Sirius is a star to be taken seriously. As the brightest in the night sky, it has no equal. We’re not talking intrinsic brightness – that award could easily go to many naked eye stars rendered faint only by distance – but Sirius wins by proximity. At just 8.7 light years from Earth, it’s the 5th closest star system beyond the sun. It also doesn’t hurt that Sirius is 1.75 times the size of the sun and 26 times as luminous.
Last night it came up bright, white and twinkling in the southeast around 10:30 p.m. Because of its brilliance, no star crackles more intensely especially when viewed low in the sky, which is where Sirius spends a fair amount of its time as seen from mid-northern latitudes.
Our atmosphere’s to blame for Sirius’ flashy personality. Starlight passes through air pockets of different density and humidity on its way to our eyes. Each pocket act like an individual lens that focuses its own image of the star. As the air churns and the winds blow, the number and positions of all those individual images are constantly changing.
We perceive these nonstop, tiny shifts as sputtering light or twinkling. The scientific description of the cause is atmospheric refraction. Refraction by moving air causes star images to constantly jump about and change brightness. Although it was calm on the ground, air currents high above made Sirius spark and sputter spectacularly last night. If you didn’t know what you were looking at, you might think it was a UFO on its way to the next abduction. Sirius has been confused with such by inexperienced observers.
The colors come from the air, too. Just as white light is composed of a rainbow or spectrum of individual colors from indigo to green to red, so is starlight. When a star is near the horizon, refraction is strong enough to create images of it in every color of the rainbow and cast them about in different directions. To our eye, the star looks like a continuous sparkle of varying colored light as split-second variations in moving air pockets make it dance about.
Because Sirius is close to Earth, its motion through space causes it to slowly move across the sky. Not much mind you, but enough to make it one of the few stars to have moved noticeably in recorded history.
In the days of ancient Egypt, the star was further northeast of where it is today. A keen-eyed skywatcher transported from that time to ours would easily see the change.
You can find Sirius anytime by facing southeast after 10 o’clock and shooting a line through Orion’s Belt down and to the left. Enjoy the sparkles!