From the December 2015 issue

See the Pup

These tips can help you observe Sirius B.
By | Published: December 28, 2015 | Last updated on May 18, 2023
Do you have an astronomical “bucket list” — a roster of must-see or must-do experiences to check off before you breathe your last? Near the top of mine is catching a glimpse of the Pup, the notoriously elusive white dwarf companion to the Dog Star, Sirius (Alpha [α] Canis Majoris). Astronomers have cataloged these stars as Sirius A and Sirius B. Double star observers refer to the brighter component as the primary and the fainter one the secondary.

The best window of opportunity to capture the Pup is around the time of apastron (the point in its orbit farthest from Sirius). This position occurs just once every half century. I blew my chance when the two were at their greatest separation in the mid-1970s, and who knows where I’ll be for the next one?

The current window recently opened up when the separation between A and B reached 10″. The two will lie at least this far apart for the next 20 years. A two-decade window sounds like an ample amount of time, but it isn’t, as I’ll explain.

Sirius B
The Pup (Sirius B) is as hard to image as it is to see. The photographer here used a 14-inch telescope and a CCD video camera to capture it.
Damian Peach
Why my obsession with the Pup, and why should it be on your bucket list? Sirius is a textbook example of a binary whose magnitudes are so different (in this case, nearly 10 magnitudes — a difference in brightness of 10,000) that the primary’s brilliance drowns out that of the secondary, especially during the three decades in their 50-year orbit when the two are less than 10″ apart. Simply stated, this is one of the sternest challenges for the backyard astronomer. No dyed-in-the-wool double star fanatic wants to go to his or her grave without notching the Pup!

How do we go about capturing it? Here’s my strategy. First, I choose at least a 6-inch telescope and an eyepiece that magnifies at least 250x.

Your telescope and eyepiece must be of high quality and have had ample time to adjust to the outside temperature.

Hexagonal aperture mask
You can make a hexagonal aperture mask to improve your chances of seeing the Pup. The mask’s round shape should fit (be the same size as) the front opening of your telescope.
Holley Y. Bakich
A night of pristine seeing conditions is essential, for even the slightest atmospheric disturbance will “boil” the image of Sirius enough to mask its tiny companion. In certain places, such an evening is a relative rarity, so a 20-year window is indeed short.

Because I live at a middle latitude and Sirius lies well below the celestial equator (at declination –16°43′), I wait until it’s at its highest point above the southern horizon. To test the waters, I’ll try splitting nearby Rigel (Beta [β] Orionis), another unequal pair (magnitudes 0.3 and 6.8, a 400-fold difference in brightness) whose 9″ separation is similar to the gap between Sirius A and B. If I can’t resolve Rigel easily, there’s no sense trying to tackle Sirius.

If the night is great but all my attempts fail, I’ll try two tricks of the trade used by experienced double star observers to ferret a faint companion out of the glare of a brilliant primary. One is using a hexagonal aperture mask to convert the bright star’s image into a six-spiked pattern. The dim partner may then appear between spikes. The other is to use an eyepiece with an occulting bar to cover the bright star, reducing its glare.

Occulting bar
The simplest type of occulting bar is a strip of tape carefully placed across the field stop of an eyepiece. Make sure the tape is straight.
Michael E. Bakich
How will I know that I’m really seeing the Pup and not a false image? Right now, the Pup lies east (and a tad bit north) of Sirius. If I let Sirius drift across my eyepiece’s field of view, the Pup will follow almost directly behind. Should my suspect behave accordingly and it’s about the same distance from Sirius as Rigel B is from Rigel A, then I’m putting a big check mark next to “See the Pup.”

Speaking of bucket lists, I highly recommend Michael E. Bakich’s 1,001 Celestial Wonders to See Before You Die (Springer, 2010). This rundown of the finest deep-sky objects is guaranteed to keep you busy until you (ahem) die. Sirius is his sixth entry. Bakich notes that the Pup will widen to a separation of 11″ in 2025, after which it will begin its slow return to the glare of Sirius. It’s a deadline I hope to beat!

Questions, comments, or suggestions? Email me at Next month: My double star observer’s answer to the old Messier Marathon. Clear Skies!