The Dog Star & the Pup

Camera : ZWO ASI071

Exposure: 300 x 2 sec., stacked

Image date: 2022-02-09

Which is the first star to become visible after sunset?

If you’re looking between December and May, the answer will be Sirius. It’s the brightest star in the whole sky. It gets is name from the Greek word “seirios”, meaning “the brilliant one”. Not named after me, then.

It can be outshone in the night sky only by the Moon, Venus, Jupiter or Mars (or that elusive supernova that we’ve been waiting four hundred years to see and still hasn’t happened).

Did you know? 1. The Sirius system is the the fifth nearest star system. 2. The Voyager 2 spacecraft, launched in 1977, is predicted to pass within 4.3 light-years of Sirius in approximately 296,000 years time.

Sirius is called the “Dog Star”, due to it being the brightest star in the constellation of Canis Major (the Great Dog). It’s a double star, only 8.6 light years away and the smaller companion, Sirius B, is known as the Pup.

What makes Sirius interesting to amateur astronomers is that the Pup is very challenging to spot, due to its very close proximity to the overwhelming brightness of Sirius A.

Component	Nickname	App. Mag	Class	Temp ºK
Sirius A	The Dog Star	-1.47	Type A	9,940
Sirius B	The Pup	+8.44	White Dwarf	25,000

The magnitude difference represents a brightness ratio of 10,000:1, where the bright glare of Sirius A overwhelms the dim Pup (Sirius B), making it a challenge to find. I recall once attempting to see it visually as an observational astronomer – and failing.

I was able to locate the Pup for the first time by stacking a number of short exposures – and it stood out immediately near the 12 o’clock position, a tiny dot just above the brilliance of Sirius A.

Because of the short exposures, no other stars appeared in the image but I had the foresight to take a long exposure to enable me to confirm that the orientation of the Pup matched its known position exactly.

I was also helped by the two stars being almost at at their furthest separation of their fifty year orbital cycle. Current separation is 11.3 arc-seconds.

Real and apparent orbital trajectories of Sirius B around Sirius A.
Because of our position relative to Sirius, we see the orbit as more elliptical than it really is.
Image credit: Wikimedia

Thanks for reading!

^{Images © Roger Powell}
amateur astronomy in australia

Cosmic Focus Observatory

34° South

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