Harry sees the faint companion of Sirius at last – how to see the white dwarf Sirius B
Two views of Sirius A & B, with and without a filter. Drawing by Harry Roberts
Alpha Canis Majoris B is a rare star, a white dwarf, and at visual magnitude 8•3 it should be an easy ‘get’, right? It would be – except for the overwhelming glare of the nearby primary Alpha CMa A, better known as Sirius, the brightest star in the night sky.
As I wrote up the log for February 10 it struck me that my search for the elusive companion of Sirius began about 1960 – half a century ago! Back then a schoolmate’s 4” reflector was used in the search – we didn’t stand a chance! Around 1965 I used an 8” F25 Dall-Kirkham (yes F25) that split doubles very nicely – yet even with that I never logged Sirius B. As B was then at wider separation than now I should have looked harder.
Sirius B was finally seen in the C8 ‘scope mainly due to the use of red filters. Two were being tested as “minus OIII” filters to suppress the nebular background of M42. They did this very well, also cutting much of the blue-violet glare of OB stars in the field, which led me to think they might help to detect faint companions of bright OB stars.
Rigel was viewed; the filters greatly reduced the primary’s glare giving a better view of the secondary – the next target was obvious: would I finally see Sirius B?
I did! There, almost due east of the brilliant primary, was a tiny but persistent dot, Sirius B!
Without the red filter Sirius A was immersed in strong glare and surrounded by many shifting diffraction rings, even during good “seeing”. But the filter cut the glare and most of the outer rings. It also dimmed star B of course, but the contrast was now much better – and B was “held” fairly easily (the Fig tries to show the effects of the filter).
The orientation of the field was known (with no star diagonal) and star B lay due east of A, with a separation similar to that of Rigel B. Next day a current map of the Sirius system was obtained (thanks Richard) confirming the position angle of B (due east) and separation ~9” arc.
I can now see Sirius B in the 10” Dobsonian, but image drift is a problem, and primary A is even brighter with diffraction spikes too. The C8 tracking gives the best view, and even without the red filter an old 6mm orthoscopic eyepiece best shows the white dwarf – its simpler design is better for gazing directly at the brightest star in the sky – and its elusive companion, Sirius B.
Sirius B facts: the brightest white dwarf visible, its Mass is ~1 solar mass, its diameter (incredibly) is only 10,300km. 97% all stars in our galaxy are white dwarfs [97% seems a little high, according to a recent paper by Napiwotzki they represent 17% of the stars in the main disc of the Milky Way - Nick] but they are seldom detected. Sirius’ distance is ~9 Ly, hence stars A and B are both bright.
White dwarfs are so-called ‘degenerate’ stars i.e. no longer undergoing fusion – and with no source of energy, they collapse due to gravity. Bessel F. found perturbations of Sirius’ orbit in 1844, predicting an unseen companion – found in 1862 by Alvan G. Clark using an 18½” refractor.
Orbit of Sirius B: has a period of 50 years – separation varies by factor of ~3.5 from 3” to 11” arc, and is currently ~9” arc, and will be widest in 2025. The average separation of Sirius A and B is 24 AU (AU = Earth/Sun distance)
Other white dwarfs:
40 Eri B diameter 17000km – mgV 9.5 (easier to see than Sirius B)
Procyon B diameter 17500km – mgV 10.7 (sep 5” max. – no chance!)
Harry Roberts, Sun and Moon observer and member of the Sydney City Skywatchers