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.