Jan 15th: Why did Betelgeuse become Alpha Orionis instead of Rigel?

By on January 15, 2015 in
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Podcaster: Avivah Yamani

Title: Why did Betelgeuse become Alpha Orionis instead of Rigel?

Organization: 365 Days Of Astronomy ; langitselatan

Link : http://cosmoquest.org/x/365daysofastronomyhttp://langitselatan.com

Source for this podcast:
Bayer’s Uranomteria and Bayers Letter: http://www.ianridpath.com/startales/bayer.htm
Bayer Designation: http://en.wikipedia.org/wiki/Bayer_designation
First Star Chart: http://en.wikipedia.org/wiki/Star_chart
Nick Cannas, Star Maps: History, Artistry and Cartography

Description:  Why Rigel is not alpha Orionis even though it is the brightest star in the Orion constellation. Why did Betelgeuse become Alpha Orionis instead of Rigel?

Bio: Avivah is a project director of 365 Days Of Astronomy and astronomy communicator from Indonesia.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2015, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.

Transcript:

Hi! Welcome to the Q&A of 365 Days of Astronomy. I’m Avivah from Indonesia and I will discuss a question about why Rigel is not alpha Orionis even though it is the brightest star in the Orion constellation. Why did Betelgeuse become Alpha Orionis instead of Rigel? Poor Rigel!

The easiest answer is because even though Betelgeuse’s magnitude is dimmer than Rigel, as a variable star, Betelgeuse’s magnitude varies between 0.2 and 1.2. At its brightest, the 0.2 magnitude, Betelgeuse is very close to Rigel’s 0.18 magnitude. Perhaps too close to be able to tell which is brighter. The star brightens and fades over a period of about 400 days.  The variation in Betelgeuse’s brightness is believed to have been observed in 1836 by Sir John Herschel, when he published his observations in “Outlines of Astronomy.”

But let’s look for another answer. I want to share a glimpse of the history from when the stars got their greek alphabet designation. In general alpha is the brightest star in the constellation. But is this always the case?

Let’s take a look to history. When you look up to the sky .. in a very dark sky you will see at least 10,000 stars above you. And all of them are only dots in the sky. How do you find the same stars again? People form a recognizable pattern in their minds from a group of stars, usually assign it to a mythological figure and name it according to the myth. This patterns are different for every culture. But one thing for sure, most observers also map the position of the stars in the sky.

The first people to map the positions of stars were the Chinese astronomers Shi Shen, Gan De and Wu Xian in the third and fourth century BC, though they mapped the star positions inaccurately. The earliest copy of a Star Map is Dunhuang Star Map from the Tang Dynasty in the 6th to 9th century AD. It was discovered in modern times in the ruins of a monastery called the Mogao Caves, or the Caves of the Thousand Buddhas in the deserts of central Asia. The Dunhuang star map contains 1,354 stars in 257 asterisms. While the earliest Western star catalog was created by the Greek astronomer Hipparchus around 129 BC, building on earlier work going back to the Babylonians.

But the golden age of pictorial celestial cartography took place in Europe, from 1600 to 1800. During this period, grand sky atlases were produced that attempted to accurately place the stars and planets in coordinate systems that paralleled those on Earth.  At this time, Tycho Brahe and Johannes Hevelius determined the locations of stars using naked eye observations.

The first atlas published was entitled Uranometria Omnium Asterismorum by Johann Bayer from Germany. His atlas was published in 1603 in Augsburg and it consisted 51 star maps engraved on copper by Alexander Mair.  The map includes 48 charts of Ptolemaic constellations, 1 chart of 12 new Southern constellations and 2 charts that were overview maps of the Northern Hemisphere and Southern Hemisphere.

The source of star positions and magnitudes for the Ptolemaic constellations in Uranometria was Tycho Brahe’s catalogue of 1,005 stars which published in 1602. Johann Bayer revised some of the magnitudes and added an additional 1,000 stars of his own. And he also included mythological figures in his star atlas.

Contrary to popular belief Bayer did not letter the stars in strict order of brightness. In fact, magnitude estimates at that time were not good enough for this to have been possible. The first time Galileo used his telescope and pointed it to the sky was a few years after the first printing of Uranometria. So all observations at that time were by naked eyes only.

In his map, Johann Bayer classified the stars into 6 magnitude classes, from first to sixth then allocated letters to the members of each class as he saw fit. Each magnitude class has a range of magnitudes and Bayer categorized each star according to its magnitude by using Greek letters with alpha as the brightest stars in the constellation, beta, the second and etc. Once he ran out of Greek letters he used Roman letter for the less bright stars that  he labeled, starting with a capital A followed by lowercase b, c, d etc.

Sometimes the brightest stars in constellation fit the same magnitude class and Bayer will consider another way to name them. That’s why alpha is not always the brightest star in constellation.

In the naming process, Bayer sometimes assigned letters to stars according to their location within a constellation (for example: the northern, southern, eastern, or western part of a constellation), according to either the order in which they rise in the east, to historical or mythological information on specific stars within a constellation, or to his own arbitrary choosing

For example, in Ursa Major the seven stars of the Plough were labelled in order of right ascension. In Gemini, the three brightest stars were labelled by declination, from north to south. It is also thought that Bayer used the “First to Rise in the East” method for Castor and Pollux of Gemini. Although Pollux is brighter than Castor, the latter was assigned alpha because it rises in the east ahead of the former. Bayer may also have assigned the stars Castor and Pollux in terms of historical or mythological knowledge. Both historically and mythologically, Castor’s name is almost always mentioned first (Castor and Pollux) whenever the twins are mentioned, and that may have compelled him to assign alpha (α) to Castor and beta (β) to Pollux.

In Cygnus the sequence of letters for the brightest stars follows the overall shape of the constellation. In many other constellations, and particularly among the fainter stars, there is no obvious pattern to the distribution of letters at all.

Another case is Orion, which Rigel the second brightest star becomes Beta and not Alpha. In the naming process, it seems Bayer first designated the two 1st-magnitude stars, Betelgeuse and Rigel, as Alpha and Beta, with Betelgeuse which in the shoulder coming ahead of Rigel which lay in the foot of the hunter, even though the latter is usually the brighter.

In Bayer catalogs, there are 16 constellations in which the star labelled Alpha is not the brightest:
Cancer, Capricornus, Cetus, Corvus, Crater, Delphinus, Draco, Gemini, Hercules, Libra, Orion, Pegasus, Pisces, Sagitta, Sagittarius, and Triangulum.

And according to the 88 modern constellations, there are at least 30 in which “Alpha” is not the brightest star, and four of those lack an alpha star altogether. Constellations with no alpha include Vela and Puppis, both formerly part of Argo Navis whose alpha is Canopus in Carina. Argo Navis was a particularly large constellation that was broken up into 3 constellations in 1752 by French astronomer La Caille.  So Carina kept the Alpha star after the, uh, divorce, so-to-speak. But it was an amicable divorce!

Thank you for listening. This is 365 Days Of Astronomy.

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About Avivah Yamani

Astronomy communicator and Project Director of 365 Days Of Astronomy.

Visit Avivah Yamani's Website

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