Natural Science Forum / Physics / General Physics / July 2008

See: http://simbad3.u-strasbg.fr/sim-id.pl?protocol=html&Ident=alpha+centauri&btnG=Search

Hmmm...
There is an ENORMOUS difference between a negative magnitude
and a positive one, and a significant difference between apparent
magnitude and absolute magnitude.
Traditionally the brightest stars were labelled 1st magnitude and
the faintest (to the human eye) were 6th magnitude, apparent versus
absolute didn't really matter much.
Then it was realised that a star further away would appear fainter
than a star close by even if they had the same intrinsic brightness,
and thus the intrinsic brightness became known as the absolute
magnitude.
The Sun (Sol) is of course the nearest star to us, and it has an
apparent magnitude of -26.73. It is nowhere near as bright as
(say) Sirius which would traditionally be a star of the first
magnitude (1.4). Alpha Centauri (I've never seen it or visited
the southern hemisphere) will appear bright because of its
proximity but it isn't especially bright as stars go, dimmer than
the Sun if both we seen at the same distance.
Astronomers make the assumption that the colour of the star
will yield its intrinsic magnitude and then get an estimate of its
distance from the observed apparent magnitude.  Unfortunately
it doesn't mean much, Sirius B is better seen in UV or X-ray.

"Alpha Centauri A, also known as Rigil Kentaurus, is the brightest star in
the constellation of Centaurus and is the fourth brightest star in the night
sky. Sirius is the brightest even thought it is more than twice as far away.
By an exciting coincidence, Alpha Centauri A is the same type of star as our
Sun, causing many to speculate that it might contain planets that harbor
life. "
  http://antwrp.gsfc.nasa.gov/apod/ap030323.html

Take careful note:
In the above picture, the brightness of the stars overwhelm the photograph
causing an illusion of great size, even though the stars are really just
small points of light.

Ref: http://www.astro.uiuc.edu/~kaler/sow/rigil-kent.html

RIGIL KENTAURUS (Alpha Centauri) with PROXIMA CENTAURI (Alpha Cen C). Among the most famed
stars of the entire sky, surely rival in renown to Sirius and Polaris even though not
visible to much of the world's population, is the "foot of the Centaur," Rigil Kentaurus,
"Rigil Kent," the first star of Centaurus, probably much better known as Alpha Centauri or
just Alpha Cen. Its fame, indeed that it is the third brightest star in the sky (after
Sirius and Canopus), lies not in its extreme characteristics but in its geometry, as it is
the closest star to the Sun, lying a mere 4.36 light years away, the distance known to 0.2
percent. Placed well down in the southern hemisphere, in fact the most southerly of naked
eye stars, it cannot be seen above about 30 degrees north latitude, making it one of the
great luminaries of the southern hemisphere. Alpha Cen deceives the eye. Through but a
modest telescope we see it as double. The brighter is a yellow class G dwarf
(hydrogen-fusing) star that, with a temperature of 5770 Kelvin (10 degrees cooler than
solar), appears almost identical to the Sun, certainly an odd coincidence. The companion,
over a magnitude fainter, is a cooler (5300 Kelvin) class K star, the two making an
obvious color contrast. The pair orbit each other every 79.9 years. Though they average 24
astronomical units apart (23 percent farther than Uranus is from the Sun), the elliptical
orbit sends them from a farthest distance of 36 astronomical units to 11, about Saturn's
solar distance. Because of Alpha Cen's proximity, the brighter component is still of the
zeroth magnitude, and would by itself still be the sky's third brightest star, the
secondary coming in at first magnitude at number 21. The orbit and orbital speeds yield
masses of 1.10 solar for the brighter star, 0.92 for the fainter (as expected for ordinary
hydrogen fusing stars on the "main sequence"). The respective luminosities of 1.57 and
0.51 times solar coupled with metal contents almost double that of the Sun lead to a
calculation of ages of between 7 and 8 billion years, notably older than the age of the
Sun. ("Astroseismology" of Rigil Kent A, wherein oscillations on the star's surface are
observed through subtle light variations, gives the same mass, a radius 1.26 times that of
the Sun, and a shorter age of 6.5 billion years.) Given Alpha Cen's mass and the higher
age, the star may be close to running out of hydrogen fuel. Alpha Centauri has yet another
member, a faint eleventh magnitude (11.05) companion called "Proxima" that is a huge two
degrees away from Alpha proper and that orbits with a period of at least a million years.
If indeed it does orbit (and that is not certain), it is now on the near side of its path
and some 10,000 astronomical units closer than the bright pair, making it actually the
closest known star (but since it is part of Alpha, surely it is still fair to call Alpha
the closest star). As a mid- class M (M5.5) dwarf star, Proxima is faint indeed, to the
eye 20,000 times dimmer than the Sun. From Alpha Cen proper, Proxima would appear as only
fifth magnitude, about as bright as the faint stars of the Little Dipper. When infrared
radiation produced by its 3100 Kelvin surface is accounted for, it is seen to be more
luminous, but still only 1/500 as bright as the Sun, the result of a mass only 20% solar.
While the chief component is best known as a solar analogue, Proxima is famed as a dwarf
class M "flare star," one that suddenly erupts with fearsome violence as a result of the
collapse of its complex and unstable magnetic fields. Only from Rigil Kentaurus would our
Sun have any kind of magnificence. Since the star is close to us, its inhabitants, were it
to have any, would see much the same constellation patterns that we do except that
Centaurus would be missing its brightest star and the stars that lie between Cassiopeia
and Perseus would be the setting for our first magnitude Sun, which would be the eighth
brightest in their sky.
Written by Jim Kaler

On Jul 17, 4:55 pm, vibh...@gmail.com wrote:

I am sorry, I jumped the gun and did not read the description in the
book. It turns out that the apparent magnitude of -0.27 is actually
the visual magnitude (ie the apparent magnitude in the visual light),
and the apparent magnitude of -0.08 is considering all the radiated
energy from the star, including infra red and ultra violet portions of
the spectrum (bolometric magnitude).