Sirius really shines this month. If you know where to look you can spot it just before sunset (more on that in a moment). Otherwise the star is plainly visible in a deep blue sky just a half-hour after sunset. Star brightness is determined by the magnitude scale, and Sirius sits at the top of the heap at magnitude –1.5.
Long ago, the brightest stars were called first magnitude and the faintest visible with the unaided eye 6th magnitude. Later, astronomers realized that not all first magnitude stars were equally bright, so the scale was extended “backwards” to zero (0) and into negative numbers.
Spica in Virgo is magnitude 1, Vega magnitude 0, Sirius –1.5, Jupiter –2.5, Venus –4.4 and so on. One magnitude corresponds to a difference in brightness of 2.5 times, making a first magnitude star 100 times brighter than one of sixth magnitude. With binoculars you can see stars as faint as magnitude 9 or 10.
Sirius stands due south a third of the way up in the sky at 8 o’clock in the evening (local time) this week. You can’t miss it. Because of its brilliance it acts as a bellwether for the state of the air aloft. If there’s a lot of atmospheric turbulence, the star twinkles wildly. If calm and serene, the star beams with a steady, languid light.
Sirius is the brightest star in Canis Major the Greater Dog, an attractive constellation that resembles a dog jumping up on its hind legs, tail wagging and tongue thrust out. In addition to Sirius the Big Dog it contains three second magnitude stars named Adhara, Wezen and Mirzam. If you ever want to learn a little Arabic, get to know the stars. While the constellations names are primarily Greek and Roman, individual star names are rooted in the Arabic culture of Africa and the Near East.
If you connect Betelgeuse in Orion, Procyon in Canis Minor (the little dog constellation about 3 fists above and left of Sirius) and Sirius, they make a neat asterism called the Winter Triangle. Each side of the triangle measures about 25° or a little more than two fists. From a dark sky, the Milky Way streams right through the middle of it. Making simple figures in the sky by connecting-the-dots helps us observers extend their knowledge of the stars and add new constellations to our life lists. I haven’t counted the constellations I know, but I’m sure I’m at least 20 short of the complete set of 88. Time in Australia would take care of that problem!
Sirius is bright because it’s close at just 8.6 light years away, larger than the sun (1.75 times its size) and 26 times more luminous with a surface temperature 7,000 degrees F hotter than old Sol. So yes, it’s a superlative star! It may also be the only star besides the sun visible in the daylight. I saw it earlier this week for the first time a couple minutes before sunset. If you’d like to give it a try, check out my article in Sky & Telescope.