Alpha Piscium, or Al Risha, is not one of the sky’s brightest stars. In fact, it’s only about 4th magnitude, which is getting down to a level of faintness that will require a dark sky to see. But Al Risha is a fascinating star in a prominent place in the zodiacal constellation Pisces the Fishes, which is one of the sky’s most graceful and beautiful constellations. Follow the links below to learn more about Alpha Piscium, aka Al Risha.
Pisces the Fishes is always shown as a pair of fish, swimming in opposite directions. The Western Fish lies in the graceful line of stars south of the Great Square of Pegasus, and the Northern Fish is another line of stars to the east of the Square. Al Risha represents the knot or cord that ties the two Fish together by ribbons at their tails. In fact, Al Risha means “the cord” in Arabic.
Northern Hemisphere autumn (or Southern Hemisphere spring) is a good time to see the constellation Pisces, with the star Al Risha at its heart, in the evening sky. As seen from across the globe, Pisces reaches its high point for the night at about 10 p.m. local standard time in early November and at about 8 p.m. in early December.
If you can find the Great Square of Pegasus – which really is very noticeable as a large square pattern on the sky’s dome, with four medium-bright stars marking its corners – you can find Pisces. You can, that is, if your sky is dark enough.
You’ll probably pick out the Western Fish first, because it contains an asterism – or noticeable pattern of stars – known as The Circlet. The little circle of faint stars forming the Circlet in Pisces can be seen easily in a dark sky on the southern edge of the Great Square.
The rest of the constellation Pisces forms a beautiful V shape – like the letter V – on two sides of the Square.
That’s surely why the German astronomer Johann Bayer, in 1603, gave this star the designation Alpha in his star atlas Uranometria (named after Urania, the Greek Muse of Astronomy), even though Al Risha is only the third-brightest star in its constellation. Bayer’s system was to assign a lower-case Greek letter (alpha, beta, gamma and so on) to each star he catalogued, combined with the Latin name of the star’s parent constellation in genitive (possessive) form. So, for example, the star Al Risha is also Alpha Piscium, the Alpha star of Pisces.
Most of the time, the Alpha star is the brightest star in a constellation, but not always. There are two brighter stars in Pisces (although not much brighter). They are Eta and Gamma Piscium. Al Risha, by the way, is also one of the only stars in Pisces with a proper name. The early Arabian stargazers, who named it, noticed it, too.
In Roman mythology, the constellation Pisces is associated with the legend of Venus and Cupid (or, in the Greek myths, Aphrodite and her son Eros). These two escaped the monster Typhon (or Typhoon) by transforming themselves into fishes and jumping into a river. Venus and Cupid are said to have bound themselves together so that, in escaping the monster, they would not be separated. The gods were pleased and placed the Fishes in the sky to commemorate the event.
Al Risha in science. Al Risha appears single, but it is a close double star, that is, two stars orbiting a common center of gravity. It consists of pair of class A stars that lie some 120 A.U. (Astronomical Units) apart, with one A.U. equally one Earth-sun distance. So the two stars that we see as Al Risha are in fact 120 times the distance between our Earth and sun, or about the distance between our sun and Pluto.
The two stars in the Al Risha system take 720 years to orbit each other. Yet these stars appear so close together from our earthly vantage point that amateur astronomers using backyard telescopes must look carefully to see both of them. Plus, from our perspective, the two stars are appearing to get closer together as they pursue their vast mutual orbit. It’s estimated they will appear closest, as seen from Earth, in the year 2060. Both stars are white, though some observers have reported subtle colors.
Al Risha’s position is: RA 02h 02m 03s, Dec +02° 45′ 50″
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.