What’s Up: Sirius and Canis Major

Mar 7, 2021 | Daily Space, Sky Watching, Stars

CREDIT: Pamela Gay / Stellarium

I fully realize that for many of you, going outside means experiencing the wonderfully nasty mud that signifies melted snow and spring rains. The skies for most of us aren’t really that great, and the nights are still cold, but if you have a telescope, there is something going on that you probably want to check out.

The brightest star in the sky, Sirius, is currently hanging bright in the southern sky in the early evening. Called the Dog Star, Sirius marks the chest of Canis Major, one of Orion’s two dogs. 

As so often happens in astronomy, with the development of the telescope, it was discovered that Sirius isn’t just one star: it is two, and these stars are orbiting each other with a roughly fifty-year period. One of these stars is a standard blue giant with a mass about twice that of the Sun. The other star, first seen in 1862, was the first white dwarf star to ever be observed. The also bright, also bluish object is the core of a now-dead star. That core is about one solar mass and is roughly the volume of the Moon. 

CREDIT: Pamela Gay / Stellarium

When first spotted by Alvin Clark, our ability to make telescopes and cameras was far from what it is today, and right now, as those stars just happen to hang out at roughly their greatest separation, you may be able to see them as separate in your own telescope using a CCD camera. The stars are roughly ten arcseconds apart. You’ll want to take a bunch of short exposures and combine them so the light of the larger Sirius A doesn’t drown out the white dwarf Sirius B.

Right now, Sirius is in a good place to start observing after sunset and will be up throughout the night for many observers in equatorial and mid-latitudes. Each night, it will rise a bit later, until, in August, this Dog Star is rising just before the Sun. This star is actually why August is referred to as the ‘dog days of summer’. In ancient times, it was thought the added light of Sirius during the summer was part of what made August so darn hot. Nope. That turns out to be thermal lag, which is a different story for another day.

More Information

See Sirius, the brightest star in the night sky (Earthsky.org)


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