Canopus: The Great Star of the South
By Joe Rao, SPACE.com
At around 8:30 in
the evening this week, anyone who looks skyward will see mighty Orion
dominating the southern sky, having just crossed the meridian on his nightly
journey westward.
Also in this part of the sky are the two
brightest-appearing stars in the night sky, Sirius and Canopus,
themselves soon to cross the meridian. Sirius is the brightest of all the known
stars. This dazzling blue-white diamond has the distinction of being visible
from every well-populated region on Earth.
In contrast, the star second only to Sirius in brilliance,
Canopus (magnitude –0.62), is only visible for those south of latitude 37.9
degrees north – a parallel passing close to San Francisco, California; Wichita,
Kansas; and Richmond, Virginia. Wrote Robert Burnham (1931-1993) in his classic
Celestial Handbook:
"Canopus
is the Great Star of the South – a name and a legend only to many North
American observers, but a dazzling gem to our more fortunately situated
neighbors to the south."
Canopus is
located in the southern constellation of Carina, the Keel of the now defunct
constellation of Argo Navis, the Great Ship of Jason
and the Argonauts. Canopus lies roughly due south of Sirius, so when the latter is at its
highest, so too is Canopus. In
fact, Canopus arrives at the
meridian about 20 minutes before Sirius.
When Sirius reaches its highest point in the south, Canopus
shines about 40 degrees below it; unfortunately out of sight for Europe,
Canada and the
northern half of the United States.
Appearing to skim just above the southern horizon from
most of the southern United States,
it usually is immersed in thick horizon haze which is probably the main reason
for the popular misconception that it's a deep yellow or even orange in color;
its true tint is a silvery white.
So it is that latitude 37.9 degrees north marks the
northernmost limit for getting just a brief view of Canopus,
presupposing that you have access to a perfectly flat southern horizon, with
absolutely no obstructions and can thus take full advantage of the
"lifting effect" of atmospheric refraction.
Interestingly, there is a sneaky way to get a view of Canopus
even farther to the north. Because the horizon appears to "dip" at
altitudes of 30,000-feet or more, airline passengers might be able to glimpse
this star peeking just above the horizon possibly from latitudes as far north
as Washington, D.C.,
or St. Louis, Missouri.
Passengers on board commercial airliners incidentally, also have an advantage
of flying above the thick haze and murk of our atmosphere.
Getting back to Canopus,
its location, roughly 15-degrees from the south pole of the ecliptic has given
it an important space-age role. Many of the space probes that have been sent
out into deep space have carried an optical Canopus
sensor to stabilize the craft in the direction perpendicular to its orbit,
which is close to the ecliptic plane. Other celestial bodies as the Sun and
Earth are similarly used for stabilizing such vehicles in other directions
during their long flight.
A long-standing controversy regarding Canopus
has been its distance. Were you to check a variety of astronomy books published
over the last 30 or 40 years, you'll likely encounter a surprising range of
distances for this star, anywhere from less than 100 light years to more than 600. So why all the discrepancy?
One major factor is that for many years Canopus
was always unavailable to the great observatories of the Northern Hemisphere,
so it really wasn't adequately observed until large observatories began to
spring up in the tropics and Southern Hemisphere. At one time Canopus
was thought to be one of the most luminous of all known stars, with figures of
up to 60,000 times that of the Sun being quoted.
More recent observations do not support such a large
estimate. But make no mistake about it; Canopus
is indeed a very large and brilliant star. Based on the latest data obtained
from the HIPPARCOS satellite, we know today that Canopus
is 313 light years away and has a computed luminosity of at least 12,000 times
that of our Sun.
