Meet Wezen, a Yellow Tail for a Big Dog

Wezen's Vitals

Official name	Wezen
Other designations	Delta Canis Majoris, HIP 34444, HD 54605, HR 2693
Nicknames	-
Apparent magnitude	1.84
Distance from Earth	1,607 light-years
Type	F8 Ia (yellow supergiant)
Color	Yellow
Mass	17 solar masses
Radius	200 solar radii
Constellation	Canis Major
Right ascension	07h 08m 23s
Declination	-26° 23’ 35”
Multiple system?	No
Variable star?	No
Exoplanets status	None known
Probable fate	Supernova

Physical Characteristics

It’s easy to look up at a sky full of stars and assume they’re all the same. But nothing could be further from the truth — just like rocks or trees, stars come in a wide range of types, colors, and sizes. Aided by telescopes and — perhaps even more importantly — spectroscopes, astronomers have carefully cataloged these differences over the past few centuries. And just like rocks or trees, some star types are more common than others. Wezen, a bright star in the constellation Canis Major, is one of the most unusual types of stars: a yellow supergiant.

Yellow supergiants are unusual because they represent a relatively brief transitional period in a star’s life. Such a giant is either crossing the threshold from a hot blue star to a cooler red one or back again, from red to blue. During that transition, the star appears yellow. Since the star should only exist in the yellow supergiant phase for a fairly short time, in Wezen we see a snapshot of a unique moment in stellar development.

On the Hertzsprung-Russel Diagram, this transitional state falls into a region called the instability strip, indicating that it’s a temporary condition. Yellow supergiants are inherently erratic, and many of them (though not Wezen) are Cepheid variable stars. The odds of finding a yellow supergiant are pretty rare, since they represent a tiny fraction of the star population. Luckily, the universe has no shortage of stars, so we can pinpoint multiple good specimens within our galaxy (not to mention other galaxies) to explore.

Polaris is probably the most famous example of a yellow supergiant, but Wezen is actually a little brighter. Its distance of 1,600 light-years prevents it from competing in apparent brightness with stars closer to Earth, but make no mistake: The luminosity of Wezen is tremendous — nearly 40,000 times brighter than the Sun. This is a star that, despite its transitional nature, is outputting a vast amount of energy.

Interestingly, a recent historical study showed that humans likely witnessed a giant star transitioning all the way from yellow to red: Betelgeuse. Chinese astronomy records from about 100 BC that characterize Sirius as white, Antares as red, and Betelgeuse as yellow. So was Betelgeuse a former yellow supergiant 2,000 years ago? It’s possible. And will astronomers of the future be able to watch Wezen slowly redden over the next few thousand years? It seems likely.

(One more color note: Even though the Sun appears yellow when viewed from Earth’s surface, it’s technically a white star because its emission spectrum peaks in the middle of the visible spectrum.)

Origin / Mythology

Much of the star lore used in modern constellations has Greek origins, but many star names have their origin in Arabic star catalogs. Wezen is an example of one of the latter, its name an adaptation of an Arabic original. The original name is “al-wazn” which means “the weight.” Why? The answer has been lost to time. The star was originally applied to a different star that was part of a pair, so perhaps it was part of a pair of scales. But that’s speculation.

One thing seems clear: “the weight” isn’t a reference to anything regarding the canine connection to Canis Major. This is also the case for other stars in the same constellation, such as “The Maidens” (Epsilon Canis Majoris) and “The Southern Shining One” (Sirius). It seems the individual star names lack a canine association.

Interestingly, Beta Columbae, a star in the nearby constellation Columba, the Dove, also possesses the Arabic name of “the weight,” though modern use spells it “Wazn,” to differentiate.

The Boorongs, an Aboriginal tribe of Australia, used the stars around the tail of Canis Major, including what we now call Wezen, to form an image of what they called Unurgunite, the Jacky Dragon, which is a small lizard common in that area. In North America, the Dakota and Lakota tribes likewise envisioned a reptile in the region of Wezen, though they saw a snake.

Chinese astronomers created a group of 28 constellations, known as the Twenty-Eight Mansions. Each of those constellations are subdivided further into asterisms. Within this organization, Wezen is the First Star of the Bow and Arrow asterism, part of a mansion called the Well.

How to See Wezen

Canis Major is well-known thanks to Sirius, whose proximity to Earth (8 light-years) makes it the brightest star in the night sky. Nevertheless, Canis Major sits fairly low in the sky from U.S. latitudes; the farther north you go, the farther south Canis Major slips in the sky. This isn’t much of an issue when it comes to observing Sirius, but Wezen is one of the southernmost stars in an already southern constellation, so it lingers fairly low at northern latitudes such as the northern states or Canada. For the best view possible, looking mid-winter is your best bet, as Wezen travels high enough to clear atmospheric haze and horizon obstructions like trees and buildings.

The easiest way to find Wezen is to locate Sirius first. If you’re just starting out in astronomy, it’s easy to confuse Sirius with a bright solar system object like Jupiter or Venus. But you can find Orion first (easy to do thanks to its three-starred belt), then follow those three stars downward toward bright Sirius. Once you have Sirius, you may locate Wezen by hopping your way down the constellation using a photo of Canis Major or a star chart. Wezen is one of a few stars that make up the tail of Canis Major.

Do take note of the star’s yellow tone while you’re there — not only is it rare, it may not be permanent!

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Daniel Johnson is a Wisconsin-based freelance writer and professional photographer and the co-author of over a dozen books. He’s a longtime amateur astronomer and fortunate enough to live in a rural region with excellent seeing conditions. You can view some of Dan’s photography (he does a lot of animals!) at www.foxhillphoto.com