Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

Stephen James O'Meara’s Secret Sky: Order from “chaos”

March 2011: The naming of stars follows a definite pattern — it just might not be the one you’re expecting.
stephen_james_o_meara_new
In my September 2010 column about observing the sky’s reddest stars, a reader noticed that Tarazed (Gamma [γ] Aquilae), had a brighter listed magnitude (2.7) than Alshain (Beta [β] Aquilae) (3.7). Shouldn’t a second-rank Beta star shine brighter than a third-rank Gamma star, he wondered?
 
On any detailed star chart, you’ll find a lowercase Greek letter associated with the brightest stars in each constellation. Renowned Bavarian astronomer Johann Bayer introduced this identification scheme in his famous 1603 star atlas, Uranometria. The letter, when paired with the Latin genitive of the constellation (for example “Aquilae” for Aquila the Eagle), helps to identify the star.
Greek-letters
Johann Bayer, a 17th-century Bavarian astronomer, was the first to use lowercase Greek letters to identify bright stars in each constellation. He traditionally assigned the most prominent star the letter Alpha (α), but several exceptions to this rule exist.
Bayer tended to label a constellation’s brightest star Alpha (α), the second brightest Beta, the third brightest Gamma, and so on through the Greek alphabet. But not always. And a study of this month’s early evening sky will show many exceptions to this imperfect stellar rule.

Disorderly dog?
Let’s start our search low in the south-southwest, where you’ll find Sirius, the brightest star in the night sky. Justifiably, Sirius shines as the Alpha star of Canis Major the Big Dog. But chaos appears to reign after that: Murzim, the constellation’s Beta star, ranks fourth in brightness; Muliphen, its Gamma star, burns in at eighth place; and Adhara, the Dog’s second-brightest star, wears the fifth-place label Epsilon (ε).

What’s going on?

Early skywatchers perceived the brightest stars of Canis Major as forming two distinct groups: A northern segment and a southern one. The northern part consists of Alpha Canis Majoris and its two bright attendants: 2nd-magnitude Beta to the west (whose rising announces the coming of Sirius), and 4th-magnitude Gamma to the east of Sirius (sweeping up behind).

The constellation’s southern section (an inferior location in the sky from the perspective of a viewer in the Northern Hemisphere) contains the constellation’s next three brightest stars: 1.8-magnitude Wezen (Delta [δ]); 1.5-magnitude Adhara (Epsilon); and 2.4-magnitude Aludra (Eta [η]). These stars form a distinct triangle about 15° southeast of Sirius. But why isn’t Adhara the Delta star?
Great bear
Ursa Major the Great Bear, which currently rises in the northeast, is home to the Big Dipper, whose Delta star seems to have an especially mismatched Greek letter. Library of Congress
Pre-telescopic astronomers had no means of determining such exact magnitudes, especially between stars of near-similar brightness twinkling so close to the horizon. So Bayer incorporated other schemes to rank brightness. As with Beta and Gamma Canis Majoris, he sometimes deferred to a star’s position in the sky or its mythological importance.

Adhara, Wezen, and Aludra, together with Omicron (ο) Canis Majoris immediately to the north, were the “Virgin” stars of Arabia — with Wezen at the group’s center; this choice location presumably earned it dominance over its near-equal, Adhara.
 
So order does exist among the brightest stars of the Big Dog after all; you just have to know a little bit about the constellation’s history to understand it.
Big-Dipper
The Big Dipper's stars are more or less equally bright, so Bayer distinguished among them by position, labeling them in order from west to east (right to left in this photo). Astronomy: Roen Kelly; Bill and Sally Fletcher
Polar importance
Perhaps the best-known exception to Bayer’s ordering scheme is the Big Dipper asterism in Ursa Major the Great Bear, now rising in the northeast. As the photo on this page shows, the Dipper’s stars are a mixture of near equals in terms of brightness. To differentiate among them, Bayer again deferred to position, this time labeling them in order of increasing celestial longitude, from west to east.
 
Now look in the sky midway between Mizar (the star in the bend of the Big Dipper’s handle) and 3rd-magnitude Pherkad (Gamma Ursae Minoris) in the Little Dipper’s bowl (not shown in the photo). There you’ll find Thuban (Alpha Draconis) glowing at magnitude 3.6.
 
Thuban is significantly fainter than Beta Draconis (2.8), which is fainter than Gamma Draconis (2.2) — the brightest star in Draco the Dragon! Bayer obviously made Thuban the Alpha star because, some 4,800 years ago, it marked the important position of the North Celestial Pole, around which all stars appear to revolve. Likewise, today’s 2nd-magnitude Pole Star, Polaris (Alpha Ursae Minoris), pulls rank on equally bright Kochab (Beta Ursae Minoris), which holds a less significant position in the sky.
 
Similarly, fainter Beta Draconis presides over brighter Gamma Draconis. Arabian observers saw it as the principal of the main herd of “sucking camels,” the four brightest stars in Draco’s head: Beta, Gamma, Nu (ν), and Xi (χ). This could be because it leads that pack in right ascension, or because the star appeared brighter than Gamma to them.
Castor-and-Pollux
Castor and Pollux, of Gemini the Twins, present another example of a Beta star outshining its Alpha neighbor. Astronomy: Roen Kelly
Identical twin stars?
Gemini the Twins, which hugs the south meridian this month after sunset, hosts another “mix up.” If you look carefully at the roughly 1st-magnitude twin stars Castor (Alpha Geminorum) and Pollux (Beta Geminorum), you’ll see they don’t burn equally bright; Beta outshines Alpha by 0.4 magnitude — enough for a careful naked-eye observer to notice the difference.

When Bayer labeled these stars, he might have considered Alpha’s superior position — not only is it the more northerly of the two, but it also precedes Beta in right ascension. On the other hand, some have suggested that Pollux, a red giant, could have brightened over the past few centuries.

Now turn your gaze toward the southwest, to Betelgeuse and Rigel — the Alpha and Beta stars, respectively, of Orion the Hunter. Betelgeuse, a red supergiant, varies in brightness. At maximum light, it can rival, or even exceed, Rigel (magnitude 0.2) in brightness; at minimum light, it drops in rank to 1st-magnitude.

When Bayer labeled these stars, Betelgeuse might have appeared superior to Rigel, especially because atmospheric hazes near the horizon can dim the latter star’s appearance. Of course, Betelgeuse’s top rank could simply reflect the star’s more northerly position. And as with the stars in the Big Dipper, Bayer also ordered the three 2nd-magnitude Belt stars of Orion — magnitude 2.2 Mintaka (Delta Orionis); magnitude 1.7 Alnilam (Epsilon Orionis); and magnitude 1.8 Alnitak (Zeta [ζ] Orionis) — not by magnitude but in order of right ascension.
 
By the way, why isn’t Gamma Aquilae the Beta star of the Eagle? The 19th-
century British observer William Henry Smyth suggested that by “several direct comparisons in 1834 and 1836, I can pronounce that [γ] is now actually brighter than β, which of course was not formerly the case.” Nevertheless, it appears that no one knows for sure.
 
Finally, I’d like to end on a missing note: Gamma Aurigae. Where is it? The answer appears below. Meanwhile, be sure to send your thoughts and observations to someara@interpac.net.

Answer: Gamma Aurigae no longer appears on star charts. It has been replaced with Beta Tauri — a magnitude 1.65 star that links the two constellations. Thus, the second-brightest star in Taurus the Bull is also the second-brightest star in the Pentagram of Auriga. But since Bayer "officially awarded” Gamma Aurigae to Taurus, third-ranking Menkalinan, by default, has become Auriga’s Beta star.
0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT

FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
Apollo_RightRail

Click here to receive a FREE e-Guide exclusively from Astronomy magazine.

Find us on Facebook