LAMBDA TAU (Lambda Tauri). Taurus is so loaded with bright and beautiful sights -- the Hyades, Pleiades, and first magnitude Aldebaran (not to mention a dim part of the Milky Way and several fine telescopic objects) that Lambda, which makes an extension of the Bull's head pointing to the west toward Cetus, is sadly overlooked. Carrying no proper name, and assigned the 11th letter of the Greek alphabet by Bayer, Lambda Tau is close to the brightest of the obvious eclipsing double stars (binaries) in the sky, a star in the mold of Algol, though at the faint end of third magnitude (3.41) notably fainter. Indeed, though actually somewhat different, Lambda -- 370 light years away -- is usually classed as an "Algol-type" binary. In an eclipsing binary, the orbital plane of the two stars is nearly in the line of sight, so that one star first blocks the light from the second, and then the second blocks the light from the first. Every 3.952955 days (3 days, 22 hours, 52 minutes), Lambda drops in brightness by well over half a magnitude to mid fourth (around 4.1), the event from start to finish taking 1.1 days. Comparison with nearby stars makes the eclipse very noticeable to the naked eye. In between the primary eclipses is a secondary eclipse a third the depth of the primary. Eclipsers reveal their secrets quickly through analysis of the variation in light and in the variation of the velocities of the orbiting stars. The system is made of a tight pair, a brighter and larger class B (B3) dwarf and a lesser class A (A4) subgiant that are only 0.10 Astronomical Units (21.9 solar radii) apart, just 27 percent Mercury's distance from the Sun. Like Algol's, the eclipses are partial, each star hiding only part of the other during each orbital pass. The primary eclipse takes place when the smaller (5.5 solar radii) and dimmer (95 times more luminous than the Sun) class A star passes in front of the larger (6.6 solar) much brighter (4000 solar) class B star (which radiates most of its light in the ultraviolet, making the visual primary eclipse only 3 times the depth of the secondary). The two stars are so close that Lambda varies somewhat even when not undergoing its eclipses. Mutual tides distort the shapes of the stars, and as they orbit they present different radiating cross sections to us. More noticeable is that when the much brighter class B star passes in front of the fainter A star, some of the light of the B star is "reflected" from the surface of the A star. Lambda Tau is thus at its brightest just before and just after the secondary eclipse. There is some evidence of gas streams and mass exchange going on between the two. There is also some evidence that these two stars, with masses of 6.8 and 1.8 times that of the Sun, are accompanied by a third much fainter star (1 solar mass) that takes 33 days to orbit at a distance of 0.4 AU. Nothing else is known about it, and it is not well confirmed. The Lambda Tau binary is still under some uncertainty, as recent analysis finds the stars more luminous than above, while the modern distance seems to make them dimmer. The system has been estimated to be 100 million years old.
Written by Jim Kaler. Return to STARS.