By Chris Vaughan, Senior Astronomer, AstroGeo.ca
The distinctive constellation Pegasus (the Winged Horse) is well placed for evening stargazing in autumn. Although the constellation’s name comes from Greek mythology, many of its main stars have names derived from Arabic words and phrases. That holds true for most constellations. A few of the star names we use today were originally coined by sky-watchers in the Arabian Peninsula before the rise of Islam, but most modern proper names for stars come from Muslim astronomers – in a round-about way.
In the second Century, Greek astronomer Claudius Ptolemy recorded the positions and brightness of 1025 stars in his book, the Almagest. Ptolemy identified stars according to their position within a familiar constellation or asterism, such as “the tail of the lion”. In the 8th and 9th centuries the Almagest was translated into Arabic. Muslim astronomers adopted some of Ptolemy’s proper names for stars, translated into Arabic, and added other names from traditional Arabic star lore. So the “tail of the lion” became Denebola, from ذنب الاسد ðanab al-asad. Around 964 CE, the Persian Muslim astronomer Abd al-Rahman al-Sufi compiled Arabic star-lore into his Book of Fixed Stars, or كتاب صور الكواكب kitāb suwar al-kawākib. His original no longer exists, but you can browse through a later replica of al-Sufi’s book here.
Image 1: al_Sufi Drawing of Pegasus
Caption: A drawing of Pegasus from a replica of Abd al-Rahman al-Sufi’s Book of Fixed Stars. This chart is tilted compared to the orientation of Pegasus we see on October evenings. The hand-drawn star positions differ a bit from photos or computer-drawn charts. Source: www.atlascoelestis.com
Pegasus is one of the largest and oldest of the 88 modern constellations – 7th in size, by area. Since it occupies a place in the sky just north of the celestial equator, he is visible from almost everywhere on Earth. Only residents of Antarctica never see it rise. Below, I’ll tell you how to find Pegasus, and I’ll tour you through the main stars – so you can see them for yourself on the next clear night.
Pegasus has traditionally been depicted as upside-down. Four bright stars outline his wings, and lines of stars stretching out to the upper right form his head and neck, and two front legs. But part of him is missing! The stars where his rear half should be are instead used by the water constellation of Pisces (the Fishes). Another aquatic constellation, Aquarius (the Water-Bearer) is situated to the right of the fish, below Pegasus’ head. According to some scholars, the ancient Phoenicians envisioned Pegasus’ stars as a bridled horse affixed to the prow of their ships – explaining his missing hindquarters and the nautical theme of the surrounding constellations.
Image 2: mid-October Arabic Sky chart for Pegasus
Caption: This sky chart shows the constellations in the eastern at 8 pm local time in mid-October every year. Pegasus is in the centre, halfway between the horizon and the zenith (green symbol at top centre). This map uses Arabic names for the constellations. Source: Stellarium Software
The Great Square
Pegasus contains one of the most obvious asterisms in the sky – a giant square made from four, equally bright stars called the Great Square of Pegasus. The shape will probably remind you of a baseball diamond when you see it, because it’s usually tilted with one corner downwards. An asterism is a shape or pattern in the sky made up of prominent stars – either stars from a single constellation, or a combination of stars from adjacent constellations. The Big Dipper is an asterism that uses only part of its large constellation Ursa Major.
Pegasus’ big square of stars is relatively easy to see, even from the suburbs. Head outside on a clear night in October, and face the eastern sky. Each of the square’s sides is about 16° long (or 1.6 fist widths when held at arm’s length). In mid-October at 8 pm local time, the centre of the square will be halfway between the horizon and straight overhead (the zenith, another Arabic word), and tilted with one corner pointing down. Those stars will climb to a maximum height in the southern sky around midnight, and then descend to set in the west just before sunrise. To confirm that you are viewing the right part of the sky, look for the very bright stars Vega and Deneb high in the sky above Pegasus. The distinctive “W”-shaped constellation of Cassiopeia (the Queen) will be toward his upper left.
The nomadic Arabs saw Pegasus’ square as “Al Dalw”, the “water bucket” being held by Aquarius. To draw water from a deep well, desert travelers would take a water skin and prop the mouth open using crossed sticks lashed to each corner, forming a square opening. When empty, the water skin was stowed flat or rolled up on the flanks of their camels. The square may also have been Al ‘Arkuwah, the well in which such a bucket was used. Using your unaided eyes only, the Great Square appears to be empty. Look carefully for two dim stars offset slightly from the centre of the square – they represent the place where the crossed sticks are tied together.
Image 3: Oct 22 at 8 pm – Pegasus Stars
Caption: A sky chart for Pegasus, showing all of the important star names, plus the globular star cluster Messier 15 at centre right. The left and right corners of the Great Square of Pegasus are separated by 20°, or two outstretched fist diameters. The star Alpheratz is actually a star in Andromeda, as shown by the red border lines. Source: Stellarium Software
A Tour of Pegasus’ Stars
Let’s tour the stars, one by one, and learn what’s special about each of them. To make things easier, I’m going to treat the tilted square as a diamond shape.
The star at the bottom of the diamond is Algenib, derived from the Arabic الجنب Al-janb or Al-jānib, “the flank”. It’s a very hot blue-white star of moderate brightness that sits about 350 light-years away from us, and actually emits 4,000 times more light than our sun!
Moving counter-clockwise, the white star at the right-hand corner is Markab “the saddle”. This star appears slightly brighter than Algenib – it emits less light, but it seems as bright because it is only 140 light-years away. In some old star atlases, Markab was labelled Matn al Faras, “the horse’s withers, or shoulder”.
From Markab, look farther to the right for the bent line of dimmer stars that trace the horse’s neck and head. A palm’s width from Markab is blue Homam from the Arabic Sa’d al Humām “man of high spirit”. Another fist’s width further on sits Biham, derived from S’ad al Biham “lucky stars of the young beasts”. Both stars are about equal in brightness. A somewhat fainter star named Al Fum al Faras “the mouth of the horse” is located a thumb’s width to the right of Biham.
From Biham look a palm’s width higher to the right for bright star Enif, named from الأنف Al’anf “the Nose”. Enif is a cool, orange supergiant star located 670 light-years away from us. It is nearing the last stages of its life cycle. You should be able to perceive that its colour is tinted. Enif is huge! Were it to replace our Sun, it would cover 40° of Earth’s sky, eighty times wider than the Sun or Moon! Enif is just at the lower mass limit for dying in a supernova explosion. Use binoculars to scan the sky about four fingers widths to the upper right of Enif. You’ll find a small, dim, but pretty, fuzzy patch of stars designated Messier 15. That globular star cluster is 33,000 light-years away from us.
Image 4: Messier 15 Globular Star Cluster in Pegasus
Caption: This image of the bright globular star cluster Messier 15 in Pegasus was taken by Ron Brecher of Guelph, Ontario in February, 2015. The image spans about one finger width (or 1°) of the sky. In binoculars, M15 will look like a little, grey, fuzzy patch. Ron’s galleries of fine astro-images can be enjoyed at his website www.astrodoc.ca
Resuming our trip around the diamond, the fairly bright, magnitude 2.4 star at the top corner is Scheat “the foreleg”, the second brightest star in the constellation. That name may arise from the expression Al Sā’id “the upper part of the arm”. Scheat is a cool, red giant star located 200 light-years away from us. Pegasus’ two front legs start at Scheat and extend upwards to the right. A slim palm’s width above Scheat is the dim yellow star Matar, from سعد المطر Al Saʽd al Maṭar “lucky star of rain”. The leg terminates a palm’s width farther in the same direction – at a double star designated Pi Pegasi or π Peg. Using binoculars you should be able to see that Pi consists of two close-together, yellow-white stars.
The second foreleg takes a jog to the right before bending back up parallel to the first one. A well-spaced pair of yellowish stars named Sadalbari سعد بارع Saʿd al-bāriʿ, “auspicious star of the splendid one” and Sadalnazi “auspicious star of the camel” mark the knee. The foreleg stretches to a faint white star a fist’s width to the upper right, and ends at a dim white star an additional palm’s width farther along that line.
There’s a dim yellow star sitting a thumb’s width just outside of the baseball diamond, midway between the top and right corners. This is the sunlike star designated 51 Pegasi. It hosts the first exoplanet ever discovered, in 1995 – a Jupiter-sized planet that orbits that star every 4.23 days at a distance much closer than Mercury does in our solar system. Planets like this are called hot Jupiters. Initially, the planet was nick-named Bellerophon, one of the original riders of Pegasus in Greek mythology. Now it is officially named Dimidium, the Latin word for “half” – since the planet has half the mass of Jupiter. Take a look at the star in your binoculars and let your imagination soar, like Bellerophon!
Image 5: Dimidium Extra-solar Planet
Caption: This artist’s conception depicts the huge planet Dimidium, formally known as Bellerophon, which orbits the star 51 Pegasi. It is half the mass of Jupiter, and orbits closer to its star than Mercury does from our sun! Source: NASA.gov
The final star of the square, at the left-hand corner, is called Alpheratz, a name derived from the Arabic phrase سرة الفرس Surrat al-faras “navel of the mare/horse”. It’s another hot, blue-white supergiant star, but it is located only 97 light-years away from us. The spectrum of this star’s light indicates that it is highly enriched in the metal Mercury. In actuality, Alpheratz does not belong to Pegasus. It’s the brightest star in Andromeda, and marks the princess’ head – but that’s a tour for another night!
Referring back to the water bucket concept, in Arabic astrology, Markab and Scheat were together called Al Fargh al Mukdim the “fore-spout (of the bucket)”. Algenib and Alpheratz were called Al Fargh al Thani the “rear spout”. Those two faint stars inside the square were Salm “a Leather Bucket” and Al Karab “the Bucket-rope”. I hope you enjoy this tour of Pegasus, and will appreciate its connections to our ancestors! Keep looking up!
CHRIS VAUGHAN is an award-winning Astronomer and Earth Scientist with a lifelong passion for visual observing. He operates the David Dunlap Observatory’s 74” telescope, volunteers for RASC, and visits schools with his Digital Starlab planetarium. His Astronomy Skylights blog at www.AstroGeo.ca is read worldwide, and he is a regular contributor to SkyNews magazine, Space.com, and popular astronomy apps.
