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.