Sirius (Alpha Canis Majoris) -- Double Star in Canis Major
Dog Star & Pup
Sirius (Alpha Canis Majoris) is the brightest star in the night sky. Its apparent brightness is
due to it being close (8.6 light years) and because it is 25x brighter than our sun. For
thousands of years Sirius has been associated with either dogs or dog-like gods. Its
closeness to the sun during northern hemisphere summers was historically thought to add
additional heat (with the sun) to create the Dog-Days-of-Summer which adversely affects both
dogs and men. The star, Sirius, marks the eye of the large hunting dog belonging to the mighty
celestial hunter Orion and thus is often referred to as the Dog Star, even though its real
translation is “the scorching one” or “the brilliant one”.
Sirius is a double star. Its much dimmer companion is a White Dwarf (the collapsed core of a
star after its nuclear life is over). It is called “Sirius B” or “The Pup”. The Pup is smaller than
the Earth in size, but its total mass is nearly equal to the mass of our sun. Its density is off the
chart, a teaspoon of this star would weigh over 2,000 pounds on Earth. The Pups surface
temperature is far hotter than our sun and even that of Sirius itself (remember no nuclear
reactions are taking place in the Pup).
The Pup circles Sirius every 50 years and its orbit is strongly elliptical. Only during the part of
the orbit that takes it away from Sirius do amateur astronomers have a chance to see the Pup.
At other times it is just too close to bright Sirius to be seen. We are currently in luck. The Pup is
now 12 arcseconds from Sirius and its distance will be growing over the next few years.
Do not be fooled by the Pup’s distance and brightness (magnitude 8.5), it is still very, very
difficult to see. The problem is that Sirius is so bright that the Pup is lost in the glare. It is
currently doable in telescopes of 100mm and above without having to resort to fancy tricks like
occulting bars or special filters. But only if everything else is perfect. If the telescope has any
tube currents, is not perfectly collimated, or is of inferior quality you will likely not see it. Even if
your telescope is perfect you are still going to need a stable atmosphere. If Sirius appears to
be changing colors or twinkling through your telescope, then don’t waste your time looking for the
Pup. If Sirius is a rock-solid white point of light, then up the magnification and go for the Pup.
It is not often I can find the Pup with my 155mm refractor; the atmosphere is rarely stable
On one exceptional night, about a year ago, I teased out the Pup with my 110mm refractor, but
I have not yet seen it with my 85mm and 60mm refractors (but I have tried). I would be curious
to hear if anyone has spotted the Pup in something less than a 100mm telescope. It should be