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.