November 2015 - Double Star of the Month

56 And (01 56 09.23 +37 15 06.5) is a very wide pair of stars which can be found 5 degrees south and a little preceding that spectacle of the autumn sky, gamma Andromedae.

It is notable for the colours of the stars involved.

Here we have a K0 giant (A) and an M0 giant (B) within 200" of each other but actually not associated at all. The M star is about 3 times further away than the other according to Hipparcos.

Although the WDS gives visual magnitudes of 5.7 and 6.0, historical reports sometimes put star B as the brighter of the two - it is likely that either or both stars are liable to some small variation in brightness of a size such as to blur the difference between the two altogether.

Burnham, who would have been interested in this system because of the large proper motion of A (0".2 per year), found a faint companion of mag 11.9 at 18" and 77 degrees from A.

Also nearby (two-thirds of a degree preceding and a little north, according to the discoverer William Herschel), is STF 179, containing stars of magnitudes 7.6 and 8.1 separated by 3".5, and the open cluster NGC 752 is less than a degree north following.

Bernhard Hildebrande Dawson was an Argentine astronomer who was born in Kansas City in 1891 and who worked initially at La Plata Observatory in Argentina. He was perhaps best known as the discoverer of Nova Puppis 1942.

His double star discoveries are denoted by a small greek delta and number 31 is perhaps his most interesting find - a visual pair with a period of 4.56 years.

He started using the 17-inch refractor at La Plata in 1912 to pursue a programme of re-measurement of the John Herschel pairs. The first star in his catalogue (DAW 1 at 02 27 57.34 -58 08 22.3) was found in 1916 and sits in a rather blank area of sky in Horologium about 13 degrees following the first magnitude Achernar. This is a triple system with the two widest components (17" apart) being found by Sir John Herschel in South Africa.

In 1916 Dawson divided A and found a separation of 0".8. The current value is 1".2 so assuming it is still widening this would explain why Herschel did not see it during his sweeps.

The WDS gives A and B as 8.0 and 8.5 whilst C is 9.6. The range of position angle of AB is close to 180 degrees, a sign that the magnitudes of the two stars must be close enough to cause uncertainty about the quadrant in which B sits.

Ross Gould from Australia using a 40-cm sees A as a bright yellow star whilst x140 will show the Dawson companion.

Bob Argyle - Double Star Section Director