IZAR (Epsilon Boo). Arcturus climbs
the eastern sky in northern spring evenings, the kite-shaped figure
of Bootes, the Herdsman, to the left.
Centered in the eastern edge of the figure lies the constellation's
second brightest star, Izar (short or long "I"). The name derives
from a short Arabic phrase meaning the "girdle" or "loin cloth,"
and means much the same as "Mizar" that
indicates the "loin" of Ursa Major, the
Greater Bear. Bayer must have had something other than brightness
in mind -- likely position -- when he gave Bootes' stars their Greek letter names, as second-magnitude
(2.59) Izar received the "Epsilon" designation and the next
brightest, third magnitude Muphrid, got
"Eta." Izar's claim to fame is that it is one of the finest double
stars in the sky. It consists of a third magnitude (2.70) class K
(K0) orange bright giant only three
seconds of arc from a fifth magnitude (5.12) white class A (A2)
main sequence dwarf. Note, however, that the magnitudes are quite
uncertain, as the proximity of the pair makes measurement
difficult. Izar B may be as bright as magnitude 4.8, and the
combined pair as bright as magnitude 2.3. The color contrast,
enhanced by the stars' proximity to each other, is so striking that
the discoverer (F. G. W. Struve) called the pair "Pulcherima" for
"the most beautiful." At a distance of around 200 light years, the
dimmer A star is found to have a total luminosity 27 times that of
the Sun, while the brighter giant, radiating
400 solar luminosities, outshines it by a factor of 15. The
uncertain distance, however, may be as great as 250 light years.
The A dwarf's 8700 Kelvin temperature show it to be about twice the
size of the Sun, while the cooler 4500 Kelvin giant is 33 times as
large. In the 170 years since discovery, the stars have completed
less than three percent of their orbit. Separated by a distance of
at least 185 Astronomical Units (the AU the distance between the
Earth and the Sun), the period is well over 1000 years long. The
pair wonderfully presents a chapter in the story of stellar
evolution. The A star is about double the solar mass, the K star
closer to quadruple. Class K giants by their natures are evolved,
fusing helium to carbon in their cores instead of hydrogen to
helium (as does the class A dwarf). More massive stars use their
fuel and evolve first. We know how rapidly stars age. The pair
was born some 300 million years ago as the white A star we see now
and a hotter, bluer, and brighter mid class B star. Ten or 20
million years ago, the brighter star's central hydrogen fuel supply
ran out and now it is a bright giant with little time left to it.
In a little over a billion years, the same thing will happen to the
smaller star, and it will become a lesser orange giant. By that
time, the bigger star will have ejected almost all of its outer
envelope leaving not much more than the old core, which will appear
as a dim dense "white
dwarf" about the size of Earth that will nearly be lost in the
orange glow of the giant-to-be (which will someday become a lesser
white dwarf itself).