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Beta Tauri
Beta Tauri
Beta Tauri
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β Tauri
Location of β Tauri in Taurus (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Taurus[1]
Pronunciation /ɛlˈnæθ/[2] or /ˈɛlnæθ/[3]
Right ascension 05h 26m 17.51312s[4]
Declination +28° 36′ 26.8262″[4]
Apparent magnitude (V) 1.65[5]
Characteristics
Spectral type B7III[6]
U−B color index −0.49[5]
B−V color index −0.13[5]
Astrometry
Radial velocity (Rv)9.2[7] km/s
Proper motion (μ) RA: +22.76 mas/yr[4]
Dec.: −173.58 mas/yr[4]
Parallax (π)24.36±0.34 mas[4]
Distance134 ± 2 ly
(41.1 ± 0.6 pc)
Absolute magnitude (MV)−1.42[1]
Details
Mass5.0±0.1[8] M☉
Radius4.79±0.13[9] R☉
Luminosity564±20[10] L☉
Surface gravity (log g)3.63[10] cgs
Temperature13,600±100[11] K
Metallicity [Fe/H]+0.2[10] dex
Rotational velocity (v sin i)59[12] km/s
Age100±10[8] Myr
Other designations
Elnath, El Nath, Alnath, β Tauri, Gamma Aurigae, 112 Tauri, BD+28°795, FK5 202, GC 6681, HD 35497, HIP 25428, HR 1791, SAO 77168, CCDM 05263+2836, WDS J05263+2836A
Database references
SIMBADdata

Beta Tauri is the second-brightest star in the constellation of Taurus. It has the official name Elnath; Beta Tauri is the current Bayer designation, which is Latinised from β Tauri and abbreviated Beta Tau or β Tau. The original designation of Gamma Aurigae is now rarely used. It is a chemically peculiar B7 giant star, 134 light years away from the Sun with an apparent magnitude of 1.65.

Nomenclature

[edit]

This star has two Bayer designations: β Tauri (Latinised to Beta Tauri) and γ Aurigae (Latinised to Gamma Aurigae). Ptolemy considered the star to be shared by Auriga, and Johann Bayer assigned it a designation in both constellations. When the modern constellation boundaries were fixed in 1930, the designation γ Aurigae largely dropped from use.[13]

The traditional name Elnath, variously El Nath or Alnath, comes from the Arabic word النطح an-naţħ, meaning "the butting" (i.e. the bull's horns). As in many other Arabic star names, the article ال is transliterated literally as el, yet overwhelmingly in Arabic pronunciation it is assimilated to the n, meaning it is omitted. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two batches of names approved by the WGSN; which included Elnath for this star.[16]

In Chinese, 五車 (Wǔ Chē), meaning Five Chariots, refers to an asterism consisting of β Tauri, ι Aurigae, Capella, β Aurigae and θ Aurigae.[17] Consequently, the Chinese name for β Tauri itself is 五車五 (Wǔ Chē Wǔ; English: Fifth of the Five Chariots.)[18]

Physical properties

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The absolute magnitude of Beta Tauri is −1.34, similar to another star in Taurus, Maia in the Pleiades star cluster. Like Maia, β Tauri is a B-class giant with a luminosity 700 times solar (L☉).[19] It has evolved to become a giant star, larger and cooler than when it was on the main sequence.[20] However, being approximately 130 light-years distant compared to Maia's estimated 360 light-years, β Tauri ranks as the second-brightest star in the constellation.

It is a mercury-manganese star, a type of non-magnetic chemically peculiar star with unusually large signatures of some heavy elements in its spectrum.[12] Relative to the Sun, β Tauri is notable for a high abundance of manganese, but little calcium and magnesium.[19][21] However, the lack of strong mercury signatures, together with notably high levels of silicon and chromium, have led some authors to give other classifications, including as a "SrCrEu star" or even an Ap star.[22][23] Its limb-darkened angular diameter has been measured at 1.085±0.026 mas. At a distance of 41.1 pc, this corresponds to a linear radius of 4.79±0.13 R☉.[9]

At the southern edge of the narrow plane of the Milky Way Galaxy a few degrees west of the galactic anticenter, β Tauri figures (appears) as a foreground object south of many nebulae and star clusters such as M36, M37, and M38.[24] It is 5.39 degrees north of the ecliptic, still few enough to be occultable by the Moon. Such occultations occur when the Moon's ascending node is near the March equinox, as in 2007. Most are visible only in the Southern Hemisphere, because the star is at the northern edge of the lunar occultation zone – but rarely as far north as southern California.[25]

Companions

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A faint star is, angularly from our viewpoint, close enough for astronomers to consider, and guides to mention, the pair as a double star. This visual companion, BD+28°795B, has a position angle of 239 degrees and is separated from the main star by 33.4 arcseconds (″).[26][27] Six angularly closer, even fainter stars have been found in a search for brown dwarf and planetary companions – all considered background objects.[28]

A very close companion was reported from lunar occultation measurements at a distance of 0.1″, but not confirmed by other observers. Radial velocity measurements indicate that Beta Tauri is a single-lined spectroscopic binary, but there is no published information about the companion or orbit.[29][11]

References

[edit]
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Beta Tauri

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from Grokipedia
Beta Tauri (β Tau), commonly known as Elnath, is a blue-white of spectral class B7III located in the constellation Taurus, marking the tip of the bull's northern horn. It is the second-brightest star in Taurus after and the 27th-brightest star in the night sky overall, with an apparent visual magnitude of 1.65. The star lies approximately 130 light-years from the Sun, based on 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. Elnath exhibits high , with components of 22.76 mas/year in and -173.58 mas/year in , classifying it as a high proper-motion star, and it has a of +9.2 km/s relative to the Sun. Physically, the star has an estimated 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 contributions. As a chemically peculiar mercury-manganese star, Elnath shows anomalous surface abundances, including levels 25 times higher than solar and deficiencies in calcium and magnesium to about one-eighth solar values. It displays no notable photometric variability and is in a post-main-sequence evolutionary phase where core fusion is nearing completion, positioning it for future expansion into an orange giant. Positioned about 3° west of the Milky Way's anticenter, Elnath provides a prominent reference point for observers in the .

Nomenclature

Traditional names

Beta Tauri is known by the traditional name Elnath, derived from the phrase al-naṭḥ, meaning "the butting one" or "the gore," in reference to the bull's horn in the constellation Taurus. 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. This name, also spelled Alnath or El Nath, was used in medieval astronomy and later adopted in European star catalogs. 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. 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. 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. The name Elnath etymologically ties to the bull imagery central to Taurus in Mesopotamian and Greek astronomical traditions, symbolizing the constellation's horned figure.

Astronomical designations

Beta Tauri bears the β Tauri, assigned by 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 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. This dual naming arose from varying interpretations of constellation outlines by astronomers like and , but was resolved when the (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 Star Catalog (1966), FK5 202 from the Fifth Fundamental Catalogue (1988), and HIP 25428 from the Catalogue (1997), which provide precise astrometric data.
CatalogIdentifier
Bayer (Taurus)β Tauri
Bayer (Auriga, historical)γ Aurigae
Flamsteed (Taurus)112 Tauri
Flamsteed (Auriga)23 Aurigae
Henry DraperHD 35497
Harvard RevisedHR 1791
SAO 77168
FK5202
HIP 25428

Location and visibility

Celestial position

Beta Tauri occupies the equatorial coordinates of 05ʰ 26ᵐ 17.5ˢ and +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 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 and directly on the boundary with Auriga, where it is sometimes denoted as γ Aurigae. Its space motion includes an annual proper motion of +22.76 ± 0.42 mas/yr in and -173.58 ± 0.18 mas/yr in , accompanied by a 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 under clear conditions and ranking it as the second-brightest star in the constellation Taurus after (magnitude 0.86), as well as the 27th brightest star in the night sky overall. Its steady brightness shows no significant photometric variability, remaining constant to within 0.01 magnitudes based on precise measurements from space-based observatories. 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 . This color is evident even without optical aid, contributing to its striking presence at the tip of Taurus's northern horn. In the , Beta Tauri offers optimal visibility during winter evenings from to , when the constellation rises prominently in the eastern sky after and reaches its highest point () around midnight. At a of +28° 36', it is well-placed for from mid-northern latitudes, achieving altitudes above 60° for viewers at 40° north; its position near the further enhances its backdrop against a relatively sparse stellar field. Through a small , 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.

Stellar properties

Primary component

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

Chemical composition

Beta Tauri is classified as a mercury-manganese (HgMn) , a non-magnetic subtype of chemically peculiar upper main-sequence B-type exhibiting overabundances of heavy elements in their photospheres due to atomic in stable radiative atmospheres. These display anomalous surface compositions resulting from the interplay of gravitational settling, radiative acceleration, and weak thermal , which segregate elements vertically and horizontally without the influence of magnetic fields that characterize related Ap . 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 , , and are hallmarks of HgMn stars like Beta Tauri, while iron shows near-solar abundance ([Fe/H] ≈ 0) and is depleted ([Ni/H] = -0.48, a factor of ~3 below solar). Calcium is also depleted ([Ca/H] ≈ -1.0). Additional depletions include ([He/H] = -0.08), carbon (-0.09), magnesium (-0.36), (-1.08), (-0.19), (-0.88), and (-0.51), all relative to solar values from Asplund et al. (2005). These patterns underscore the role of 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.

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 in the . However, no published information exists on the , eccentricity, or properties of the companion such as 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 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.
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