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The Goat Star

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Darrell Heath | Astronomy Columnist

During the late fall you
can see a golden-yellow
star shining brightly in
the northeast at around
9:00PM. The star’s name
is Capella, a word that
has its origins in a Latin
phrase meaning “the little
she-goat.” Capella is the
third brightest star in our
northern hemisphere sky
and the sixth brightest in
the night sky. It belongs to
the very ancient constellation
known as Auriga,
the Charioteer. Auriga is
sometimes portrayed as a
goat herder or as the Greek
hero Erichthonius, son of
the god Hephaestus, and
the inventor of that car and
war machine of the ancient
world, the chariot. In some
of the ancient depictions
of Auriga we see him with
a young goat across his
shoulder, this is Capella,
which in some versions of
the myth is the she-goat
Amalthea who is supposed
to have suckled the infant
Zeus.
Capella is some 42 lightyears
away from Earth and
while it looks like a single
star we should always keep
in mind that, with stars,
looks can be deceiving.
As it turns out, about half
of the stars in our galaxy
exist as a binary pair,
two stars orbiting around
one another. Or, in some
cases, you might find stars
that exist as multiple systems.
Capella falls into
this latter category. It
consists of two bright primaries
(Capella A and B)
orbiting very close together
(they are 67 million miles
apart, a little more than
the Venus-Sun distance)
and two fainter red dwarfs
located much further out.
The bright primaries orbit
around one another once
every 104 days.
Stars come in a variety
of sizes, colors, temperatures,
chemical make up,
and ages; astronomers can
glean a lot of these vital
statistics by looking at a
star’s spectra, which is its
light spread out via a prism
into its constituent colors.
A star’s spectra looks like a
rainbow-colored bar scanning
code and, much like
the code you see on a label
in the store, it contains the
information we need to figure
out its stats. Once an
astronomer has all of this
information they can then
assign a star a classification
based upon its spectral
type. This classification
scheme makes use of the
letters O, B, A, F, G, K, and
M; where an O-type star is
a very hot star and M-type
stars are the coolest. Each
spectral type has subdivisions
designated with
both Roman and Arabic
numerals to convey even
more information about the
star’s brightness, size, and
temperature. By using such
a system astronomers can
just look at a bit of shorthand,
like “G2V”, and immediately
know what kind
of star it is, its brightness,
color, temperature, where
its at in its life history and
what will happen to it in
the future. By the way, our
Sun is a G2V spectral type
star; Capella’s primaries are
both G8 type stars. They
are similar to the Sun but
differ by being a bit more
massive: Capella A is 2.7
x Sun and Capella B is 2.5
x Sun. This difference in
mass also results in differences
in luminosity and
temperature. Capella A is
80 times as bright as the
Sun with a surface temperature
of about 8,900F
while Capella B is 75 times
as bright as the Sun with
a surface temperature of
10,000F. By comparison
the Sun’s surface temperature
is about 9,941F, pretty
similar to Capella A and B.
But why are two stars that
are significantly bigger and
brighter than the Sun so
cool? Shouldn’t they be
hotter? Well, this is another
one of those counterintuitive
things about stars.
Capella A and B are only
500 million years old (the
Sun is 4.6 billion years old),
so, by star standards they
are just babies. But the
more massive a star is the
shorter its lifespan because
it burns through its fuel
supply much more quickly
than do smaller mass stars.
Capella A and B have exhausted
their primary fuel
supplies of hydrogen and
are just hanging out while
their cores shrink down
to the point where they
can become hot enough to
start burning the next fuel
supply: helium. As the core
starts to heat up it makes
the star’s outer layers swell
outwards. This increase in
size (Capella A has a diameter
of 10.4 million miles – 12
x Sun while Capella B has
a diameter of 7.8 million
miles – 9 x Earth) means
that the star will become
brighter simply because its
surface area increases. At
the same time, as the outer
layers swell, the surface
temperature cools down
because that’s just what
happens to a heated gas
that is expanding.
On a dark, moonless
night be sure and look just
to the lower right of Capella
to see a triangle pattern
of three fainter stars. If
Capella is the nanny goat
then these three stars can
be thought of as her kids.
The she goat and her babies
are on display in our
evening sky from late fall
to the spring.