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*The 10 Brightest Stars*
*By Pedro Braganca *
Special to SPACE.com
posted: 07:00 am ET
15 July 2003
Each star is an individual with its own personality. Thousands are
visible on any clear night far removed from city lights. Together, with
the faint glow of myriad others, the tapestry of the celestial sphere is
fashioned.
Stars come in different colors, sizes, shapes and ages. One trait that
makes a star unique is its brightness.
/Below: The Brightest Star <#sirius>/
Astronomers measure the brightness of a celestial object according to a
system originally devised by Hipparchus in 120 B.C. Hipparchus ranked
the brightness of stars in the sky on a scale of 1 to 6 as seen from the
Earth. The brightest stars he could see were classified as first
magnitude and the faintest were sixth magnitude.
Centuries later we still use the magnitude scale of Hipparchus, although
it has since been modernized.
The magnitude scale is logarithmic; one magnitude difference is equal to
a brightness difference of about 2.5 times. So a magnitude 1 star is
about 100 times brighter than a magnitude 5 star. The brighter planets
and stars have negative magnitudes. The Sun, being the brightest object
in the sky, has a magnitude of –26, followed by the full Moon at
magnitude –11. Objects with a magnitude of 6 or less can be seen without
optical aid under ideal observing conditions away from all local lighting.
The following is a catalog of the ten brightest stars that grace the
celestial sphere, an imaginary projection of Earth into space. All the
stars are drawn on the inside of this sphere, even though stars of
course exist in space at varying distances. As on Earth, the celestial
sphere is split into northern and southern half’s, called hemispheres.
As seen from our corner of the galaxy, these are lighthouses of the
heavens and can be enjoyed even from the heart of metropolitan areas.
*SCIENCE TUESDAY *
*The Lick Observatory's 3-meter telescope reveals the faint companion of
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.
------------------------------------------------------------------------
/Pedro Brananca is an astronomy support associate at Starry Night/.
*1. Sirius *
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 Earth’s 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 Earth’s 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 it’s 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. [Sirius Map
]
[Map Sirius from your location with Starry Night Software
]