabic sources referred to her as al-Mar'at al Musalsalah. Other appellations, such as Persea or Cepheis, tie the figure to her roles as Perseus's wife and Cepheus's daughter. The hero Perseus ultimately rescued her—either by wielding Medusa's severed head to petrify the beast or, in Ovid's Metamorphoses, by striking it down with a diamond sword. After their marriage and the founding of Mycenae, Athena placed Andromeda among the stars. Three neighboring constellations—Perseus, Cassiopeia, and Cepheus—represent the other characters in that same story, while Cetus itself retreats beyond Pisces. Delta Andromedae, as part of the body, anchors this mythological tableau in the autumn sky.
A Star in Many Skies
Delta Andromedae belongs to a constellation that has been catalogued across cultures for millennia. Ptolemy included Andromeda among his forty-eight patterns in the Almagest, and the International Astronomical Union still recognizes it today, assigning the three-letter code 'And' in 1922 and fixing its precise polygonal boundary—thirty-six segments—through Eugène Delporte's 1930 work. Yet the star's cultural context shifts dramatically depending on the observer's tradition. Arab astronomers, while aware of Ptolemy's layout, wove δ And into a separate figure called al-Hut, meaning 'the fish,' a constellation that also incorporated the hazy patch we now identify as the Andromeda Galaxy. In traditional Chinese astronomy, the stars of Andromeda were distributed among four distinct groupings, each carrying its own astrological and mythological weight; one such grouping, Kui, represented either a walking person's foot or a wild boar. A related constellation also appears in Hindu mythology. In 1787, German astronomer Johann Bode attempted to merge several Andromeda stars with most of Lacerta into a tribute called Honores Friderici, honoring King Frederick II of Prussia, though the figure quickly faded from use. Delta Andromedae thus sits at the intersection of Greek, Arab, Chinese, and Hindu celestial traditions.
Scale and Southern Silence
The constellation that houses delta Andromedae is one of the most expansive figures on the celestial map, spanning 722 square degrees of sky. To put that in perspective, it covers more than 1,400 times the apparent area of the full moon, amounts to roughly 55 percent of the size of Hydra—the single largest of all eighty-eight constellations—and dwarfs Crux, the smallest, by a factor of over ten. Despite its vast footprint, the figure is locked to the northern celestial hemisphere. Because of its declination range, which stretches from 21.68° to 53.19°, observers situated south of 40° south latitude will never see it rise above the horizon; for them, Andromeda simply does not exist in the visible sky. For northern observers, however, the constellation commands the autumn evening sky, sharing that seasonal prominence with the other figures drawn from the Perseus legend. Its right ascension spans from 22 hours 57.5 minutes to 2 hours 39.3 minutes, a wide longitudinal reach that reflects its sheer size. The constellation is also linked to Pegasus, and its stars form part of a broader zone that Ptolemy's attribution to a wider area has preserved since antiquity.
A Neighborhood of Galaxies and Meteors
The region around delta Andromedae is arguably the most observationally rich patch of sky in the northern hemisphere. The constellation's most celebrated deep-sky object is the Andromeda Galaxy, catalogued as M31, which is the nearest spiral galaxy to our own Milky Way and ranks among the brightest entries in the Messier catalogue—visible to the unaided eye. Its two companion galaxies, M110 and M32, also fall within Andromeda's borders, as does the more distant edge-on spiral NGC 891. The Blue Snowball Nebula, a planetary nebula, rewards telescope users with a small blue circular glow. Among the brighter stars that frame this neighborhood, Alpheratz (alpha) is a binary that has also been counted as part of Pegasus, Almach (gamma) is a colorful binary beloved by amateur astronomers, and Mirach (beta) is a red giant whose warm hue is discernible without optical aid. The constellation also serves as the radiant point for the Andromedids, a modest meteor shower that peaks in November. Together, these objects make the area around delta Andromedae a focal point for both casual stargazers and serious observers throughout the autumn season.
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