THE IMPERIAL STAR -
ALPHA CENTAURI : Part 1

ARTICLE PAGES

INTRODUCTION

ALPHA CENTAURI is one of the most impressive stellar ornaments in the sky and remains a major observational highlight for any amateur astronomer. Every amateur in the southern hemisphere must have seen it; and likely most of our fellow northern amateurs and much of the world’s population have at least heard about it. Alpha Centauri lies some two hours in Right Ascension east of the Cross and for deep-sky observers, merely 13.5'ESE (PA 127^o) from the faint by interesting Henize bi-polar planetary nebulae, He2-111.

Alpha Centauri is often referred by its proper name as Rigel Kent or less frequently by the long name, Rigel Kentaurus, but is is more commonly referred just as Alpha Centauri or α Cen. Rigel Kent, surprisingly, is not a very new name - unlike all the other bright 1st magnitude stars in the sky, as it was named this in the 20th Century by aviators who used it as a guiding beacon for visual navigation. The star is also catalogued as HIP 71683 and HIP 71681 or together as SAO 252838. As a double star it is sometimes referred after Father Ricard, the discoverer, as RHD 1 - or by James Dunlop as Δ165 To me it remains Imperatorius Astrum - the Imperial Star.

Very distinctly rich yellow in colour α Centauri is ranked as the third brightest star in the sky at -0.29 magnitude. This bright beacon ranks falls in brightness by only behind Sirius and southern Canopus, but holds a particular reverence over all other stars as our closest stellar neighbour. (1). I have read many accolades about this particular star over the years but the one that really sticks in my mind is by E.J. Hartung who describes Alpha Centauri in his the definitive southern observer”s handbook “Astronomical Objects for Southern Telescopes” (AOST2) using just one word - “brilliant”. (2) Often, for some reason or another, several northerners have placed Rigel Kent behind the first magnitude star Arcturus - and this is seemingly often slyly done by listing the two stars individually. Either way, α Centauri (AB) and α Centauri “A” is brighter. Navigators amazingly did not name it Rigel Kent until the early days of aviation, who often used it as a stellar beacon for global positioning. α Centauri lies within a bright part of the Milky Way - so the telescopic field contains many many background stars.

TELESCOPIC APPEARANCE OF
ALPHA CENTAURI

The third brightest star in the sky, lying two hours in Right Ascension east of the Cross is Alpha (α) Centauri. For some odd reason northern authors often place Rigel Kent behind the 1st magnitude star Arcturus - and this is often slyly done by just individually listing the two stars. Either way, α Cen and α Cen “A” are brighter than Arcturus. (Aviators and their navigators amazingly did not name it Rigel Kent until the early days of aviation when they were using the star for a global positioning beacon.)

Alpha Centauri in the sky lies within a bright part of the Milky Way - so the telescopic field contains many background stars. Hartung must have been a modest personality judging by his elegant literary style, as he describes Alpha Centauri in “Astronomical Objects for Southern Telescopes” using one word “brilliant”.

Statistical Data on Alpha Centauri

COMPONENT	Alpha Cen A HIP71683 SAO252838A	Alpha Cen B HIP71681 SAO252838B	Alpha Cen C Proxima HIP70890
R.A. (2000)	14h 39′ 40.90″	14h 39′ 39.39″	14h 29′ 47.750″
Dec. (2000)	-60° 50′ 00.65″	-60° 50′ 22.10″	-60° 42′ 52.90″
Solar Mass (M)	1.07	0.87	0.4
Spectra	G2 V	K1 Vd	M5 VII E
Total Mag.	-0.04	-0.04	12.1 - 13.12 B
App.Mag. (v.)	-0.29	1.35	11.01(var)
Abs.Mag. (MV)	4.38	5.74	15.4
B-V Mag.	0.71	0.9	1.807
Luminosity (L*/L)	1.5	0.4	0.00001
Radii (R)	1.22	0.92	0.35
Separation (AU)	35 (Max)	55 (Min)	13 000
Radial Velocity (RV) kms.-1	-26	-18	-16
Proper Motion (pmRA.) (mas.)	-3 678.19±1.510	600.35±26.10	-3 775.64
Proper Motion (pmDec.) (mas.)	+481.84±1.24	+952.11±19.75	+768.16±1.82
Distance (pc.)	1.3478±0.002 5	1.3478±0.002 7	1.295
Distance (ly.)	4.3964	4.3964	4.223
Period (Years)	79.92	79.92	100 000
Parallax (mas)	742.12±1.40	742.12±1.41	772.33±2.42
Note 1: Distances here are quoted from calculations made by Jahreiss and Morrison (1993) using the Gliese Catalogue. (The Gliese Catalogue specialises with the closest stars to the Sun.)
Note 2: Proper Motion can be either expressed in terms of mas (milli.arcsec.) per year, ″ per Century OR ″ in decayears (10 years) (″d.yr-1)
Note 3 :Proxima is a UV Cet Tye Variable named V645 Cen.

ENDNOTES (Part 1)

1. My fellow Australians, however, seem to have forgotten this reverence, as it is the only star of the “Cross and the Pointers” left off the Australian Flag!

2. He must have been a modest man in my opinion judging by his particular elegant literary style.

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Last Update : 29th October 2005

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