PHI SGR (Phi Sagittarii). Some stars are known for their very
ordinariness. Yet even then, they can be interesting and
instructive. Mid-third magnitude (3.17) Phi Sagittarii sits
quietly at the edge of one of the brightest parts of the Milky Way, and marks the joint where the handle
of the Sagittarius's famed asterism, the
"Little Milk Dipper," joins the bowl.
Listed as a giant (but see
below) at the cool end of blue-white class B (B8), with a
temperature of 12,300 Kelvin, Phi Sgr is still plenty warm. At a
distance of 239 light years, its light has a very clear pathway
with minimal absorption by interstellar dust. Distance and a
goodly correction for ultraviolet radiation give it a luminosity
475 times that of the Sun, which leads to a
radius of 4.8 times solar. Considerably understudied, the star's
projected rotation speed is not well know, two disparate values
giving an average of 52 kilometers per second, which in turn leads
to a rotation period of under 4.6 days. The rotation is slow for
a star of this class, and one might then expect some chemical
peculiarities, a lack of atmospheric stirring allowing some
elements to settle downward under the force of gravity, while
others are pumped upward by radiation (as happens, for example, for
Epsilon Serpentis). Complete lack of any
such effect suggests that the star is really spinning more rapidly
but is seen with its pole tilted more or less toward us. The
rotation in turn makes the pole warmer than the rest of the star,
leading to errors in stellar parameters. That aside, luminosity,
temperature, and theory give a mass between 4 and 4.2 times that of
the Sun and show the star to be not so much a giant as a subgiant
that has just given up hydrogen fusion (or will do so very
shortly), its age around 165 million years. Phi Sagittarii is
listed as a spectroscopic double,
but there seems to be no data on the "companion" at all. A
lingering occultation by the Moon, one that lasted longer than it
should have, also suggested a companion. But here we come to one
of the underappreciated problems in observational astronomy. The
Earth's atmosphere is a refracting medium that causes stars to
appear higher in the sky than they really are, the effect
increasing with decreasing altitude above the horizon. Along with
refraction goes dispersion,
the spreading of light into its component colors. If you look with
a telescope at stars near the horizon, they appear as little spectra with green on top (the blue
absorbed by the air) and red on the bottom, a very pretty sight,
but one that is very disturbing in making precise observations.
From 60 years ago, "the apparent duplicity observed by (an earlier
author) must undoubtedly be ascribed to the effect of atmospheric
dispersion," which stretched out the occultation and shows vividly
how cautious the astronomer must be. Not double at all, Phi Sgr
appears to be quite alone.
Written by Jim Kaler 8/14/09. Return to STARS.