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All
stars shine but none do it like Sirius, the brightest star in the night
sky. Aptly named, Sirius comes from the Greek word Seirius, meaning,
"searing" or "scorching." Blazing at a visual magnitude of 1.42, it is
twice as bright as any other star in our sky.
Sirius resides in the constellation Canis Major, the Big Dog, and is
commonly called the Dog Star. In ancient Greek times the dawn rising of
Sirius marked the hottest part of summer. This is the origin of the phrase
"dog days of summer."
Because of Earths 26,000 year precession cycle, in which the planet's axis
slowly wobbles due to the gravitational attraction of the Sun and Moon on
the Earths equatorial bulge, Sirius no longer marks the hottest part of
summer, rising later in the year. Precession gradually changes the
location of stars on the celestial sphere.
Sirius is best seen at a favorable time during the winter months for
northern hemisphere observers. To find the Dog Star, use the constellation
of Orion as a guide. Follow the three belt stars -- obvious targets even
for casual skywatchers -- 20 degrees southeast to the brightest star in
the sky. Your fist at arm's length covers about 10 degrees of sky.
Sirius, the red giant star Betelgeuse, and Procyon in Canis Minor form a
popular asterism known as the Winter Triangle.
Intrinsically, Sirius is 23 times more luminous and about twice the mass
and diameter of the Sun. Of course its farther away from Earth than the
Sun. But not too far, cosmically speaking. At a mere 8.5 light-years away,
Sirius seems so bright in part because it is fifth closest star to the
Sun.
The
brilliance of Sirius illuminates not only our night skies, but also our
comprehension of them. While observing it in 1718, Edmund Halley, of comet
Halley fame, discovered that stars move in relation to one another a
principle now known as proper motion.
In
1844, German astronomer Friedrich Bessel observed that Sirius had a
wobble, as if being tugged by a companion. While testing his new 18.5-inch
lens in 1862 (the largest refracting telescope in the world at that time),
Alvan Clark solved this mystery by discovering that Sirius was not one
star but two; the first compact stellar remnant had been discovered, and
it would prove to be a pioneer of what would be later referred to as a
whole class of white dwarf stars.
The
companion, dubbed Sirius B, has the mass of the Sun in a package as small
as the Earth, having collapsed after depleting its hydrogen. A single
cubic inch of matter from this companion star would weigh 2.25 tons on
Earth. At magnitude 8.5, it is 1/400th as luminous as the Sun. The
brighter and larger companion is now known as Sirius A.
Terms & Definitions
Luminosity: The intrinsic brightness of a star -- as it would appear if
you orbiting it -- compared to the Sun. The Sun's luminosity is 1. Sirius
has a luminosity of 23 and Betelgeuse 55, 000.
Magnitude: A logarithmic brightness scale; the difference between
magnitude 1 and magnitude 5 is 100 fold. The larger the magnitude, the
fainter the object. The lower the magnitude, the brighter the object. The
brightest stars have negative magnitudes.
Brightness: A measure of a star's magnitude or brightness as seen from the
Earth. Brightness is dependent on luminosity and distance.
Degrees: The separation between two points of light on the celestial
sphere is measured in degrees. A closed fist held at arms length is about
10 degrees while a finger would be 1 degree or two moon widths.
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