# Lambda Tauri

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Triple star system in the constellation Taurus

λ Tauri Location of λ Tauri (circled) Observation data Epoch J2000.0 Equinox J2000.0 Constellation Taurus Right ascension 04h 00m 40.81572s[1] Declination +12° 29′ 25.2259″[1] Apparent magnitude (V) +3.37 (- 3.54) - 3.91[2] Characteristics Spectral type B3 V + A4 IV[3] U−B color index −0.62[4] B−V color index −0.12[4] Variable type Algol[2] Astrometry Radial velocity (Rv) +17.8[5] km/s Proper motion (μ) RA: −8.02[1] mas/yr Dec.: −14.42[1] mas/yr Parallax (π) 6.74±0.17 mas[1] Distance 480 ± 10 ly (148 ± 4 pc) Absolute magnitude (MV) −2.45[6] Orbit[7] Primary λ Tau A Name λ Tau B Period (P) 3.9529552 days Semi-major axis (a) 21.91 R☉[8] Eccentricity (e) 0.025±0.015 Inclination (i) 76+1 −2[9]° Longitude of the node (Ω) 15±2 or 195±2[9]° Periastron epoch (T) 2,444,667.3±2.1 HJD Semi-amplitude (K1) (primary) 56.9±0.6 km/s Semi-amplitude (K2) (secondary) 215.6±0.7 km/s Details λ Tau A Mass 7.18[10] M☉ Radius 6.40[10] R☉ Luminosity 5,801[3] L☉ Surface gravity (log g) 3.38[11] cgs Temperature 18,700[3] K Rotational velocity (v sin i) 85[8] km/s Age 33.2±3.9[12] Myr λ Tau B Mass 1.89[10] M☉ Radius 5.30[10] R☉ Luminosity 128[3] L☉ Temperature 8,405[3] K Rotational velocity (v sin i) 76[8] km/s Other designations Bibing, λ Tau, 35 Tau, BD+12 539, FK5 150, HD 25204, HIP 18724, HR 1239, SAO 93719[13] Database references SIMBAD data

**Lambda Tauri**, [Latinized](/source/Romanization_of_Greek) from λ Tauri formally named **Bibing**,[14] is a [triple star system](/source/Triple_star_system) in the [constellation](/source/Constellation) [Taurus](/source/Taurus_(constellation)). In 1848, the light from this system was found to vary periodically and it was determined to be an [eclipsing binary](/source/Eclipsing_binary) system—the third such discovered.[7] The components of this system have a combined [apparent visual magnitude](/source/Apparent_visual_magnitude) of +3.37 at its brightest, making it one of the [brighter members](/source/List_of_stars_in_Taurus) of the constellation. Based upon [parallax](/source/Parallax) measurements from the [Hipparcos](/source/Hipparcos) mission, the distance to this system is approximately 480 [light-years](/source/Light-year) (150 [parsecs](/source/Parsec)).[1]

## Nomenclature

*Lambda Tauri* ([Latinized](/source/Romanization_of_Greek) from λ Tauri, abbreviated λ Tau) is the star's [Bayer designation](/source/Bayer_designation). In the *Calendarium of Al Achsasi Al Mouakket*, this star was designated *Sadr al Tauri*, which was translated into [Latin](/source/Latin) as *Pectus Tauri*, meaning "the bull chest".[15]

In [Chinese astronomy](/source/Chinese_astronomy), this star is part of the constellation Bi (畢, [Net](/source/Net_(Chinese_constellation))), one of the [lunar mansions](/source/Twenty-Eight_Mansions); it is named Bibing (畢柄), the handle of the net. The [IAU Working Group on Star Names](/source/IAU_Working_Group_on_Star_Names) adopted the name Bibing for λ Tauri Aa on 17 May 2026.[14]

## System

A [light curve](/source/Light_curve) for Lambda Tauri, plotted from *[TESS](/source/Transiting_Exoplanet_Survey_Satellite)* data[16]

The inner pair of this triple star system, Lambda Tauri AB, orbit around each other with a [period](/source/Orbital_period) of 3.95 days and a low [eccentricity](/source/Orbital_eccentricity) of about 0.025.[7] Their [orbital plane](/source/Orbital_plane_(astronomy)) is inclined by around 76° to the line of sight from the Earth,[10] so it is being viewed from nearly edge on and the two stars form an [Algol](/source/Algol)-like [eclipsing binary](/source/Eclipsing_binary) system. The combined brightness of the pair varies from magnitude +3.37 to +3.91 as first one star and then the other pass in front of its companion. The primary member, λ Tau A, undergoes a decrease of 0.435±0.050 in magnitude during an eclipse, while the secondary component, λ Tau B, decreases by 0.09–0.10 in magnitude.[17] The mean physical separation between these two stars is estimated at 21.91 times the radius of the Sun, or 0.1 [Astronomical Units](/source/Astronomical_Units).[8]

The primary component has a [stellar classification](/source/Stellar_classification) of B3 V, making this a massive [B-type main sequence star](/source/B-type_main_sequence_star). It has over seven times the [mass of the Sun](/source/Mass_of_the_Sun)[17] and 6.4 times the [Sun's radius](/source/Sun's_radius).[8] This star is the brightest member of the system, radiating about 5,801[3] times the [luminosity of the Sun](/source/Luminosity_of_the_Sun) from its outer envelope at an [effective temperature](/source/Effective_temperature) of 18,700 [K](/source/Kelvin), which gives it a blue-white hue common to the B-type stars.[18] Lambda Tauri A is rotating rapidly with a [projected rotational velocity](/source/Projected_rotational_velocity) of 85 km/s.[8] It, along with [δ Librae](/source/Delta_Librae), were the first stars on which rotational line broadening was observed, by [Frank Schlesinger](/source/Frank_Schlesinger) in 1909.[19]

The third component, λ Tau C, is orbiting the inner pair over a 33.025 day period with an [eccentricity](/source/Orbital_eccentricity) of roughly 0.15. The orbital plane of this component is nearly coplanar with the orbit of Lambda Tauri AB, differing by no more than 7°. It has about half the mass of the Sun.[17] The orbit of this star causes perturbation effects on the orbit of the AB pair, resulting in periodic changes in their orbital eccentricity and other [orbital elements](/source/Orbital_elements).[7]

## Physical characteristics

The spectrum of Lambda Tauri A shows an under abundance of carbon relative to the norm for this category of star. A possible explanation for this is a loss of mass by the star some time in the past. An inner region of the star became depleted by the conversion of carbon into nitrogen during the nuclear fusion process, and this region was later exposed when the outer envelope of the star was lost. Alternatively, the star may have undergone a period of convective mixing, bringing the carbon-depleted material to the surface. However, the cause of such a fully convective behavior in a [main sequence](/source/Main_sequence) star of this mass is unclear.[11]

The secondary companion has a stellar classification of A4 IV,[3] suggesting that it is a [subgiant star](/source/Subgiant_star) that has nearly exhausted the supply of hydrogen at its core and is in the process of [evolving](/source/Stellar_evolution) into a [giant star](/source/Giant_star). It has nearly 1.9 times the mass of the Sun, 5.3 times the Sun's radius,[8] and is radiating 128 times the Sun's luminosity at an effective temperature of 8,405 K.[3] As with the primary, this star is spinning rapidly with a projected rotational velocity of 76 km/s.[8] The side of the secondary facing the more massive star is being heated by an additional 1,440 K, which produces a rotational effect that causes the strength of the secondary's [spectral lines](/source/Spectral_line) to vary over the course of its orbit.[7]

A conundrum with this system is the large radius of the secondary star. In [stellar evolutionary](/source/Stellar_evolution) terms, the more massive primary should be the first to reach the subgiant stage. Hence the enlarged radius of the secondary must be caused by a means other than the star's age. This suggests that the pair Lambda Tauri AB form a [semidetached binary](/source/Semidetached_binary) with the secondary filling its [Roche lobe](/source/Roche_lobe), giving it a distorted shape.[7]

## References

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v t e Constellation of Taurus Golden Gate of the Ecliptic List of stars in Taurus Sterope Taurus in Chinese astronomy Stars Bayer α (Aldebaran) β (Elnath) γ (Prima Hyadum) δ1 (Secunda Hyadum) δ2 δ3 ε (Ain) ζ (Tianguan) η (Alcyone) θ1 θ2 (Chamukuy) ι κ1 κ2 λ (Bibing) μ ν ξ ο π ρ σ1 σ2 τ (Gaja) υ φ χ ψ ω1 ω2 Flamsteed 5 (f) 6 (t) 7 10 16 (Celaeno) 17 (Electra) 18 19 (Taygeta) 20 (Maia) 21 (Asterope) 22 23 (Merope) 27 (Atlas) 28 (Pleione) 32 36 37 (A1) 39 (A2) 40 41 44 (p) 47 57 (h) 66 (r) 71 72 75 (Shakata) 79 (b) 88 (d) 90 (c) 97 (Lembu) 104 (m) 109 (n) 111 114 (o) 119 126 132 134 136 139 44 Eri Variable T Y RV RW RY SZ UX XX XZ AA BP CI CM (Crab Pulsar) CQ DF DG B DH DL EQ EU FU GG GI GK GV HK HL HP HU IK V411 V471 V711 V773 V830 V833 V834 V987 V1079 V1298 HR 1172 1427 1436 HD 23514 24040 24496 32963 36112 37124 Other 2MASS J03552337+1133437 2MASS J04070752+1546457 2MASS J04202144+2813491 2MASS J0441+2301 2MASS J04442713+2512164 CoKu Tau/4 Gliese 176 HH 30 IRAS 04125+2902 K2-25 K2-136 K2-288 KPNO-Tau 12 L1551 IRS 5 NGC 1988 NLTT 11748 PSR J0337+1715 PSR J0348+0432 PSR J0538+2817 Teide 1 WD 0343+247 WISE 0410+1502 Exoplanets 2M0437 b ε Tauri b (Amateru) BD+20 594b Gliese 176 b HD 24040 b HD 37124 b c d IRAS 04125+2902 b K2-155d K2-288Bb Kojima-1Lb TOI-4603 b Star clusters NGC 1647 1746 1807 1817 1996 Other Hyades Pleiades Nebulae NGC 1435 1514 1554 1555 Other Crab Nebula GN 04.32.8 IC 349 IRAS 05437+2502 Simeis 147 Taurus molecular cloud Galaxies NGC 1349 1409 1410 1550 1589 1590 1633 1634 1642 Other 1ES 0414+009 3C 99 3C 109 3C 120 3C 132 3C 138 Abell 478 BCG MS 0440+0204 BCG PKS 0428+205 QSO J0439+1634 Galaxy clusters Abell 478 Astronomical events GRB 070714B SN 1054 Category

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