DELTA VOL (Delta Volantis). Well to the west of Crux, the famed Southern Cross, only 20 degrees from the southern pole, lies the relatively obscure modern constellation of Volans, the Flying Fish. It is outlined by half a dozen fourth magnitude stars, of which the double star Gamma Volantis (3.60) leads, followed by Beta Vol. At 3.98, Delta just barely loses to Zeta for fourth place. To the eye it's hard to tell the differences among the set, which is probably why Alpha Vol rather oddly comes in fifth. What Delta Vol lacks in apparent brightness, though, it makes up for in its rather rare -- and luminous -- nature. Not far from the edge of the Milky Way, and at a substantial distance of 738 light years (give or take 18), this class F (F6) bright giant appears dimmed by just over a magnitude by interstellar dust. If that is the case, with a temperature of 6000 Kelvin it radiates with the power of 2900 Suns, which gives it a radius of 50 solar (60 percent the size of Mercury's orbit). With a measured projected rotation speed of 12 kilometers per second, it could take as long as 200 days to make a full rotation. On the HR diagram of absolute visual magnitude (what the magnitude would be at 32.6 light years) plotted against spectral class, Delta Vol then falls almost exactly between Polaris and Mirfak (Alpha Persei), both magnificent yellow supergiants, suggesting that Delta Vol is one too; indeed, it's at times been called an F8 lesser supergiant. Since Polaris is a modest Cepheid-type variable and Delta Vol is not, our star then becomes important in defining the warm-side limit of the zone of Cepheid instability. As a mid-temperature star, it consistently seems to show a bit of magnetic activity. The evolutionary status is unclear. It could be cooling at its surface with a dead helium core and a hefty mass of 6.5 Suns, or it could be stably burning helium to carbon with a slightly lower mass of 6 solar. In either case, Delta Vol is about 50 million years old and will die as a heavy white dwarf with a mass close to that of the Sun. There is, however, some controversy as to the amount of interstellar dimming, one author offering zero! If that is the case, Delta Vol radiates at a much lower rate of 1100 Suns, its mass comes in at 5 solar, and it is cooling with a dead helium core. If the star were in the northern hemisphere, you can be sure the issue would either be quickly resolved, or it never would have arisen in the first place.
Written by Jim Kaler 4/30/10. Return to STARS.