DELTA HYA (Delta Hydrae). Hydra, the Water Serpent, begins its lengthy inhabitation of the sky at its raggedly round head. Going clockwise from the northeast turn, it consists of Zeta, Epsilon, our fourth magnitude (4.16) Delta, Sigma, and Eta, with Rho (just to the southeast of third magnitude Epsilon) thrown in for good measure. According to Allen, the six stars represent the Arab's "Min al Az'al," "belonging to the uninhabited spot," certainly a mysterious representation. Delta, the class A (A1) hydrogen-fusing dwarf star itself, has a number of interesting features. First the basics. From a distance of 160 light years (give or take 5), the star shines with the total radiance of 47 Suns, which leads to a radius of 2.6 solar and (from theory) a mass of 2.4 times that of the Sun. Theory also shows that the star is clearly a dwarf, as indicated by its spectrum, and that it is well along its path of core hydrogen fusion, which it will give up at an age of 630 million years. Delta's first outstanding characteristic is its rapid projected equatorial rotation speed of 267 kilometers per second, which gives it a rotation period of under half a day. The rotation also distorts the star into an oval that makes the nominal temperature of 9400 Kelvin rather suspect, as such stars are hotter near their flattened poles than they are at their distended equators. The shape also probably accounts for the larger radius of 2.9 solar derived from measures/estimates of angular diameter. The rapid spin, though, stirs up the outer layers and prevents chemical abundance anomalies caused by radiative uplift of some elements and gravitational settling of others (though no one has yet done an actual chemical analysis of the star). Accompanying Delta Hya are up to three companions. The most distant, 11th magnitude Delta Hya C, 240 seconds of arc away, has moved too much over the past nearly-two centuries to really belong, and is clearly a line of sight coincidence. But most likely real is Delta B, an 11th magnitude neighbor 2.6 seconds of arc distant. From its absolute brightness it is probably a class K5 dwarf. A minimum separation of 130 Astronomical Units leads to an orbital period greater than 820 years. The pair needs to be followed over the years to really tell. Finally, there is a notation that the star is a spectroscopic double with a closer companion, but it is unconfirmed, and such a mate is probably not real. That leaves us with two. The Sun is layered with a stable envelope above the hydrogen-fusing core that conducts its heat via radiation. On top of that, about two-thirds of the way out, the envelope roils in stirring convection that with rotation generates solar activity seen as sunspots, flares, and coronal mass ejections, which create auroral displays at low latitudes. As the masses of dwarf stars increase, the convective layer thins until by hot class F and into class A, it disappears. Class A stars should not then generate X-rays (from magnetic activity), but there they are from some of them including Delta Hydrae. More likely, though, they are coming from a cooler companion, perhaps from class K Delta B (or even from the mysterious closer unconfirmed neighbor).
Written by Jim Kaler 4/13/12. Return to STARS.