Sky & Telescope - June 2021 - 45

Evenings with the Stars by Fred Schaaf

The Many
Distinctions
of Arcturus
The leading light of Boötes
rides high in the June sky.

A L A N DY ER

A

rcturus, plainly visible high in the
south on June evenings, may be the
most distinctive of all the sky's bright
stars. That's a big claim. But given the
star's characteristics, I think it holds up.
The star's name means " the Bear
Watcher, " which is apt considering
its relationship to Ursa Major, the Big
Bear. The well-known " arc to Arcturus "
curves from the Big Dipper's handle and
leads right to Arcturus. That arc can be
extended onward from Arcturus with a
straight line to " spike to Spica. " You can
continue even farther with " the curve
to Corvus. " I made up that last one, but
try it yourself - it works nicely.
Arcturus gleams at magnitude -0.05,
making it the fourth-brightest nighttime star in all the heavens. However,
sitting at number three is Alpha Centauri, which shines with the combined
light of its two component suns of
magnitude -0.01 and 1.35. If you treat
the Alpha Centauri pair individually,
Arcturus rises to number three on the
list. For observers at mid-northern latitudes, however, Arcturus is second only
to the brightest star in the night sky,
Sirius (magnitude -1.44). Arcturus rules
the evening sky from late spring to midautumn - the time of year when Sirius
is absent in the evening hours.
A further remarkable indicator of
Arcturus's brightness is its daytime
visibility. A keen-eyed friend of mine,
Chuck Fuller, reported seeing Arcturus
with the naked eye right at sunset. More
impressive was the sighting by the great
19th-century German visual observer
J. F. Julius Schmidt, who glimpsed
Arcturus without optics 24 minutes

before sunset. And
which star (other than
the Sun) was the ﬁrst
viewed in a telescope in
daytime? Arcturus. It
was observed by French
astronomer Jean-Baptiste Morin, in 1635.
For skywatchers at
mid-northern latitudes,
Arcturus crosses the
meridian at an altitude
of roughly 65°. However, the star passes
just about overhead as
seen from Honolulu,
Hawai'i. That made
Arcturus an important navigational aid
to early Polynesian
sailors, who determined
their latitude by noting
which stars appeared
at the zenith. Between
about AD 1219 and
1266 some of these
South Paciﬁc sailors
crossed Earth's equator
and happened upon the
Hawaiian island chain.
They noted Arcturus as
the most prominent zenith star - one
that could be used to guide a return visit
to these new lands. (They found their
way back home to Tahiti by sailing south
to where Sirius appears overhead.)
It's not just Arcturus's brightness
that stands out - it also has a distinctive color. The difference in brightness
between starlight measured through
ﬁlters that isolate the blue light, B, and
visible (yellow-green) light, V, gives the
B-V color index. The higher the resulting value, the redder a star appears.
Arcturus's B-V color index of +1.23 lies
in a big gap between slightly orange
Pollux (+1.00) and more deeply orange
Aldebaran (+1.54). But Arcturus is
much brighter than those other stars, so
its hue stands out more strongly.
Finally, Arcturus is also remarkable
for its origin and age. Whereas our Sun
is roughly 4.6 billion years old, astrono-

S ARCING TO ARCTURUS Following the
curve of the Big Dipper's handle leads the
observer's eye to Arcturus, the brightest star in
the constellation Boötes.

mers estimate Arcturus to be 7.1 billion
years old - more than half the age of the
universe itself. And among the countless stars (including our Sun) that travel
within the Milky Way in orbits tilted
only slightly to the galaxy's equatorial
plane, Arcturus (and perhaps only 52
other much dimmer stars) have orbits
that are tilted out of the plane and elongated. This raises the distinct possibility
that Arcturus (and the other 52 stars)
are from another galaxy that collided
with the Milky Way long, long ago.
 FRED SCHAAF first identified Arcturus
when he was seven or eight years old.
He's come to refer to it as " the great
glad star of spring. "
sk yandtelescope.org * JUNE 2 021

45
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Sky & Telescope - June 2021

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