The Andromeda constellation consists of 16 stars visible in the northern sky. Named by ancient Greeks after the mythological Andromeda princess, the constellation contains the Andromeda Galaxy, the nearest full-fledged galaxy to our own Milky Way.
Andromeda is one of 48 constellations described by the Ancient Greek astronomer Claudius Ptolemy in 150 AD in his famous work "The Almagest."
Ptolemy's list was the first official description of constellations, although these stellar groupings had been known to ancient Greeks, Babylonians, Egyptians and other cultures, who frequently saw them as embodiments of their mythical figures.
Andromeda is one of the largest of Ptolemy's constellations.
Another 40 constellations were added to Ptolemy's tally over the centuries. Many of these additions reside in the southern sky, which was not visible to Ptolemy, who spent most of his life in Egypt.
Today, Andromeda is the 19th largest of the 88 constellations recognized by the International Astronomical Union.
Related: Constellations of the western zodiac
When to observe the Andromeda constellation
The Andromeda constellation can be best viewed from the Northern Hemisphere, as it moves lower toward the horizon the farther south you stand. At about 40 degrees south latitude it disappears completely, according to EarthSky. (The 40th parallel south lies just below the southern tips of Africa and Australia and crosses the southernmost part of South America).
In the Northern Hemisphere, Andromeda is best observed from August to February, while in the Southern Hemisphere, the visibility period is much shorter, from October to December, according to PlanetGuide.
From August to September, in the Northern Hemisphere, Andromeda emerges on the north-eastern horizon at about 10 p.m. local time, then gradually rises overhead. From October to November, it emerges in the eastern sky at around 8 p.m. From December to January, Andromeda rises at around 6 p.m. and moves along the northwestern horizon, according to Planet Guide.
How to find the Andromeda constellation
Andromeda is part of a big prominent group of constellations named after mythological figures related to the Perseus and Andromeda myth. The pronounced W-shaped Cassiopeia constellation points directly at Andromeda.
With a bit of imagination, you can see the constellation as a sprawled figure with its arms extended. The figure's feet point in the direction of the Perseus constellation, right next to Cassiopeia. The figure's "head" connects the constellation to the neighboring square of Pegasus.
The Andromeda myth
In ancient Greek mythology, Andromeda was a beautiful daughter of the king Cepheus and queen Cassiopeia. When her mother bragged that Andromeda was better-looking than the famously beautiful Nereid sea nymphs, the sea god Poseidon sent his pet sea monster Cetus to destroy Cepheus' kingdom.
It was said, however, that sacrificing Andromeda could save the country and its people. So, the loving parents had their daughter chained to a rock by the sea, so that the sea monster could get her easily. Fortunately, the charming prince Perseus flew past on his winged horse Pegasus and fell in love with Andromeda's beauty. He killed Cetus and married Andromeda. The two lived happily ever after.
Stars of the Andromeda constellation
The Andromeda Constellation contains nine main named stars, according to Constellation Guide. Three of these stars are brighter than magnitude 3.00, which puts them into the top 100 brightest stars in the sky.
In addition to the visible stars, one can find many intriguing objects within the Andromeda constellation that are only visible with telescopes or binoculars. The most famous one of these is the Andromeda Galaxy, the closest full-fledged galaxy to the Milky Way. Andromeda is currently about 2.5 million light-years from the Milky Way, and the two galaxies are on a collision course; a few billion years from now, they will smash into each other.
Alpheratz (Alpha Andromeda)
The brightest star in the Andromeda Constellation is Alpheratz, also called Alpha Andromedae. Alpheratz is essentially Andromeda's head, but it's also part of the neighboring square of Pegasus.
Astronomers know today that Alpheratz is located 97 light-years from Earth and is, in fact, a binary star, a system of two stars orbiting a common center of mass. The bigger of those two stars has a unique chemical composition and is the brightest known star with high levels of mercury and manganese. A so-called subgiant, this star is 200 times brighter than the sun while being less than four times as massive.
The smaller star of the Alpheratz binary system is about 10 times as luminous as the sun and orbits the larger star every 97 days, according to the Constellation Guide.
Mirach, also known as Beta Andromedae, is nearly as bright as Alpheratz and located about 200 light-years away from Earth. It is a giant star 1,900 times as bright as the sun, according to the Constellation Guide, and three to four times as massive.
Within the Andromeda Constellation, Mirach forms the chained figure's left hip. Close to Mirach, astronomers can observe two distant dwarf galaxies located some 10 million light-years from Earth.
Almach, or Gamma Andramodae, is Andromeda's foot. It is a multi-star system featuring a central giant orbited by a pair of white dwarfs. The third brightest star of the Andromeda Constellation, Almach is about 350 light-years away from Earth. The star's central giant is 2,000 times more luminous than the sun.
Other stars in the Andromeda Constellation:
Other stars in the Andromeda Constellation are Delta Andromedae, Iota Andromedae, Upsilon Andromedae, Adhil (or Xi Andromedae) and Mu Andromedae.
The Andromeda Galaxy, the Milky Way's nearest major galactic neighbor, can be found within the Andromeda Constellation. The galaxy, located some 2.5 million light-years away from Earth, is barely visible to the naked eye but can be found with binoculars.
The Andromeda Galaxy, also known as M31, can be seen as a misty patch on the edge of the Andromeda Constellation to the right of the sprawled figure's right hip, which is right next to the bright star Mirach. The right V shape of the Cassiopeia constellation points vaguely in the direction of the galaxy.
Andromedids meteor shower
The constellation also appeared to be the source of the Andromedids Meteor shower that used to occur every few years in November. But the stars of the constellation, obviously, had nothing to do with these shooting stars. The actual source of the space rocks was a trail of debris left behind by Comet Biela, and which Earth used to intersect every November. However, in recent decades, the once mesmerizing display nearly ceased, as our planet's orbit moved out of the path of the cometary debris.
Read more about Andromeda in the Constellation Guide or the Planet Guide. Universe Today also provides a deep dive into this constellation.
Swinburne University of Technology, Constellation
Britannica, Andromeda Constellation
University of St. Andrews, Claudius Ptolemy https://mathshistory.st-andrews.ac.uk/Biographies/Ptolemy/
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