Double Star of the Month - May 2007
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In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month. |
xi Boo (14 51 23.3 +19 06 02)
was discovered by Herschel the elder on 1780, Apr 9 with the note ``Double L.
(large star) pale r. or nearly r. S. (small star) garnet, or deeper r. than the
other'' (my brackets). With a parallax of 0".149 it is a nearby system and
so the large apparent orbit means that for most of the 151 year orbit the pair
is well within range of the small telescope. It is also a beautiful pair - the
colours are yellow and orange, but unequally bright, the apparent V magnitudes
being 4.76 and 6.95. The stars are presently closing slowly and in
mid-2007 there are to be found at PA 311 degrees and separation 6".2. Closest
approach occurs in 2066 when the stars are barely 2" apart. The WDS lists 1351
measurements of this pair and only 432 for alpha Centauri - a measure of how
much the southern hemisphere is missing its double star observers.
alpha Centauri (14 39 40.9 -60 50 07) is the nearest binary star to the Sun and
the most spectacular visual system in the sky. Discovered by Father Richaud from
Pondicherry in 1689 whilst observing a comet (``je remarquai que le pied le plus
oriental & le plus brillant etoit un double etoile aussi bien que le pied de la
Croissade - I noted that the brightest star at the easternmost foot (of
Centaurus) was a double star as good as that at the foot of the Cross'' i.e.
alpha Crucis.
In 1838 Henderson first measured the parallax at about 1 arc second but it was
left to Bessel to publish the parallax of 61 Cygni first. A modern value
obtained by the Hipparcos satellite (0''.74212) is equivalent to 4.395 light
years with a formal error of 0.008 light years. The stars are yellowish and
orange-yellow reflecting the spectral types of G2V and K1V and the orbital
period is 80 years. Because the system is so close the apparent separation of
the orbit varies from 1.7 to 21.7 arc seconds so that they can be seen by small
telescopes throughout the orbital cycle. At present the stars are closing (235
degrees and 8''.67 for 2007.5). In 1915, Robert Innes found a third star of V=11
some 150 arc minutes away which shares the proper motion of alpha but which has
a slightly larger parallax. The star became known as Proxima Centauri and is our
nearest stellar neighbour. In 2006, a search was made in the infra-red around
alpha Cen B using adaptive optics on one of the VLT telescopes. The modelled
mass for this star is about 0.027 solar mass less than that derived from the
orbit and it was thought that this might be explained by the presence of a
sub-stellar object circling B. In all 252 faint background stars were found
within 15'' of alpha Cen B but nothing co-moving and therefore nothing
physically connected to B itself.
Bob Argyle