February 2025 - Double Star of the Month
Castor (07 34 35.86 +31 53 13.8) has already been discussed in this series (Feb 2007) but since then considerable advances have been made in our knowledge of this famous sextuple system.
It is eighty years since systematic radial velocity measurements of the two bright components, both of which are spectroscopic binaries, were made. Recent observations using a 1.5-metre telescope and an echelle spectrograph have refined the periods to within 4 seconds for AaAb (period = 9.2 days) and 0.1 second for BaBb (period = 2.9 days).
Combining these data with new and direct images of the spectroscopic pairs using ground-based interferometry has produced masses for the four main stars to remarkable accuracy. They are 2.37 suns for Aa, 0.39 suns for Ab, 1.79 suns for Ba and 0.39 suns for Bb, the errors in each case being below 0.02 sun.
The work was described by Dr. Guillermo Torres and collaborators in a 2022 paper which appeared in Astrophysical Journal. The paper also gives the elements of an orbit for AB with a period of 459 years.
The pair was measured in May 2024 with the Cambridge 8-inch Cooke, and a mean of three nights gave 50.6 degrees and 5".76 as the stars continue to widen.
DUN 38 (07 03 57.32 -43 36 28.9) is a physical quadruple star which is located in southern Puppis about 4 degrees east of the 3.2 magnitude star nu Puppis. Three of the four components can be easily seen in a small telescope.
This is a bright and pretty pair easily seen in small apertures with the components A and B being magnitude 5.6 and 6.7 and spectral types G1V and K1V, which suggest colours of deep yellow and orange which is indeed what is observed by Ross Gould using 175-mm. He also notes that star C is bright orange.
The current separation is 21" and the position angle is 125 degrees. Component C is 185" away in PA 335 degrees. A and B are both 55.6 light-years away, and whilst the C star is somewhat more distant it shares the large proper motion with A and B and is considered to be a physical companion. C is a rapid, close and unequally bright binary with a period of about 4.1 years.
Bob Argyle - Double Star Section Director
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
