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DELTA HYA (Delta Hydrae).Hydra, theWater Serpent, begins its lengthy inhabitation of the sky at itsraggedly roundhead. Going clockwisefrom the northeast turn, it consists ofZeta,Epsilon,our fourth magnitude (4.16) Delta, Sigma, and Eta, with Rho (justto the southeast of third magnitude Epsilon) thrown in for goodmeasure. According to Allen, the six stars represent the Arab's"Min al Az'al," "belonging to the uninhabited spot," certainly amysterious representation. Delta, the class A (A1) hydrogen-fusingdwarf star itself, has a number of interesting features. First thebasics. From a distance of 160 light years (give or take 5), thestar shines with the total radiance of 47 Suns, which leads to aradius of 2.6 solar and (from theory) a mass of 2.4 times that oftheSun. Theory also shows that the star isclearly a dwarf, as indicated by itsspectrum, and that it is well along itspath of core hydrogen fusion, which it will give up at an age of630 million years. Delta's first outstanding characteristic is itsrapid projected equatorial rotation speed of 267 kilometers persecond, which gives it a rotation period of under half a day. Therotation also distorts the star into an oval that makes the nominaltemperature of 9400 Kelvin rather suspect, as such stars are hotternear their flattened poles than they are at their distendedequators. The shape also probably accounts for the larger radiusof 2.9 solar derived from measures/estimates of angular diameter. The rapid spin, though, stirs up the outer layers and preventschemical abundance anomalies caused by radiative uplift of someelements and gravitational settling of others (though no one hasyet done an actual chemical analysis of the star). AccompanyingDelta Hya are up to threecompanions. The most distant,11th magnitude Delta Hya C, 240 seconds of arc away, has moved toomuch over the past nearly-two centuries to really belong, and isclearly a line of sight coincidence. But most likely real is DeltaB, an 11th magnitude neighbor 2.6 seconds of arc distant. From itsabsolute brightness it is probably a class K5 dwarf. A minimumseparation of 130 Astronomical Units leads to an orbital periodgreater than 820 years. The pair needs to be followed over theyears to really tell. Finally, there is a notation that the staris aspectroscopic double with acloser companion, but it is unconfirmed, and such a mate isprobably not real. That leaves us with two. The Sun is layeredwith a stable envelope above the hydrogen-fusing core that conductsits heat via radiation. On top of that, about two-thirds of theway out, the envelope roils in stirring convection that withrotation generates solar activity seen as sunspots, flares, andcoronal mass ejections, which createauroral displays at low latitudes. Asthe masses of dwarf stars increase, the convective layer thinsuntil by hot class F and into class A, it disappears. Class Astars should not then generate X-rays (from magnetic activity), butthere they are from some of them including Delta Hydrae. Morelikely, though, they are coming from a cooler companion, perhapsfrom class K Delta B (or even from the mysterious closerunconfirmed neighbor). Written byJim Kaler 4/13/12. Return toSTARS.
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