60 LEO (60 Leonis). The triangle of bright stars that marks the
hindquarters of Leo, the Lion, is almost
as well known as the foreparts, the so-called "Sickle." At the eastern end of the triangle lies
second magnitude Denebola (the Lion's
"tail"), while Zosma sits at the northwest
apex, Chertan at the western end, both
third magnitude (and respectively Beta, Delta, and Theta Leo). In
the neighborhood, centered to the northwest of Zosma, lies a
fainter triangle of Flamsteed stars: 60 Leo (three degrees due west
of Zosma, 72 Leo (almost the same angle to the north), and (six
degrees to the northwest, nearly on the border with Leo Minor) the pretty visual binary 54 Leonis. The
most southerly of them, 60 Leo, at 127 light years (give or take
just 1) also the closest, is a fourth magnitude (at 4.42 on
the faint side) class A (A1) dwarf, but one with a
(small) bit of a difference. With a temperature of 9150 Kelvin,
the star shines with the light (including a bit of ultraviolet) of 22 Suns, which with temperature leads to a radius
of 1.9 times solar. Bringing in structure and evolution theory,
the star then carries a mass of 2.2 times that of the Sun,
appearing to be roughly halfway through its core-hydrogen-fusing
lifetime of 900 million years: which is less than ten percent of
the similar lifetime of the Sun, the dramatic difference caused by
60 Leo's higher mass, which causes it to run through its internal
fuel much faster than does the Sun, the star soon to become a
helium-fusing giant.
The key issue with 60 Leo is its relatively slow (projected) rotation speed
of 23 kilometers per second (while more than 10 times the solar
rate, slow for the class), which gives a rotation period of under
4 days. Rapid rotation leads to atmospheric stirring (the stellar
"atmosphere" the thin semi-transparent outer layer where the spectrum is formed) in which the chemical
elements are mixed together in true proportions. Slow it down, and
some of the heavier elements (in 60 Leo notably calcium and
scandium) sink downward under the force of gravity, while several
others rise under the pressure of absorbed radiation (in 60 Leo
strontium, yttrium, zirconium, barium, and some others) to become
massively overabundant. The result is a "metallic-line star"
("line" referring to the narrow spectral absorptions). The odd
composition is thus consistent with a true slow rotation, the
star's spin axis probably pretty much perpendicular to the line of
sight. Given the temperatures for the "Am" stars, 60 Leo then
becomes a "hot metallic-line star." The skewing of the chemistry
of such stars sometimes drives astronomers to distraction when
attempting to assess true abundances for purposes of stellar or Galactic research.
Written by Jim Kaler 5/04/12. Return to STARS.