BETA TUC (Beta Tucanae). Not one star, but three, no five, maybe
six, two of them a fine naked eye pair not unlike Mizar and Alcor, but in reverse, in the far
southern hemisphere in modern Tucana (the Toucan) rather than in the far northern and
ancient Ursa Major (Tucana the home
of the Small Magellanic Cloud, a nearby satellite galaxy). With
the naked eye, we see fifth magnitude (5.09) Beta-3 tucked up 10
minutes of arc from fourth magnitude (3.70) combined Beta-1 and
Beta-2, which are only 27 seconds of arc apart and separable only
by telescope (making Beta the second brightest star in the
constellation after Alpha Tuc).
Beta-1, the western-most and brighter, comes in at magnitude 4.37,
while Beta-2 slips to fifth magnitude (4.54). At a distance of 151
light years, Beta-3 is measured at seven light years farther than
Beta-1,2 (the errors allowing a closeness of five light years).
Nevertheless, the stars all seem joined by a common motion through
space, and almost certainly all belong to one another. Beta-1, a
class B (B9) hydrogen-fusing dwarf is the hottest, radiating 40
Suns of energy from a 10,990 Kelvin surface, which gives a radius
of 1.74 solar, and from the fast rotation speed of at least 107
kilometers per second, a rotation period of under a day. Stellar
structure theory gives a mass of 2.5 times that of the Sun, and
reveals extreme youth, the star effectively of "zero age" and just
starting its career. Orbiting it some 2.5 seconds of arc away is
a 13th (13.5) magnitude red dwarf (class M3?) that, with a minimum
separation of at least 110 Astronomical Units, must take at least
700 years to circle its much larger mate. Beta-2 is a different
kind of double,
consisting of class A (A2 and A7) dwarfs of magnitude 4.8 and 6.0
only half a second of arc apart. With temperatures of 9100 and
8000 Kelvin, Beta-2 A and Beta-2 B respectively shine with the
light of 17 and 8 Suns with radii of
1.7 and 1.5 solar, their masses 2 and 1.7 times that of the Sun.
Consistently, theory shows them as young as Beta-1, as expected.
Orbiting with a period of 43.43 years, the two are measured to
average 16.5 AU apart, a high eccentricity taking them from 30 to
3.3 AU from each other. However, the orbit then gives a combined
mass of just 2.4 solar, well under that found from luminosity and
temperature, suggesting that the orbit, or distance, is in some
error (a mean separation of 19.5 AU reconciling the two). In
between, at class A0, is Beta-3. With a temperature around 10,000
Kelvin and luminosity of 20 solar, it weighs in with a mass 2.2
solar, and again a very youthful age. It too may be a close
double, consisting of rather similar stars. However, then the
luminosities and temperatures are inconsistent with theory,
suggesting that Beta-3 is really single. With a projected distance
of 1160 AU, Beta-1 and Beta-2 would orbit with a period of at least
155,000 years, while Beta-3, with its fragile bond, would take tens
of millions of years to orbit the inner quadruple -- if it does at
all. Both Beta-1 and Beta-3 are surrounded by what appear to be
cool disks, as might be expected from the very young. The complex
system seems to belong to the Toucan-Horologium moving group, a
loose association of the just-born.