Alpha Piscium
Alpha Piscium
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α Piscium
Location of α Piscium (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Pisces
Right ascension 02h 02m 02.81972s[1]
Declination +02° 45′ 49.5410″[1]
Apparent magnitude (V) 3.82 (4.33 + 5.23)[2]
Characteristics
Evolutionary stage main sequence[3][4]: 16 
Spectral type kA0hA7 Sr + kA2hF2mF2 (IV)[5]
Variable type α2 CVn[6]
Astrometry
Proper motion (μ) RA: +32.45[1] mas/yr
Dec.: +0.04[1] mas/yr
Parallax (π)20.76±0.15 mas[4]
Distance157.1 ± 1.1 ly
(48.17±0.34 pc)[4]
Absolute magnitude (MV)+0.50[7]
Orbit[4]
Primaryα Psc A
Nameα Psc B
Period (P)2,822±215 yr
Semi-major axis (a)7.45±0.35″
Eccentricity (e)0.4650±0.0145
Inclination (i)113.01±2.45°
Longitude of the node (Ω)3.09±6.28°
Periastron epoch (T)2188.6[8]
Argument of periastron (ω)
(secondary)		150±6°
Orbit[4]
Primaryα Psc Ba
Nameα Psc Bb
Period (P)24.999428±0.000033 days
Semi-major axis (a)(5.175±0.018)×10−3"
(0.2493±0.0020 AU)
Eccentricity (e)0.6004±0.0018
Inclination (i)65.2±0.4°
Longitude of the node (Ω)73.08±0.63°
Argument of periastron (ω)
(primary)		140.14±0.42°
Semi-amplitude (K1)
(primary)		61.17±0.35 km/s
Semi-amplitude (K2)
(secondary)		61.97±0.47 km/s
Details
α Psc A
Mass2.49±0.05[9] M☉
Radius2.33±0.26[4] R☉
Luminosity53.7+8.0
−6.9[9] L☉
Surface gravity (log g)4.01±0.14[3] cgs
Temperature10,000±710[3] K
Rotation1.491±0.022[4] days
Rotational velocity (v sin i)81[10] km/s
Age330±150[3] Myr
α Psc Ba
Mass1.668±0.033[4] M☉
Radius1.76±0.41[4] R☉
Surface gravity (log g)4.0[4] cgs
Temperature8,000[4] K
α Psc Bb
Mass1.646±0.029[4] M☉
Radius1.55±0.41[4] R☉
Surface gravity (log g)4.0[4] cgs
Temperature8,000[4] K
Other designations
Alrischa, Kaitain, Okda, Okdah, Syndesmos, α Psc, 113 Piscium, BD+02°317, FK5 28, HIP 9487, SAO 110291, WDS J02020+0246AB[11][12]
α Psc A: HD 12447, HR 596[13]
α Psc B: HD 12446, HR 595[14]
Database references
SIMBADα Psc
α Psc A
α Psc B

Alpha Piscium (α Piscium) is a triple star[4] system in the equatorial constellation of Pisces. Based upon dynamical parallax measurements, it is about 157 light-years from the Solar System.

The three components are designated Alpha Piscium A (officially named Alrescha /ælˈriːʃə/, the traditional name of the system),[15][16] Ba and Bb.

Nomenclature

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α Piscium (Latinised to Alpha Piscium) is the star's Bayer designation. The designations of the two components as Alpha Piscium A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[17]

The system bore the traditional name Alrescha (alternatively Al Rescha, Alrischa, Alrisha) derived from the Arabic الرشآء ar-Rishā’ "the cord" and less commonly Kaitain and Okda, the latter from the Arabic عقدة ʽuqdah "knot" (see Ukdah).[18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] to catalog and standardize proper names for stars. The WGSN approved the name Alrescha for the component Alpha Piscium A on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[16]

In Chinese, 外屏 (Wài Píng), meaning Outer Fence, refers to an asterism consisting of Alpha Piscium, Delta Piscium, Epsilon Piscium, Zeta Piscium, Mu Piscium, Nu Piscium and Xi Piscium. Consequently, the Chinese name for Alpha Piscium itself is 外屏七 (Wài Píng qī, English: the Seventh Star of Outer Fence).[20]

Properties

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Alpha Piscium comprises an hierarchical architecture. The outer system comprises the star Alpha Piscium A and the subsystem Alpha Piscium B, separated by 1.85".[4] Alpha Piscium A, the primary, is of magnitude +4.33[2] and spectral type kA0hA7 Sr,[5] while the components of Alpha Piscium B have combined magnitude 5.23[2] and combined spectral class kA2hF2mF2 (IV).[5] The two bodies take roughly 2,800 years to orbit one another[4] and they will make their closest approach to each other around 2188.[8] The components of Alpha Piscium B, called Ba and Bb, have an orbital period of roughly 25 days and a semi-major axis of 0.2493±0.0020 AU.[4]

A light curve for Alpha Piscium, plotted from TESS data[21]

Alpha Piscium is catalogued as an α2 Canum Venaticorum variable, a type of variable star where the brightness changes are caused by the rotation of the star. The brightness varies by about 1/100 of a magnitude, identified from Hipparcos photometry.[6] The primary component is thought to be the source of the variations, and it has a period of 0.845 days which was believed to be the rotation period of the star,[22] but the latter has been revised to 1.49 days.[4] Variations with a period of 6.65 days have also been identified.[22]

Alpha Piscium composes a four-star system together with the G1/2V star HIP 9519, which is separated by 404.7".[4]

Long-exposure observation

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Stars that can set (not in a circumpolar constellation for the viewer) culminate at midnight – where viewed away from any polar region experiencing midnight sun – when at opposition, meaning they can be viewed from dusk until dawn. This applies to α Piscium on 21 October, in the current astronomical epoch.[23]

Half of the year from this date, 22 April, the star will be at conjunction above or below, the sun – apart by the star's declination (angle set out in table, right). The nearby days and months have most of the star's risen time during daylight.[citation needed]

References

[edit]
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Alpha Piscium

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Alpha Piscium, also known as Alrescha, is a binary star system located in the constellation Pisces, marking the point where the two fish are tied together by a cord in mythological depictions.[1] It is the third-brightest star in Pisces, with a combined apparent visual magnitude of 3.94, making it visible to the naked eye under dark skies.[1] The system lies approximately 151 light-years from Earth and consists of two white main-sequence stars orbiting each other with a period of about 3,270 years.[1][2] The primary component, Alpha Piscium A, is an A0p spectral type star with an apparent magnitude of 4.3 and is classified as a chemically peculiar Ap star, exhibiting strong magnetic fields roughly 1,000 times that of Earth, which cause spectral line variations over a 0.85-day rotation period due to enhanced elements like silicon and strontium.[1][3] The secondary, Alpha Piscium B, is an A3m metallic-line star with a magnitude of 5.23, showing abundance anomalies in elements such as copper and zinc.[1] The two stars are currently separated by an angular distance of 1.8 arcseconds (corresponding to about 83 AU), and the system is cataloged as a visual binary (Struve 202).[1] Alrescha's proper name derives from Arabic "al-rišā," meaning "the cord" or "the knot," reflecting its position at the binding point of Pisces.[3] The system's coordinates are right ascension 02h 02m 02.8s and declination +02° 45′ 50″ (J2000 epoch), with a parallax of 21.66 mas.[3] As an Alpha2 Canum Venaticorum variable, the primary's brightness shows small fluctuations due to its magnetic activity.[3]

Nomenclature and Etymology

Bayer Designation and Catalog Entries

α Piscium (α Psc) is the Bayer designation for a prominent star in the constellation Pisces assigned by Johann Bayer in his influential 1603 star atlas Uranometria, where it marks the "knot" connecting the tails of the two fish in the Pisces figure.[4][5] This binary system appears in numerous astronomical catalogs under various identifiers, including the Flamsteed designation 113 Piscium, Henry Draper Catalogue entry HD 12447, Bright Star Catalogue number HR 596, Hipparcos Input Catalogue number HIP 9487, and Gaia Data Release 3 source identifier 2545924696372820096.[6][7][8] In 2016, the International Astronomical Union (IAU), through its Working Group on Star Names, formalized the designations of the system's components as Alpha Piscium A (the primary) and Alpha Piscium B (the secondary) to standardize nomenclature for binary stars.

Traditional and Cultural Names

Alpha Piscium holds significant traditional names rooted in Arabic astronomy, reflecting its position in the constellation Pisces. The primary name, Alrescha (also spelled Al Risha, Alrischa, or Alrisha), derives from the Arabic term al-rišāʾ, meaning "the cord" or "the well rope," symbolizing the binding element between the two fish in the mythological depiction of Pisces.[9] Alternative Arabic designations include Kaitain, from al-ḥayṭ al-kattāniyy meaning "the thread," and Okda, from al-ʿuqdah meaning "the knot," both emphasizing the star's role as a connector in ancient sky maps.[10] In the mythology of Pisces, which portrays two fish tied together by a cord to evade the monster Typhon, Alrescha marks the knot where the ribbons from the fishes' tails meet, serving as the symbolic binding point of the constellation.[11] This imagery underscores the star's cultural importance as a unifier in Greco-Roman and Arabic traditions, where the constellation represented linked entities fleeing peril.[12] In Chinese astronomy, Alpha Piscium is designated as Wài Píng qī (外屏七), or "the Seventh Star of the Outer Fence," forming part of the ancient asterism Wài Píng (外屏), a linear enclosure of seven stars from Alpha to Delta Piscium intended to screen off the nearby "cesspit" asterism Tiānhùn in Cetus.[13] This naming reflects the systematic organization of the sky into protective barriers in traditional Chinese celestial lore. The International Astronomical Union (IAU) formalized Alrescha as the proper name for the primary component, Alpha Piscium A, through its Working Group on Star Names (WGSN) on August 21, 2016, standardizing it in the official IAU List of Approved Star Names to preserve historical nomenclature.[1]

Stellar Characteristics

Primary Component (Alpha Piscium A)

Alpha Piscium A is a chemically peculiar upper main-sequence star classified as spectral type A0p, exhibiting significant overabundances of elements including silicon, strontium, and chromium due to diffusion processes in its stable atmosphere influenced by a strong magnetic field.[1] This Ap (peculiar) classification reflects anomalous spectral lines from these chemical enhancements, with the star also showing variability in line strengths as its magnetic field rotates into view.[1] The star has a visual magnitude of +4.33 and is categorized as an α² Canum Venaticorum-type variable, displaying small photometric variations with an amplitude of 0.01 magnitudes and a period of 0.845 days.[14] These changes arise from rotational modulation, where uneven distributions of chemical elements on the stellar surface—coupled with the oblique magnetic field—alter the brightness as the star rotates.[1] The strong magnetic field, measured at several kilogauss, drives these spectral and photometric effects by organizing atmospheric elements into spots that become visible periodically.[1] Physical parameters place Alpha Piscium A at approximately 2.0 solar masses, with a radius of about 2.5 solar radii, a surface temperature around 10,000 K, and a luminosity of roughly 40 solar luminosities.[1] These properties align with models of young, magnetically active A-type stars on the main sequence. The binary system containing Alpha Piscium A lies at a distance of 151 ± 7 light-years from Earth, determined from Gaia parallax data as of DR2.

Secondary Component (Alpha Piscium B)

Alpha Piscium B, the fainter companion in the binary system, is classified as an A3m star, a subtype of metallic-line (Am) stars exhibiting anomalous spectral lines due to underabundances of calcium and scandium, along with overabundances of certain heavy metals including copper, zinc, strontium, zirconium, and barium.[1] These chemical peculiarities arise from mild diffusion processes in the stellar atmosphere, where gravitational settling and radiative acceleration lead to selective enrichment of heavier elements in the outer layers, a phenomenon common in slowly rotating A-type main-sequence stars.[15] The star has a visual magnitude of +5.23 and shows no significant photometric variability.[1] As a main-sequence dwarf, Alpha Piscium B possesses a mass of approximately 1.8 solar masses, a surface temperature of around 8,600 K, and a luminosity of about 12 solar luminosities; its radius is estimated at roughly 1.6 solar radii based on these parameters.[1] These attributes place it as a stable, hydrogen-fusing companion contrasting with the more active primary.

Binary System Dynamics

Orbital Parameters

Alpha Piscium is a wide visual binary system whose orbital parameters have been estimated from long-term visual observations and astrometric data. The orbital period is approximately 720 years, reflecting the system's large physical scale and slow relative motion of its components.[1] This estimate is derived from historical visual position measurements spanning centuries. The orbit exhibits moderate eccentricity, with components experiencing variation in separation from a minimum at periastron to a maximum at apastron. The current angular separation is 1.8 arcseconds. Early position measurements are compiled in catalogs like the Washington Double Star Catalog. Astrometric data from the Hipparcos mission (1997) and Gaia mission (up to DR3 in 2022) provide precise positions and proper motions, but the short observational baseline relative to the long period limits full orbit determination—only a small arc is covered.[8] Due to this, orbital elements remain tentative. At the system's distance of 151 light-years (46.2 pc from parallax 21.66 mas), the current projected linear separation is approximately 83 AU.[3]

Physical Separation and Evolution

The binary components of Alpha Piscium are currently separated by approximately 83 AU, with the average physical distance estimated at 120 AU and varying between 50 and 190 AU over the orbital cycle due to eccentricity.[1] This wide separation ensures negligible tidal interactions, allowing each star to evolve independently as isolated main-sequence objects without significant mutual influence on their structures or angular momentum. The system is estimated to be around 330 million years old, placing both components on the main sequence, with the primary (A-type chemically peculiar star) approaching the end of core hydrogen fusion.[14] With metallicity [Fe/H] ≈ -0.1, consistent with the Galactic thin-disk population, the stars show abundances typical of relatively young, metal-enriched objects in the solar neighborhood.[14] The primary is projected to exhaust core hydrogen in roughly 1 billion years, evolving into a subgiant and eventually forming a white dwarf, with the secondary following on a slightly delayed timescale due to its lower mass. The wide orbit prevents common-envelope evolution or mass transfer, as the stars remain distant during post-main-sequence expansion. Binary evolution models for Ap-Am pairs, such as those by Netopil et al. (2017), indicate that magnetic fields and peculiar abundances affect rotational and chemical evolution without major binary influences.

Visibility and Observation

Celestial Position and Coordinates

Alpha Piscium is located in the equatorial constellation of Pisces, with its J2000.0 equatorial coordinates given by a right ascension of 02h 02m 02.81972s and a declination of +02° 45′ 49.5410″. These positions, determined through high-precision astrometry, place the star near the celestial equator, making it accessible from both hemispheres. In galactic coordinates, Alpha Piscium resides at longitude 155.35° and latitude -55.60°, situating it in the southern galactic hemisphere, well below the plane of the Milky Way.[7] Within the constellation Pisces, which depicts two fish connected by a cord, Alpha Piscium marks the southeastern knot where the cord binds the tails of the fish together, serving as a key navigational point in the asterism.[11] The star's proper motion is +32.45 mas/year in right ascension and +0.04 mas/year in declination.[16] Due to the precession of Earth's rotational axis, the equatorial coordinates of Alpha Piscium shift gradually over time, completing a full cycle approximately every 25,772 years. This luni-solar precession causes the right ascension to increase by about 50.3 arcseconds per year on average, while the declination varies depending on the star's position; for Alpha Piscium, near the ecliptic, these changes accumulate to alter its coordinates by roughly 1.4° in right ascension and 0.4° in declination over a century.[17] Astronomers account for such effects using standard precession models to update positions for specific epochs, ensuring accurate cataloging and observation planning across centuries.[17]

Optimal Viewing Conditions

Alpha Piscium, with a combined apparent magnitude of 3.82 (primary 4.33, secondary 5.23), is visible to the naked eye under dark skies but requires binoculars or a small telescope to resolve its binary nature from locations with moderate light pollution.[9] In the Northern Hemisphere, optimal viewing occurs during autumn evenings, when the star rises in the eastern sky by late September and becomes prominent after sunset.[18] From northern mid-latitudes, it culminates at local midnight around late October, specifically during the last week of the month, allowing for the longest observation window free from horizon obstruction.[19] The star's low declination of +2.76° results in a maximum altitude of approximately 53° at culmination for observers at 40°N latitude, reaching this peak due south (azimuth 180°), though it appears lower—around 42°—from higher latitudes like 51°N.[7] Visibility is best from September through December, with the star accessible in the evening sky for several hours; however, it becomes unavailable for about one month around late April due to solar conjunction on approximately April 22, when proximity to the Sun's glare renders it unobservable.[9] Amateur observers should prioritize clear, moonless nights during this period to appreciate its position at the "knot" of Pisces.[1] For planning observations from 2025 to 2030, Alpha Piscium's visibility follows a consistent annual cycle due to its fixed sidereal position, with culmination shifting minimally due to precession (less than 0.01° per year). The table below outlines basic seasonal ephemeris highlights for northern mid-latitudes (e.g., 40°N), assuming standard time and dark skies:
YearRise in Evening SkyCulmination at MidnightSets After MidnightNotes
2025Late SeptemberOctober 21–28Early MayFull Moon interference minimal in October
2026Late SeptemberOctober 21–28Early MaySimilar to 2025; avoid April conjunction
2027Late SeptemberOctober 21–28Early MayPeak autumn clarity expected
2028Late SeptemberOctober 21–28Early MayLeap year; slight daylight shift
2029Late SeptemberOctober 21–28Early MayConsistent visibility window
2030Late SeptemberOctober 21–28Early MayPrecession negligible for planning
Professional observers may note that the binary's current 1.8 arcsecond separation allows resolution with 4-inch telescopes under steady seeing, enhancing autumn viewings.[1]

Advanced Observational Techniques

Observing the binary components of Alpha Piscium visually requires telescopes with apertures of at least 100 mm, as the current angular separation is approximately 1.8 arcseconds, making it a challenging split for smaller instruments under typical seeing conditions.[20] The pair was first resolved by William Herschel on October 19, 1779, using his 157 mm reflecting telescope, marking it as one of the early visual binaries in his catalog (H I 81).[21] Long-exposure imaging techniques are essential for capturing the faint magnitude 5.23 companion against the glare of the brighter primary (magnitude 4.33), often employing charge-coupled device (CCD) cameras to accumulate signal over several minutes while minimizing atmospheric distortion. Adaptive optics systems, which correct for wavefront aberrations in real-time, further enhance resolution and contrast, allowing clearer separation in ground-based observations with larger telescopes. Spectroscopic observations have revealed radial velocity variations in the system, with early measurements indicating orbital motion in the primary component, as documented in 1913 using photographic spectroscopy.[22] Modern high-resolution spectroscopy, such as that possible with instruments like HARPS, can probe chemical peculiarities in the Ap-type primary, including enhanced elements like silicon and strontium, while the secondary shows Am-type metallic-line anomalies. Although no dedicated interferometric observations of Alpha Piscium have been reported, facilities like the CHARA array offer potential for precise angular diameter and separation measurements at sub-milliarcsecond resolution, leveraging long baselines to resolve the binary's dynamics beyond traditional imaging limits. A key challenge in photometric studies arises from the primary's intrinsic variability as an Alpha2 Canum Venaticorum star, which introduces noise in brightness measurements of the companion; differential imaging techniques, subtracting the primary's point-spread function from companion frames, help mitigate this glare and variability for accurate flux ratios.
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