Here are a couple of new astronomical mnemonics by Peter Hobbs to help remember the names of the ten brightest stars in the Earth sky. Firstly, imagine a VCR (video-cassette recorder) being set up:

" Son: 'Sir Can Rig A VCR, Pa..' "

Son - Sun (app. mag. -26.7)
Sir - Sirius (-1.43) in Canis Major
Can - CANOPUS (-0.73) in Carina
Rig - RIGIL KENT (-0.27) in Centaurus
A - Arcturus (-0.06) in Bootes
V - Vega (0.04) in Lyra
C - Capella (0.09) in Auriga
R - Rigel (0.15) in Orion
P - Procyon (0.37) in Canis Minor
A - ACHERNAR (0.53) in Eridanus

Numbers indicate the relative brightness (or "apparent magnitude") of each star. Names in CAPITAL letters are those of Southern Latitude stars not visible from Europe and North America. For the two similar "rigs" in the list (Rigil/Rigel), imagine the rigging of a Kentish boat:

"Sir Can Rig Kent Ark!"

to distinguish between the two star names
i.e. - Rigil Kent (aka. Alpha Centauri) and Rigel (in Orion).

Stars can be identified by their level of brightness as seen from the Earth. Their unit of brightness is called apparent magnitude. All stars that can be seen with the naked eye have a magnitude of 6 or less (magnitude 6 being just visible to the naked eye on a very clear night, with each next magnitude up being about 2½ times brighter than the one below it).

Polaris (the Pole Star) is the defining star of the magnitude scale. It was set to represent magnitude 2.0 exactly and all other brightnesses were compared to it using a logarithmic scale of 2.512 (the fifth root of 100). Thus a star of first magnitude (ie. 1) is 100 times brighter than a sixth magnitude star but only 40 times brighter than a fifth magnitude star, and just 16 times brighter than a third magnitude star.

There are nearly 3000 stars of differing magnitudes 0-to-6, but only the Sun and four other stars are brighter than magnitude 0 (and thus have negative magnitudes). These are Sirius (-1.43), Canopus (-0.73), Rigil Kent (-0.27) and Arcturus (-0.06). A further eleven have a magnitude of less than 1.

Some stars are variable in brightness. Each is usually a binary star (a bright star with a second eclipsing dim star orbiting around it) or else a truly pulsating star (eg. a 1-to-70 day Cepheid variable or a long-period Mira variable). Thus the red supergiant LPV star Betelgeuse (in Orion) is the 11th brightest star with an average magnitude of 0.90, but when its cycle is at maximum brightness it becomes brighter than several of the first ten stars.

Apparent magnitude is "Earthly brightness" and should not be confused with absolute magnitude, which is the calculation (from spectrum considerations) of a star's "true" luminosity in space at a standard distance of 10 parsecs (or 32.6 light years) from the star. One parsec (pc) is the distance to an imaginary nearby star whose parallax (or sideways "shift" in 6 months as viewed against a background of distant unmoving stars) is one second of arc (or 1/3600 of a degree). At 10pc, apparent magnitude equals absolute magnitude. Stars greater than 10pc from Earth have apparent magnitudes numerically bigger than absolute magnitudes (and the bigger the number, the fainter they appear). Stars closer than 10pc have apparent magnitudes lower than absolute magnitudes. Thus Sirius is 2.7pc (8.6 light years) away with an apparent magnitude of -1.5 but an absolute magnitude of only +1.4, while the supergiant Rigel (app.mag. +0.12) has an absolute magnitude of -8.1 and is therefore about 260pc (850ly) distant.

A separate mnemonic deals with the spectral classification of stars (by temperature type).


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