Double Star of the Month in Cancer

• February 2021 - Double Star of the Month

  In December 1968 I was observing with friends in the back garden of a house in Newcastle-upon-Tyne using a 12-inch reflector. We looked at a number of double stars that night, including phi² Cancri = STF 1223 (08 26 47.08 +26 56 07.8) and 24 Cancri = STF 1224 (08 26 39.82 +24 32 03.7). These two pairs form a kind of wide double-double and can be found in northern Cancer.

  

  Starting with the beautiful pair iota Cancri, move about 4 degrees SE to find phi². This is a pair of magnitude 6.9 and 7.5 stars separated by 5".7 and currently at PA 53 degrees. With the 12-inch I noted that the stars appeared white and lilac at a power of x208. Gaia indicates that these stars are at the same distance from us (347 light-years).

  STF 1224 is the brighter of the two pairs with the components having V = 5.2 and 6.2. The separation is 5".2 and the position angle 219 degrees. I recorded both stars as being blue-white. Robert Aitken found that the B component was a very close binary of short period. The BC pair revolves in just 21.8 years and the separation stays close to 0".15 throughout the cycle. The Gaia EDR3 catalogue gives a parallax for the A component of 14.429 mas giving a distance of 226 light-years.

  The magnitude 2.5 star pi Puppis (07 17 08.56 -37 05 50.9) lies in a rich area of the Miky Way which is part of the Vela-Puppis star forming region. It is surrounded by a number of naked-eye stars and the open cluster Collinder 135.

  

  Pi, which is distinctly red, is a close, very unequal double star which was discovered by Hipparcos in 1991 but whose nature does not appear to have been confirmed since. Along with a magnitude 7.9 star some 67" distant in PA 213 degrees it also forms the pair DUN 43.

  To the north of pi are the bright stars upsilon 1 (V = 4.7) and upsilon 2 Puppis (V = 5.1) which are 4 arc minutes apart. Both these bright stars are variables. Upsilon 1 is also known as NV Puppis whilst upsilon 2 is NW Puppis. A V = 8.8 star lies at 119" and 215 degrees from B whilst William Jacob, observing from India, discovered a fainter companion (V = 9.1) to C at 3".1 and 209 degrees; this pair is now known as JC 10. There is another open cluster nearby called UBC7 and its possibly binary relation with Collinder 135 has been discussed.

  Bob Argyle - Double Star Section Director

• March 2015 - Double Star of the Month

  With Cancer now moving inexorably towards the western horizon, its stars are becoming more comfortably accessible to the classical refractor user and one of its finest doubles is 57 Cnc (08 54 14.70 +30 34 45.0).

  According to Webb, it is a pair of crocus-yellow stars which are not quite equal in brightness (magnitudes 6.1 and 6.4). There has been little motion of any kind save a small increase in both position angle and separation with the result that the separation is now well over 1".

  It is an ideal test for a 7.5-cm refractor. Webb was able to divide it at x144 on his 3.7-inch Tulley OG, and the writer found 1".55 with the Cambridge 8-inch in 2006.

  The pair is almost certainly binary in nature as the proper motion of the system would have separated them a long time ago. A faint component can be found at 55" and PA 202° which appears fixed. The WDS gives its magnitude as 9.2.

  57 Cnc is about 2.5 degrees north following the spectacular wide pair iota Cnc.

  Gamma Sextantis, also called 8 Sextantis (09 53 30.47 -08 06 17.7) was discovered on 1852 Apr 7 by Alvan Clark with a 12-cm aperture. As it happens he observed the star at a suitable part of its orbit but this discovery is a good indication of the quality of Clark's telescopes.

  He said, reporting his finds in Monthly Notices for 1857, Notwthstanding the moderate meridional altitude of of 8 Sextantis at Dorpat (about 24°), it may reasonably be doubted whether its duplicity would have been left to be discovered with a 4 3/4-inch object-glass, however perfect, if no change had occurred in its appearance since Struve's scrutiny of that part of the heavens.

  The separation of this close pair is never wider than 0".6. It reached maximum separation in its 77.8 year cycle in 2002 and is now closing again. By Spring 2015 it will be at 44°, 0".54 and the low declination means that it will be a significant test for 20-cm and its more likely that a good, steady air will be essential to resolve it, especially as there is a considerable difference in brightness between the components - A is 5.4 and B is 6.4. Hipparcos places the system at 278 light years distance and the primary is an A1 dwarf.

  To find gamma Sextantis, locate alpha Hya and move 5 degrees east.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2007

  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.

  The constellation of Cancer is a sprawling area of sky with few naked eye stars to betray its presence. The main object of note is the open cluster M44, well seen to the naked eye. Zeta Cnc (8 12 12.7 +17 38 53) was noted as a 6 arc second double by Flamsteed in 1680 and a century later William Herschel divided the brighter of the two stars. It turned out to be a binary with a period of about 60 years and attracted the attention of many 19th century observers because of it's relative ease of measurement (the distance varies from 0.6 to 1.0 arc second - it is currently nearest widest separation). Measurements of the third star with respect to star A indicated a long period of revolution - today this is thought to be over 1000 years but more intriguingly, the motion was not smooth. The apparent path appeared sinusoidal and repeated every 18 years. It was thought, correctly, that this was due to an unseen companion (D) rotating around C. It was not until 2000 that the 4th star was first detected - in the infra-red and at a distance of about 0.2 arc second. The three visible stars are F and G spectral type so appear slightly yellowish, and present a beautiful sight in a 10-cm telescope or bigger.

  Vela straddles the southern Milky Way between Puppis and Centaurus and is a rich hunting ground for the deep-sky observer. The brightest star is gamma Velorum (8 09 32.0 -47 20 12). James Dunlop found it was double so it has the catalogue number Dun 65. With magnitudes of 1.8 and 4.3, and a separation of 43 arc seconds, this is one of the most spectacular and easy doubles in the sky. Both stars are very hot and luminous and appear white in the telescope; gamma A or more correctly gamma ² is the brightest known Wolf-Rayet star. Hipparcos puts the system at a distance of 257 parsecs with an uncertainty of about 15%. This corresponds to a true luminosity of 10,000 sun power (assuming no absorption of the light by interstellar material along the way). In reality gamma ² is a massive spectroscopic binary, the companion an O7 star with revolves around the common centre of gravity in 78 days. Two further stars of magnitudes 7.3 and 9.5 respectively can be seen at distances of 63 and 94 arc seconds.

  Bob Argyle - Double Star Section Director