No Kidding, Let’s Talk Sirius


The sky’s brightest star Sirius is also the Alpha star in the constellation Canis Major the Large Dog. Back in ancient Egypt, its rising at dawn signaled the start of the flooding of the Nile River. The name Sirius comes from the Latinized version of the Greek “seirios” meaning scorching and referring to its brilliance. Photo: Bob King

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.

Sirius has an Earth-sized white dwarf companion named Sirius B. White dwarfs are hot, incredibly dense stars that represent the end point of evolution for stars like the sun and Sirius. The right panel shows Sirius compared to the sun – it’s hotter, brighter and nearly twice as big. Credit: NASA/ESA (left) and Wikipedia

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.

The color and brightness of Sirius can change rapidly as it twinkles. I shot these slightly out of focus to spread the color out and show it more clearly. Not only are the hues striking but brightness changes are obvious, too. Photo: Bob King

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.

Look southeast after 10 p.m. for the three stars in Orion’s Belt. Draw a line through them toward the horizon and you’ll arrive at Sirius. Photo: Bob King

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.

Like all the stars, Sirius moves through space around the center of the Milky Way galaxy. It’s one of the few that moved noticeably over recorded history. Diagram created with Chris Marriott’s SkyMap software.

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!

22 Responses

  1. thomas s

    howdy Bob, interesting comments on Sirius. learned something new: didn’t know that it’s movement combined with its proximity has been such as to make the change detectable over historical time. but what about A. Centauri and P. Centauri (sp?). they are closer (a binary system as I recall): is there any detectable change in their position? or is the direction of their movement relative to us such that we don’t “see” any change? also, a question: do I recall correctly that Sirius is a double star? and if so, is it a true binary or an apparent double?

    1. astrobob

      Yes, Alpha Cen has an even larger motion across the sky than Sirius. The star that’s moving fastest however is 9th magnitude Barnard’s Star in Ophiuchus. Through a telescope you can detect its movement in a year or two. Sirius is indeed a double star – its companion is the white dwarf in the photo in the blog.

  2. Kc

    Bob – one thing ive been curious about is whether Sirius B has the potential to go Type 1a supernova. Is that a realistic possibility?

    1. astrobob

      Very good question. I’ve never heard of it being likely. In the distant future, Sirius will expand and become a red giant. According to NASA astronomer Sten Odenwald, it’s expected to expand to a few times the Earth-sun distance or roughly 300 million miles. While huge, Sirius’ outer atmosphere would still be too far from Sirius B, located 23 times the Earth-sun distance from Sirius A, for the white dwarf to steal it away. Without the steady siphoning of material from Sirius A, B would not hit the mass limit and explode as a supernova.

  3. Kevin Rodgers

    How can Sirius be so much brighter than Alpha Centauri if it is twice the distance from the Sun? Alpha Centauri is only 4.3 light years from Earth, so shouldn’t it be brighter in the sky than Sirius, which is more than 8 light years from Earth? Something seems strange about this. Perhaps Sirius is closer than we are being told?

    1. astrobob

      Hi Kevin,
      Don’t worry – no one’s trying to pull the wool over your eyes. Star brightness has to do with distance as well as the star’s intrinsic brightness or luminosity. Sirius is nearly twice the size of Alpha Centauri and also hotter which makes it brighter. If you put them side by side Sirius would shine 15 times brighter. That’s why it appears brighter than Alpha Centauri despite its greater distance. If you could place both stars at a distance of 10 parsecs (32.6 light years – an astronomical standard) Sirius would shine at magnitude 1.4, Alpha Centauri at 4.3 or three magnitudes fainter.
      Deneb in the Northern Cross is a little fainter than Alpha Centauri but it’s 1,500 light years away. To be that bright at such an incredible distance, Deneb much be incredibly luminous. Indeed it outshines Alpha by 24,000 times!

  4. Dee

    Hi Bob, you make astronomy so much fun thank you. Anyway big question here…. why oh why did someone call the most beautiful star in the heavens *dog star* . Wow I’m sure they could have come up with a more fitting name for the queen of the night sky…. surely?

    1. astrobob

      Hi Dee,
      Thank you! It all started with the constellation’s name, Canis Major which means the Greater (Larger) Dog. It was natural to refer to its brightest member as the Dog Star.

    1. astrobob

      Thank you for your confidence but I doubt I’m a genius. All the same, Cruttenden’s “theory” is pseudoscience. Precession is well-explained by the moon and Sun tugging on Earth’s equatorial bulge. There’s no scientific evidence that the Sun and Sirius form a binary star.

  5. Garrett Fischer

    Astro Bob,

    I’m doing an 8th grade school project about Sirius. I found most of the information I need but I can’t seem to find anything about the star’s life span and how much longer it will be around. Any help would be greatly appreciated!

    Thank you sir,

    1. astrobob

      Hi Garrett,
      Here’s what I found. Sirius is about 200-300 million years old (much younger than the Sun!). Because it’s also larger and more massive than the Sun it burns its fuel more quickly. In just one billion years it will turn into a red giant star and about 1-2 billion years later become a white dwarf star. White dwarfs are pretty much the endpoint for a star; dwarfs no longer burn fuel to shine. Instead, they gradually cool to become black dwarfs, a process lasting many billions of years. So to sum up, Sirius will shine as a normal star for about 2-3 billion years and then as a white dwarf for billions. The Sun will take nearly five billion years to evolve into a similar red giant and a couple billion years to evolve into a white dwarf. Good luck on your project!

  6. me

    I also have proof of a star brighter then Sirius and it moves across the sky. Also as well as blink. Also comes back every night. Have you seen that one. ?

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