37 and 39 TAU (37 and 39 Tauri), a two for one special. Halfway between the Hyades and Pleiades of Taurus lies a pair of unequal stars that is something of a faint version of Mizar and Alcor in the Big Dipper, the two separated by only 10 minutes of arc (the Dipper's pair by 11.8 minutes). The brighter, 37 Tau (the number from the Flamsteed list), is a fourth magnitude (4.36) common class K (K0) helium fusing giant. Its significance is more that it leads the eye to the fainter, sixth magnitude (5.90) 39 Tau, which is the kind of star that fascinates, a near solar class G (G5) dwarf. Other than that, the two have nothing to do with each other, 37 Tau 187 light years away (give or take 3), 39 at a distance of 55.2 light years (plus or minus 0.3).

With a temperature of 4760 Kelvin, from which we can gauge the amount of infrared radiation, 37 Tauri shines with a total radiance of 70 times that of the Sun, from which we find a radius of 12.3 solar and a mass of about 2.5 Suns. A direct measure of angular diameter gives a radius of 10.5 times solar, showing that something is as bit off. With a current age of nearly 600 million years, the star quit hydrogen fusion some 70 million years ago. After getting rid of its outer layers following the cessation of helium fusion, it will turn into a white dwarf of about 0.67 solar masses. Hovering 134 and 236 seconds of arc away are tenth and thirteenth magnitude "companions," 37 Tau B and C, which from their motions are just line of sight coincidences.

The fainter, 39 Tau, is not far from being another Sun, though no planets have ever been detected. Even though of a cooler G5 spectral class (the Sun G2), the measured temperature of 5860 Kelvin is a bit warmer than the solar value of 5780 Kelvin, which might have something to do with a somewhat higher metal content (up 25 percent) for 39 Tau. The star's luminosity is but 6 percent higher than solar, the mass right on a solar mass, and the age a bit higher at 6 billion years as opposed to the Sun's 4.6 billion. Another analysis suggests a higher mass (1.1 Suns) and a much lower age of only a billion years. The lower age is supported by a higher equatorial rotation velocity of at least 5.5 kilometers per second, which gives a rotation period of under 9 days (as opposed to the Sun's 25 days). Two companions of 8th and 12th magnitude, 39 Tauri B and C, lie 177 and 149 seconds of arc away, but they too are just line of sight happenstances. While 39 Tauri does not exactly light up the sky, neither would the Sun as seen from 39 Tauri. At magnitude 6.0, our seemingly brilliant Sun would be near lost amidst the stars of Scorpius near Delta Sco.

Written by Jim Kaler 3/21/14. Return to STARS.