Are You Ready For A Siriusly Good Time?

Sirius is that unmistakably bright star shining in the lower half of the southern sky on Februrary evenings. Photo: Bob King

Sirius glinted off the ice on our rural road last night. At 8.6 light years away, it reminded me of what life was like 8.6 years ago, when texting didn’t exist and my children were still children. I like this star. I like how it scintillates and dances. How pure and white its light shines.

Sirius is the brightest star in the heavens. You can’t miss its beacon-like presence in the south in mid-February. At nightfall, the star is already well up in the southeast, reaching its highest point in the south around 9 o’clock. Here in Duluth, Minn., Sirius peaks at 25 degrees or a little more than two fists held at arm’s length at that time. In southern Arizona, it does much better, hovering some 45 degrees high or halfway to the zenith.

Stars are binned in brightness according to the magnitude scale, devised by the Greeks around 150 B.C. The brightest stars were said to be of ‘first magnitude’ while fainter stars were magnitude two, three and so on down to 6th magnitude, the faintest visible with the naked eye.

Sirius is the brightest star in the constellation of Canis Major the Greater Dog, here shown around 9 p.m. The dog's head is composed of fainter stars to Sirius' left, while his hind legs form a triangle at the bottom. Created with Stellarium

Magnitudes have been refined since ancient times. In particular, the scale now extends into negative numbers for the brightest of stars. The higher the negative number, the brighter the object. Sirius shines at magnitude -1.5 while its next closest rival, Canopus, visible in the far southern U.S., checks in at -0.7.

Here are magnitudes of some familiar celestial objects rounded to the nearest tenth:

* Arcturus — 0.0
* Vega — 0.0
* Spica — 1.0 or first magnitude
* Regulus — 1.4
* Alkaid (star at end of Big Dipper’s handle) — 1.9
* Jupiter around -2.5
* Space station around -3
* Venus around  -4.5
* Full moon -13
* Sun -27
* Faintest star seen by Hubble Space Telescope — 30

Sirius' tiny companion star orbits it at about the same distance Uranus orbits the sun. Credit: NASA/ESA

For most sky watchers in mid-northern latitudes, Sirius is outshone only by Jupiter, Venus, the moon, sun and the International Space Station. Sirius’ name comes from the Greek word for ‘scorching’, appropriate given its searing brilliance. It’s nickname, the Dog Star, is also fitting as it heads up the constellation Canis Major the Greater Dog. Sirius is a white star similar to the sun but almost twice as large and 26 times brighter. It spins on its axis once every 5 1/2 days compared to the sun’s 27 day period.

Though Sirius has celebrity enough, it’s nearly eclipsed in notoriety by its tiny companion, a white dwarf star slightly smaller than the Earth called Sirius B. Known informally as the ‘Pup’, the star orbits Sirius once every 50 years. At magnitude 8.4, it’s almost always lost in the glare of its brilliant sibling.

1891 photo of Alvin Clark, the first person to see Sirius B

What it lacks in brightness, the Pup makes up for in temperature and weight. Its surface cooks at 44,500 degrees vs. Sirius’ 17,500 degrees, and a sugar cube-sized chunk of its matter is so dense it weighs as much as an average-size compact car. Despite its size, Sirius B has enough gravity to tug on Sirius, causing it to wobble back and forth. The wobble was first detected by astronomer Friedrich Bessel back in 1844, but it wasn’t until 1863 until Alvin Clark, a famous telescope maker, first spotted it through his new 18 1/2-inch refracting telescope. He was testing the instrument at the time, pointed it at Sirius, and was surprised to see it accompanied by a faint companion. It was the first white dwarf discovered.

Sirius B will get easier to see in the coming decade as its orbit takes it farther from its much brighter sibling. Their separation varies between 3" and 11"; they're currently about 9" apart.

Amateur astronomers are looking forward to seeing Sirius’ companion this season and in the coming years. The two stars were closest and nearly impossible to separate in amateur telescopes in 1994. Since then, they’ve been opening up and can now be split in 3 and 4-inch telescopes with superb optics using high magnification in excellent seeing conditions.

I know, that’s a few caveats, but on the right night, it’s a worthy challenge and an opportunity to see a unique star.

If you’re game for a look, you can use the diagram to help you spot Sirius and its famous little sidekick. Despite numerous attempts, I’ve yet to see the Pup – mostly due to bad seeing from turbulent air – but I don’t plan on giving up anytime soon.

2 Responses

  1. Jim

    Hi Bob,
    I tried to see the pup last night from a fairly stable sky here in Florida in the 6 inch scope at 250X. But I could’t see it. I may try again tonight.

    The sun sure put on a nice show this past week. I saw a flare in sunspot group AR1158-a first for me. It was cool!

    How’s it going back in Duluth?

    1. astrobob

      Nice coincidence Jim. You’re the bellwether for Sirius B since you’ve got good seeing (generally) and a good scope. When you do split it, please let us know.

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