S = 10 SGE (S Sagittae = 10 Sagittae). Among the most important
kinds of stars in the sky are the Cepheid variables, named after
the prototype Delta Cephei. Mid-
temperature evolving supergiants, they have lost
their sense of stability. All fall within the famed "instability
strip" that runs down the middle of the HR
diagram (a plot of absolute magnitude against spectral class).
Expanding and contracting, Cepheids typically vary by one or more
magnitudes over a period of a few days. Eta
Aquilae and Mekbuda (Zeta Geminorum)
are among their ranks, as is Polaris
(which has uniquely almost stopped varying). In 1912, Henrietta
Leavitt of the Harvard College Observatory discovered that the
Cepheids' absolute brightnesses are intimately tied to their
oscillation periods. Measure a Cepheid's period, get the
luminosity, compare it to the apparent brightness, and out falls
the distance. Cepheids thus become "standard candles" that are
used to find the distances of other galaxies and that are crucial
links in measuring the expansion rate and age of the
Universe. Their discovery and measure in other galaxies was among
the "key projects" for the Hubble Space Telescope. They were first
used by Edwin Hubble to establish the existence of external
galaxies. Seventeen are of magnitude 6.0 or brighter, hence are
more or less visible to the naked eye (25 of them brighter than
6.5), including X and W Sagittarii. Eight degrees almost due
north of Altair in Aquila lies another, S Sagittae, in Sagitta, the Arrow, the star also known by its Flamsteed number, 10 Sge.
Nominally a class G (G5) supergiant, the star varies between fifth
and sixth magnitudes (5.3 and 6.1) over a period of 8.382 days, the
class running as early as F5. S Sge is so far away that parallax is pretty useless,
giving us 3200 light years with an uncertainty of a couple
thousand. It's far better to use the standard relation between
period and luminosity, which results in an absolute magnitude (what
the apparent magnitude would be at a distance of 32.6 light years)
of minus 4.0. Allowing for a few tenths of a magnitude of dimming
by interstellar dust, comparison
with the apparent magnitude then gives a much more certain distance
of 2300 light years. With a temperature averaging around 5500
Kelvin, S Sagittae glows with a luminosity (which changes with the
cycle) of around 3500 times that of the Sun.
Temperature and luminosity then conspire to yield a radius of 66
times solar, 0.3 Astronomical Units, which approaches the size of
Mercury's orbit. Theory gives a mass around 6 or 7 times that of
the Sun and an age of at least 43 million years. Not massive
enough to explode as a supernova, S Sge will die as
a fairly hefty white
dwarf. As a bit of an anticlimax, the star has a lesser companion as found through Doppler shifts in its spectrum. With a period of 1.85
years, at low mass the neighbor would orbit at an average distance
of 2.9 Astronomical Units, a safe distance at which to watch the
Cepheid show proceed.
Written by Jim Kaler 10/04/13. Return to STARS.