Lambda Tauri (λ Tau, λ Tauri) is a triple star system in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Taurus. In the ''Calendarium of Al Achsasi Al Mouakket'', this star was designated ''Sadr al Tauri'', which was translated into

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

as ''Pectus Tauri'', meaning "the bull chest". In 1848, the light from this system was found to vary periodically and it was determined to be an

eclipsing binary A binary star or binary star system is a Star system, system of two stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved ...

system—the third such discovered. The components of this system have a combined

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

of +3.37 at its brightest, making it one of the brighter members of the constellation. Based upon

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements from the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

mission, the distance to this system is approximately .

System

The inner pair of this triple star system, Lambda Tauri AB, orbit around each other with a period of 3.95 days and a low

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

of about 0.025. Their

orbital plane The orbital plane of a revolving body is the geometric plane in which its orbit lies. Three non-collinear points in space suffice to determine an orbital plane. A common example would be the positions of the centers of a massive body (host) a ...

is inclined by around 76° to the line of sight from the Earth, so it is being viewed from nearly edge on and the two stars form an

Algol ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...

-like

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 in magnitude during an eclipse, while the secondary component, λ Tau B, decreases by 0.09–0.10 in magnitude. The mean physical separation between these two stars is estimated at 21.91 times the radius of the Sun, or . The primary component has a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of B3 V, making this a massive B-type main sequence star. It has over seven times the

mass of the Sun The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies a ...

and 6.4 times the

Sun's radius Solar radius is a unit of distance used to express the size of objects in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: 1\,R_ = 6.957\ti ...

. This star is the brightest member of the system, radiating about 5,801 times the

luminosity of the Sun The solar luminosity () is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal s ...

from its outer envelope at an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of , which gives it a blue-white hue common to the B-type stars. Lambda Tauri A is rotating rapidly with a

projected rotational velocity Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bu ...

of . It, along with Delta Librae, δ Librae, were the first stars on which rotational line broadening was observed, by Frank Schlesinger in 1909. The third component, λ Tau C, is orbiting the inner pair over a 33.025 day period with an

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. 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.

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 star of this mass is unclear. The secondary companion has a stellar classification of A4 IV, suggesting that it is a subgiant star that has nearly exhausted the supply of hydrogen at its core and is in the process of stellar evolution, evolving into a giant star. It has nearly 1.9 times the mass of the Sun, 5.3 times the Sun's radius, and is radiating 128 times the Sun's luminosity at an effective temperature of . As with the primary, this star is spinning rapidly with a projected rotational velocity of . The side of the secondary facing the more massive star is being heated by an additional , which produces a rotational effect that causes the strength of the secondary's spectral lines to vary over the course of its orbit. A conundrum with this system is the large radius of the secondary star. In stellar evolutionary 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 with the secondary filling its Roche lobe, giving it a distorted shape.

References

{{DEFAULTSORT:Lambda Tauri Algol variables Bayer objects, Tauri, Lambda B-type main-sequence stars Taurus (constellation) Triple star systems A-type subgiants Flamsteed objects, Tauri, 035 Hipparcos objects, 018724 Bright Star Catalogue objects, 1239 Henry Draper Catalogue objects, 025204 Durchmusterung objects