Beta Tauri

Beta Tauri (β Tau), commonly known as Elnath, is a blue-white giant star of spectral class B7III located in the constellation Taurus, marking the tip of the bull's northern horn.^[1] It is the second-brightest star in Taurus after Aldebaran and the 27th-brightest star in the night sky overall, with an apparent visual magnitude of 1.65.^[2]^[1] The star lies approximately 130 light-years from the Sun, based on Gaia parallax measurements of about 24.5 mas, and it also holds the designation Gamma Aurigae (γ Aur), serving as a navigational "linking star" between the constellations Taurus and Auriga.^[1]^[3] Elnath exhibits high proper motion, with components of 22.76 mas/year in right ascension and -173.58 mas/year in declination, classifying it as a high proper-motion star, and it has a radial velocity of +9.2 km/s relative to the Sun.^[1] Physically, the star has an estimated mass of about 4.5 solar masses, a radius of about 5 solar radii, and an effective surface temperature of 13,600 K, resulting in a bolometric luminosity approximately 600 times that of the Sun when accounting for ultraviolet contributions.^[3] As a chemically peculiar mercury-manganese star, Elnath shows anomalous surface abundances, including manganese levels 25 times higher than solar and deficiencies in calcium and magnesium to about one-eighth solar values.^[3] It displays no notable photometric variability and is in a post-main-sequence evolutionary phase where core hydrogen fusion is nearing completion, positioning it for future expansion into an orange giant.^[3] Positioned about 3° west of the Milky Way's anticenter, Elnath provides a prominent reference point for observers in the northern celestial hemisphere.^[3]

Nomenclature

Traditional names

Beta Tauri is known by the traditional name Elnath, derived from the Arabic phrase al-naṭḥ, meaning "the butting one" or "the gore," in reference to the bull's horn in the constellation Taurus.^[4] This name was officially approved by the IAU Working Group on Star Names (WGSN) on July 20, 2016, as the proper name for the star.^[5] This name, also spelled Alnath or El Nath, was used in medieval Arabic astronomy and later adopted in European star catalogs.^[3] In ancient catalogs, the star appears without a proper name but is described by Ptolemy in his Almagest (2nd century CE) as the prominent star marking the tip of the northern horn of the Bull.^[6] The Arabic designation likely originated from translations and adaptations of Ptolemy's descriptions by astronomers like al-Sufi in the 10th century, who aligned traditional names with Greek constellations.^[7] Due to its position on the border between Taurus and Auriga, Johann Bayer assigned it dual designations in his 1603 atlas Uranometria: Beta Tauri for its role as the bull's horn tip and Gamma Aurigae for its proximity to the charioteer's foot, reflecting the ambiguous boundaries in earlier maps.^[8] The name Elnath etymologically ties to the bull imagery central to Taurus in Mesopotamian and Greek astronomical traditions, symbolizing the constellation's horned figure.^[4]

Astronomical designations

Beta Tauri bears the Bayer designation β Tauri, assigned by Johann Bayer in his 1603 Uranometria atlas as the second-brightest star in the constellation Taurus. It also holds the Flamsteed designation 112 Tauri, from John Flamsteed's 1725 Historia Coelestis Britannica, which numbers stars sequentially by right ascension within each constellation. Due to its position near the historical border between the constellations Taurus and Auriga, Beta Tauri was alternatively designated γ Aurigae (Gamma Aurigae) in some early catalogs, reflecting ambiguous boundaries in pre-modern star atlases.^[9] This dual naming arose from varying interpretations of constellation outlines by astronomers like Ptolemy and Tycho Brahe, but was resolved when the International Astronomical Union (IAU) formalized constellation boundaries in 1930, placing the star firmly within Taurus and retaining β Tauri as the primary designation while deprecating γ Aurigae. In Auriga, it corresponds to the Flamsteed number 23 Aurigae. Beta Tauri appears in numerous modern astronomical catalogs with additional identifiers. The Henry Draper Catalogue assigns it HD 35497, based on its spectral classification and position in Annie Jump Cannon's 1918-1924 survey. The Harvard Revised Catalogue of 1982 lists it as HR 1791. Other entries include SAO 77168 from the Smithsonian Astrophysical Observatory Star Catalog (1966), FK5 202 from the Fifth Fundamental Catalogue (1988), and HIP 25428 from the Hipparcos Catalogue (1997), which provide precise astrometric data.

Catalog	Identifier
Bayer (Taurus)	β Tauri
Bayer (Auriga, historical)	γ Aurigae
Flamsteed (Taurus)	112 Tauri
Flamsteed (Auriga)	23 Aurigae
Henry Draper	HD 35497
Harvard Revised	HR 1791
Smithsonian Astrophysical Observatory	SAO 77168
FK5	202
Hipparcos	HIP 25428

Location and visibility

Celestial position

Beta Tauri occupies the equatorial coordinates of right ascension 05ʰ 26ᵐ 17.5ˢ and declination +28° 36′ 27″ in the J2000.0 epoch. The star lies at a distance of approximately 134 light-years, or 41.1 parsecs, as determined from the revised Hipparcos parallax measurement of 24.36 ± 0.34 mas. Within the constellation Taurus, Beta Tauri marks the tip of the bull's northern (left) horn, positioned near the Hyades open cluster and directly on the boundary with Auriga, where it is sometimes denoted as γ Aurigae.^[10]^[11] Its space motion includes an annual proper motion of +22.76 ± 0.42 mas/yr in right ascension and -173.58 ± 0.18 mas/yr in declination, accompanied by a radial velocity of +9.2 ± 2 km/s indicating recession from the Solar System.

Observational characteristics

Beta Tauri, also known as Elnath, has an apparent visual magnitude of 1.65, rendering it visible to the naked eye under clear conditions and ranking it as the second-brightest star in the constellation Taurus after Aldebaran (magnitude 0.86), as well as the 27th brightest star in the night sky overall.^[12] Its steady brightness shows no significant photometric variability, remaining constant to within 0.01 magnitudes based on precise measurements from space-based observatories.^[13] The star appears bluish-white to observers, a consequence of its hot surface temperature associated with its B7III spectral classification, which produces a prominent blue hue in the visible spectrum.^[10] This color is evident even without optical aid, contributing to its striking presence at the tip of Taurus's northern horn. In the Northern Hemisphere, Beta Tauri offers optimal visibility during winter evenings from December to March, when the constellation rises prominently in the eastern sky after dusk and reaches its highest point (culmination) around midnight.^[14] At a declination of +28° 36', it is well-placed for observation from mid-northern latitudes, achieving altitudes above 60° for viewers at 40° north; its position near the galactic anticenter further enhances its backdrop against a relatively sparse stellar field.^[13] Through a small telescope, Beta Tauri reveals a faint visual companion, designated BD+28° 795B, separated by 33.4 arcseconds at a position angle of 239°, appearing as a 10th-magnitude point of light that contrasts sharply with the primary's brilliance.^[12]

Stellar properties

Primary component

The primary component of Beta Tauri is classified as a blue-white giant star with spectral type B7III.^[15] This star possesses an effective temperature of approximately 13,600 K, a radius of 4.8 solar radii, and a mass of 4.6 solar masses.^[16] Its luminosity measures 700 times that of the Sun, yielding an absolute bolometric magnitude of -2.37 and a surface gravity of log g = 3.63.^[16]^[3] Positioned in the transitional phase between the main sequence and giant stages of stellar evolution, the primary has an estimated age of 100–200 million years and rotates with a projected equatorial velocity of 59 km/s.^[3]

Chemical composition

Beta Tauri is classified as a mercury-manganese (HgMn) star, a non-magnetic subtype of chemically peculiar upper main-sequence B-type stars exhibiting overabundances of heavy elements in their photospheres due to atomic diffusion in stable radiative atmospheres. These stars display anomalous surface compositions resulting from the interplay of gravitational settling, radiative acceleration, and weak thermal diffusion, which segregate elements vertically and horizontally without the influence of magnetic fields that characterize related Ap stars.^[17] High-resolution spectroscopic analysis of Beta Tauri, based on Dominion Astrophysical Observatory (DAO) CCD spectrograms at a dispersion of 2.4 Å mm⁻¹, identifies strong Mn II lines and the characteristic Hg II resonance line at 3984 Å, confirming its HgMn classification despite the latter's weak or variable intensity in some observations. Abundance determinations, derived from equivalent widths via the curve-of-growth method and synthetic spectrum fitting with ATLAS9 model atmospheres, reveal a manganese overabundance of [Mn/H] = +1.77 (a factor of ~60 relative to solar), with mercury typically enhanced by factors of 10–100 in this class, though not quantified here due to non-detection. Elevated levels of gallium, yttrium, and xenon are hallmarks of HgMn stars like Beta Tauri, while iron shows near-solar abundance ([Fe/H] ≈ 0) and nickel is depleted ([Ni/H] = -0.48, a factor of ~3 below solar). Calcium is also depleted ([Ca/H] ≈ -1.0).^[17] Additional depletions include helium ([He/H] = -0.08), carbon (-0.09), magnesium (-0.36), silicon (-1.08), sulfur (-0.19), titanium (-0.88), and chromium (-0.51), all relative to solar values from Asplund et al. (2005). These patterns underscore the role of diffusion in creating chemical inhomogeneities, analyzed through detailed line profile fitting that accounts for the star's projected rotational velocity of 59 km s⁻¹. The peculiarities distinguish Beta Tauri from normal B stars and highlight diffusion-dominated evolution in non-magnetic environments.^[17]

Companions

Spectroscopic binary

Radial velocity measurements indicate that Beta Tauri is a single-lined spectroscopic binary (SB1), with the presence of an unseen companion inferred from periodic Doppler shifts in the spectral lines of the primary star. Only the lines from the brighter primary component are detectable, as the secondary is too faint to resolve individually. The companion orbits too closely to be visually separated from the primary, with a projected separation of less than 1 arcsecond. This close configuration was detected through Doppler spectroscopy in the 20th century. However, no published information exists on the orbital period, eccentricity, or properties of the companion such as mass or spectral type.

Visual companion

Beta Tauri possesses a faint visual companion, cataloged as BD+28 795B (also known as CCDM J05263+2836B), separated from the primary component by 33.4 arcseconds at a position angle of 239 degrees. This companion is significantly dimmer than the primary and is regarded as an optical double, with no evidence of physical association or common proper motion with the Beta Tauri system. Observations indicate that the companion's distance and kinematics differ from those of the main star, confirming it as a line-of-sight alignment rather than a bound member.^[12]