It has no equal. Sirius is the brightest star in the entire sky, twice as bright as its nearest competitor, Canopus, in the southern constellation Carina. As the most brilliant, it’s spectacularly easy to identify. A winter star for northern hemisphere skywatchers, it first appears in the late evening sky in November. Now in February, Sirius already stands two fists high in the southeastern sky in evening twilight.
It would be a fun observing project to see how soon after sunset you might see it. I suspect that once you’re familiar with where it is in the sky, you might catch it before sunset this time of year. Sirius gets its name from the Greek word for “searing” or “scorching” and refers to its pre-dawn rising in August during the hottest time of the year. It heated shimmer was thought to parch the land, cause lethargy and drive dogs mad, hence it’s nickname, the Dog Star.
But if the ancients knew how far Sirius lay from Earth, they might have seen this stellar diamond differently. At 8.7 light years or some 52,200,000,000,000 (trillion) miles distant, the trickle of heat added by Sirius to the Earth is virtually nil. Yet as stars go, it’s close to Earth, one of the main reasons it dazzles. Sirius is also whiter and hotter than the sun with a surface temperature of 17,300°F (9,600°C) compared to the sun’s 10,000°F (5,500°C). When you combine that extra heat with the star’s girth — 1.75 times that of the sun — it’s easy to understand why the star appears so bright in the night sky.
Sirius has a companion star called the Pup, a tiny but exceedingly dense white dwarf only about 7,000 miles (11,200 km) across (smaller than the Earth!) but with 98% the mass of the sun. With its matter squeezed so tightly, the Pup’s gravitational pull is 350,000 times greater than Earth’s. A 150 pound person standing on the star would weigh 50,000,000 pounds!
A white dwarf is the end of the road for average-size stars like the sun. In the distant future, when the sun runs out of nuclear fuel to “burn,” its core will contract to form a white dwarf, while the solar atmosphere will poof off into space to form a temporary wreath of glowing gas called a planetary nebula. Someday, the same will happen to our brilliant pal, Sirius, and instead of one tiny star orbiting a big one, twin white dwarfs will whirl about the other.
Stars and constellations only appear steady and unchangeable because we see them during a lifetime, a mere blink compared to all the time that came before and all that will come to be. Not only are all stars in motion but they’re also growing old and being born just like people.
Sirius heads up the constellation Canis Major the Greater Dog. With a little imagination and a reasonably dark sky, you can easily picture a dog jumping up on its hind legs to greet neighboring Orion the Hunter. Much of Canis Major lies within the fuzzy band of the Milky Way with lots of star clusters and nebulae visible in both binoculars and telescopes. Some dark night, when Sirius is twinkling, take out your binoculars, start at Dog Star and slowly sweep across and up and down the area.
Twinkling is what Sirius is most famous for. As the brightest star, it displays the ever-present turbulence in the atmosphere best. From mid-latitudes, the star spends a good amount of time in the lower part of the southern sky, and the lower a star is, the more atmosphere its light has to penetrate to reach our eyes. Various pockets of air at different temperatures “focus” starlight this way and that like small lenses. We see the shifts as twinkling. Since white light is made of every color of the rainbow, one pocket might send a bit of red our way, then blue, yellow or green in totally random order. That’s the reason that Sirius not only twinkles but does it in color.
On especially turbulent nights, the effect is mesmerizing. Clear skies!