Andromeda (constellation)

Andromeda  is one of the forty-eight constellations enumerated by the second-century Greco-Roman astronomer Claudius Ptolemy, and is currently one of the eighty-eight modern constellations. Located to the north of the celestial equator, it bears the name of Andromeda, the daughter of Cepheus and Cassiopeia, who was chained to a rock to be devoured by the sea monster Ceto. It is most visible on autumn nights in the northern hemisphere, like other constellations named after characters from the Perseus myth. Given its northern declination, Andromeda is only visible north of 40° south latitude. For more southern observers, it is always below the horizon.

It is one of the largest constellations, with an area of 722 square degrees. Thus, it is equivalent to more than fourteen hundred times the size of the full moon; 55% of the area of Hydra, the largest constellation; and at more than ten times the area of Crux, the smallest.

In Chinese astronomy, the stars that make up Andromeda were part of four different constellations that had astrological and mythological significance. There is also a constellation related to Andromeda in Hindu mythology.

Notable features

Constellation Andromeda AlltheSky.com

Andromeda's main star, Alpheratz (α Andromedae), is a bluish-white subgiant of chemically peculiar spectral type B8IVp. It is the brightest member of the mercury-manganese group of stars, with its mercury content several thousand times that of the Sun. The distribution of this element is not uniform, being concentrated in areas close to the equator. Doppler imaging studies show that these areas move slowly over the stellar surface. Located on the border between the constellations of Andromeda and Pegasus, Alpheratz was sometimes included within the latter constellation.

Next in brightness is β Andromedae, called Mirach or Mirac, a red giant of type M0III whose color is visible to the naked eye. Distant almost 200 light years from Earth, it has a radius 86 times greater than the solar radius. The third brightest star is γ Andromedae, called Almach or Alamak. A popular object for amateur astronomers, it is a multiple star whose brightest component is a luminous giant of type K3IIb. The secondary component is a binary star composed of two main sequence stars of type B8V and A0V with an orbital period of 63.67 years; in turn, the B8V star is a spectroscopic binary. λ Andromedae is also a spectroscopic binary and one of the most important RS Canum Venaticorum variables. Its brightness varies 0.225 magnitudes, reaching a maximum of 3.70, with a period of 53.952 days.

Among the variables of the constellation, it is worth mentioning Z Andromedae, prototype of symbiotic stars. It consists of a close binary system containing a hot white dwarf and a red giant; the red giant loses matter that appears to form a disk around the stellar remnant. Every 10 to 20 years, the system experiences a sudden period of high activity in which the brightness increases by 3 magnitudes.

Extrasolar planets have been discovered on several stars in Andromeda. Titawin (υ Andromedae) is a yellow dwarf of type F8V with four extrasolar planets; the system is completed by a red dwarf whose separation from the yellow dwarf is not known. 14 Andromedae—officially known as Veritate—is an orange giant of type K0III that has a planet in a circular orbit at a distance of 0.83 AU. Another star with an exoplanet is Buna, the name given to HD 16175; the planet moves in a markedly eccentric orbit (ε = 0.59) at an average distance of 2.1 AU. Finally, κ Andromedae is a hot subgiant accompanied by a distant brown dwarf.

The closest Andromedan star to Earth—at 10.6 light-years away—is Ross 248, a red dwarf type M6.0V. It is a BY Draconis type variable whose brightness fluctuates by 0.13 magnitudes, being the first star where small brightness variations were attributed to spots on its photosphere (1950). Another near-Earth system is Groombridge 34, composed of two red dwarfs and flare stars with an orbital period of approximately 2,600 years. Two planets have been detected around one of them.

Andromeda Galaxy (M31), the largest galaxy in our Local Group.

However, the most important astronomical object in the constellation is the Andromeda galaxy (M31), the largest and brightest of the Local Group galaxies. Distant 2.5 million light-years from us, it is easily visible to the naked eye under truly dark skies. With a diameter of 220,000 light-years, it contains a trillion stars, at least twice as many as the Milky Way. The mass of the Andromeda Galaxy is estimated to be 1.5 × 10¹² solar masses, while that of our galaxy is estimated to be 8.5 × 10¹¹ masses. solar.

Several fainter galaxies, including M31's companions M110 and M32, as well as the more distant NGC 891, lie in the Andromeda area.

In this constellation falls the open cluster NGC 752, with an age of about 1.6 billion years and located about 400 parsecs away. It contains a relatively small number of stars, including the variable QX Andromedae.

Also, the planetary nebula NGC 7662 —known by the nickname of the Blue Snowball nebula—, is located in the west-southwest of the constellation. Lastly, the Andromedidas, meteor shower in November, has its radiant in Andromeda.

Main Stars

Alpheratz (α Andromedae), a distant 97 light years.

• α Andromedae (Alpheratz or Sirrah), of magnitude 2,07, white-suclid binary star. She's the brightest representative of mercury-manganese stars.
• β Andromedae (Mirach), also of magnitude 2,07, red giant to 200 light years and semi-regular variable.
• γ Andromedae (Alamak or Almach), of magnitude 2,10 with a small telescope can be solved in a double system (γ1 and γ2): in turn γ2 is a triple star system.
• δ Andromedae, an orange giant of magnitude 3,28 with two tenuous visual companions at 31 and 38 seconds of arc.
• ε Andromedae, with magnitude 4,34 is a yellow giant with a very eccentric orbit around the center of the galaxy.
• γ Andromedae, binaria eclipsante and variable RS Canum Venaticorum of magnitude 4,10.
• . Andromedae, white-smooth subjugant of magnitude 4,30.
• λ Andromedae, one of the most brilliant variables RS Canum Venaticorum, with a variation in its brightness of 0.225 magnitudes in a period of 54 days.
• μ Andromedae, white star of the main sequence of magnitude 3,87.
• . Andromedae, binary star of magnitude 4,53 whose components are very close to each other.
• κ Andromedae, white-azulada star of magnitude 4,1 accompanied by a brown dwarf.
• ? Andromedae, quadruple star system of magnitude 3.62.
• π Andromedae, binary star with a blue component and the other white one separated about 36 seconds of arc.
• Δ Andromedae, white-azulada star of magnitude 4,96.
• ♫ Andromedae, yellow dwarf hotter than the Sun with four extra-solar planets.

Artistic representation of Saffar, one of the planets that orbits around Upsilon Andromedae.

• φ Andromedae, binary system formed by two white-blue stars.
• ω Andromedae, white-yellow subjugant of magnitude 4,83.
• 6 Persei, a yellow-orange giant of magnitude 5,31; despite his name Flamsteed, it is framed in Andromeda.
• 7 Andromedae, white-yellow star of magnitude 4.54.
• 8 Andromedae, a red giant of magnitude 4,85, has four visual companions whose numbers range from 10.7 to 16.
• 14 Andromedae, an orange giant star of magnitude 5,22 with an extrasolar planet.
• 15 Andromedae (V340 Andromedae), Delta Scuti variable and Star Lambda Bootis.
• 28 Andromedae (GN Andromedae), Delta Scuti variable of magnitude 5.22.
• 51 Andromedae, the fifth brightest star of the constellation with magnitude 3.57, in the past was framed in the constellation of Perseus.
• S Andromedae, supernova that took place in 1885 in the Andromeda galaxy, the first supernova observed outside the Milky Way.
• Z Andromedae, cataclysmic variable star that every 10 or 20 years increases its brightness about 3 magnitudes. It's the prototype of the symbiotic stars.
• SS Andromedae, semi-regular variable whose brightness varies between magnitude 9 and 10.1.
• VX Andromedae, carbon star and variable semi-regular variable between magnitude 7.5 and 9.7.
• AQ Andromedae, carbon star and semi-regular variable of average magnitude 7.81.
• FF Andromedae, binary composed of two red dwarfs with chromosopheric activity.
• GO Andromedae, variable Alpha2 Canum Venaticorum of magnitude 6,13.
• GY Andromedae (HR 465), variable star with a peculiar emission spectrum.
• KZ Andromedae, star system of magnitude 7,93 formed by a yellow dwarf and a binary with chromosopheric activity.
• PW Andromedae, variable RS Canum Venaticorum member of the AB Doradus Association.
• QX Andromedae, eclipsant binary in the NGC 752.
• V405 Andromedae, variable RS Canum Venaticorum formed by two very active red dwarfs.
• V439 Andromedae (HR 8), variable BY Draconis of magnitude 6,13 distant 44.6 light years.
• HD 10307 (HR 483), binary system at 41.2 light years of the solar system, composed of a solar analogue star and a red dwarf.
• HD 16175, yellow star with an extrasolate planet.
• Groombridge 34, binary star formed by two red dwarfs at 11.62 light years of the Earth.
• Ross 248, close and tenuous red dwarf at 10.3 light years away.

Deep Sky Objects

Image of the NGC 7662 planetary nebula (Caldwell 22) obtained with the Hubble space telescope.

• Andromeda Galaxy (M31), (NGC 224). Recta Ascension: 00h 42m 42.0s Declination: +41°16'00" (Epoca 2000). At a distance of 2.56 million light years this spiral galaxy is the largest galaxy closest to the Milky Way. At first glance it is seen as a cloud cell, its center is defined with the use of binoculars. In the amateur telescopes it is seen as a yellowish cloudy stain. It has two companions: M32 (NGC 221) Ascension Recta: 00h 42m 42.0s Declination: +40°52'00" (Epoca 2000) and M110 (NGC205) Ascension Recta: 00h 40m 24.0s Declination: +41°41'00" (Epoca 2000). They must be observed with binoculars or telescopes at low power, the first is brighter than the second.
• Andromeda I and Andromeda II, also satellite galaxies of the Andromeda galaxy, but of such dim shine that they cannot be observed by the amateur astronomer.
• NGC 752. Ascension Recta: 01h 57m 48.0s Declination: +37°41'00" (Epoca 2000). Cluster open to 4.5° of γ Andromedae.
• NGC 891. Ascension Recta: 02h 22m 36.0s Declination: +42°21'00" (Epoca 2000). Spiral galaxy that shows a dark central line in large-open telescopes. 4th east of Gamma And.
• NGC 7662. Ascension Recta: 23h 25m 54.0s Declination: +42°33'00" (Epoca 2000). Also known as a blue snowball, it is a planetary nebula that is located at 2.5° southwest of ι Andromedae. 3' south is NGC 7640 Ascension Recta: 23h 22m 06.0s Declination: +40°51'00" (Epoch 2000) is a spiral galaxy barred.

Mythology

Andromeda at the Uranographia by Johannes Hevelius

Cassiopeia and Cepheus, kings of Ethiopia were the parents of Andromeda. Queen Cassiopeia boasted of the beauty of her daughter, and compared her to the Nereids, daughters of the sea god Nereus. The sea goddesses, in response to this affront, demanded revenge from Poseidon and that is how he sent a sea monster (Cetus) to destroy the country's coasts.

Cepheus went to the oracle of Zeus who advised him that the only way to calm the anger of the gods and get rid of the monster was to sacrifice his daughter Andromeda, tying her to a rock on the cliff so that the monster could take her away.

At that time Perseus was coming back from his expedition against the Gorgon, and seeing the victim he fell in love with her. He went to the king and proposed to free her in exchange for her hand. Perseus killed the monster by showing him the head of Medusa, which he brought as a trophy from his last adventure, turning it into coral.

Phineus, the princess's uncle and to whom she had engaged in marriage, swore revenge on Perseus and with his supporters they cornered the hero to kill him. He had no recourse but to use Medusa's head, turning them into stone.