Sagittarius (constellation)

Sagittarius (the archer) is a constellation of the zodiac, usually depicted as a centaur holding a bow. Sagittarius lies between Scorpius to the west and Capricornus to the east.

The brightest stars form an easily recognizable asterism, “the Teapot”, whose handle is formed by the stars ζ Sagittarii, τ Sagittarii, σ Sagittarii and φ Sagittarii, the cover by φ Sagittarii, λ Sagittarii and δ Sagittarii, the The body of the Teapot is made up of ζ Sagittarii, φ Sagittarii, δ Sagittarii and ε Sagittarii, the "spout" being the stars δ Sagittarii, ε Sagittarii —the brightest star in the constellation— and γ Sagittarii.

Notable features

AlltheSky.com Sagittarius Constellation Image Orientation taking as reference north latitude: Above: East. Down: West. Left: North. Right: South

The brightest star in Sagittarius is Kaus Australis (ε Sagittarii), a bluish-white giant of spectral type B9.5III and a temperature of 9200 K located 145 light years in the direction of the center of the Milky Way. Shape a binary system with a main sequence star slightly less massive than the Sun.

Next in brightness is Nunki, a name of Babylonian origin for σ Sagittarii. It is a hot (20,000 K) bluish-white star of type B2V with a mass almost eight times that of the Sun. Studies by interferometry suggest the presence of a close companion. Its distance from Earth —from the parallax measured with the Gaia space probe— is 227 light-years.

The third brightest star is ζ Sagittarii, known as Askella or Ascella, a binary formed by two white main sequence stars whose separation varies between 10.6 and 16.1 au due to the eccentricity of the orbit (ε = 0.211). The fourth and fifth stars in terms of brightness are two orange giants but with very different parameters. While Kaus Media (δ Sagittarii) is 347 light-years from Earth and is 1180 times more luminous than the Sun, Kaus Borealis (λ Sagittarii) is a giant of type K1IIIb similar to Pollux (β Geminorum) 23 times less brighter than Kaus Medius. γ2 Sagittarii, called Alnasl or Nash, is also an orange giant similar to Kaus Borealis that has a radius 12 times greater than the solar radius.

Another interesting star in Sagittarius is Polis, a Coptic term for μ Sagittarii, a distant bluish-white supergiant, type B8Iap, which is at least 3,000 light-years away. It is an eclipsing binary: every 180.6 days its brightness decreases by 0.08 magnitudes when a dimmer companion passes in front of it. In addition, four other stars appear to have a weak physical link to the binary, although the whole cannot be considered a true star system.

Another star with a proper name is Albaldah (π Sagittarii), a luminous white-yellow giant of spectral type F2II with an estimated age of 67 million years. Rukbat —official name of α Sagittarii— is a main sequence star of type B8V and 12,400 K effective temperature. On the other hand, υ Sagittarii is a prototype binary for very hydrogen-deficient stars ("HdB stars"). Difficult to classify, it is thought that what is observed is the visible component of the system, a supergiant of spectral type A, which has an invisible companion detected only in the ultraviolet region. Furthermore, the system appears to be surrounded by an envelope or disk, which gives rise to uncertainty as to the spectral type and causes it to vary with time. Another star in the constellation, 9 Sagittarii, is one of the hottest visible to the naked eye: it is a binary whose main component, type O3.5V, has a surface temperature of 43,850 K.

In Sagittarius are two Cepheids visible to the naked eye, X Sagittarii and W Sagittarii. They have maximum apparent magnitude +4.20 and +4.29 respectively. Another variable of interest is KW Sagittarii, one of the largest known stars, approximately 1000 times larger than the Sun. Also, the so-called Pistol Star (in English Pistol Star) is found in this constellation. Invisible to the naked eye, it is nevertheless one of the most luminous stars in the galaxy. With a mass of about 27.4 solar masses, the Gun Star is classified as a blue luminous variable and has a luminosity equivalent to 3.3 million suns.

WR 104 (V5097 Sagittarii).

Another peculiar star is the Wolf-Rayet star WR 104, whose most notable feature is that the dust that flows from it forms a kind of "pinwheel" that rotates along with it every 220 days, which is why it is also called called a Spiral Star.

More than twenty stars in this constellation have extrasolar planets. HD 179949 —called Gumala according to the IAU— is a yellow dwarf hotter than our Sun with a "hot Jupiter" type planet whose orbital period is just over 3 days; the presence of monoxide has been detected of carbon and water vapor in the atmosphere of this planet. Very similar to this star is HD 169830, which hosts a planetary system with two giant planets. Similarly, Sika (HD 181720) is a metal-poor G1V type star where an extrasolar planet has also been discovered.

Ross 154 is the closest star in the constellation and the eighth closest to the solar system (at 9.68 light years). It is a red dwarf of spectral type M3.5Ve and a flare star; the Chandra observatory noted a strong flare that increased the flux of X-rays from the star by a factor of more than 100.

To the west of γ Sagittarii is the galactic center of the Milky Way. S2 is the name of the closest star to the supermassive black hole that seems to exist in the center of our galaxy, being one of the brightest of the so-called star cluster-S, made up of the closest stars to the Sagittarius A* radio source.

Central region of the M22 cluster. (Image of Hubble Space Telescope)

Sagittarius is a constellation very frequented by amateur astronomers, since it contains a large number of deep-sky objects. M23 is an open cluster, 2,150 light-years distant, with more than 150 stars whose cluster membership has been proven. Among the globular clusters, M22 stands out, one of the closest to Earth, located 10,400 light years near the galactic bulge. Two black holes between 10 and 20 solar masses have been discovered in this cluster. M54, by contrast, is a very distant cluster, lying 87,000 light-years away. It is currently thought to belong to the Sagittarius Dwarf Elliptical Galaxy (SagDEG) and not to the Milky Way. Cluster M55 contains about 100,000 stars and is 17,600 light-years from the Solar System; its mass is approximately 269,000 times that of the Sun.

Among the nebulae, the Trifid Nebula (M20) and the Omega Nebula (M17) stand out. The first is an HII region discovered by Guillaume Le Gentil in the year 1750. Its name means "divided into three lobes," since its outstanding feature is its three bright lobes separated by dark dust lanes. It is located 5,500 light years from Earth. For its part, the Omega Nebula has a diameter of approximately 15 light years and is associated with a molecular cloud of about 40 light years in diameter and a mass of 30,000 solar masses. It is considered the brightest and most massive HII region in our galaxy. Another object of interest is the Lagoon Nebula (M8), discovered by Giovanni Hodierna before 1654; it is a giant interstellar cloud classified as an emission nebula and HII region.

Barnard Galaxy, irregular dwarf galaxy of the Local Group. (Image obtained with the ESO WFI)

Sagittarius contains three galaxies belonging to the Local Group. The first of these, the Sagittarius Dwarf Elliptical Galaxy (SagDEG), is a small satellite galaxy of the Milky Way with at least four known globular clusters (the aforementioned M54, Terzan 7, Terzan 8 and Arp 2). It has multiple stellar populations, ranging from the oldest globular clusters—nearly as old as the universe itself—to populations several hundred million years old.

The second is Barnard's Galaxy (NGC 6822), considered the prototype of the primitive galaxies that inhabited the early universe. It lies 1.5 million light-years away and is about 7,000 light-years across. With active star formation throughout its entire disk, NGC 6822 is home to some of the brightest HII regions in the local universe. It has a low metal content ([Fe/H] ≈ -1.2).

Finally, the Sagittarius Dwarf Irregular (SagDIG) appears to be the member of the Local Group furthest from the Local Group's barycenter and is, in fact, very close to the edge of the surface of zero relative velocity of our group of galaxies.

Main Stars

False color image of the Pistol Star and the surrounding nebula.

Star of the Peony Nebula. Image of the Spitzer space telescope.

• α Sagittarii (Rukbat), with apparent magnitude 3,97, is overcome in brightness by numerous constellation stars despite having the Bayer denomination Alpha.
• β Sagittarii (Arkab), in reality two distinct stars: β1 (Arkab Prior) is a white-smoiled binary star; β2 (Arkab Posterior) is a white-yellow star of magnitude 4.28.
• γ Sagittarii, also two stars: γ1 is a strained variable called W Sagittarii with a period of 7.59 days; γ2 receives the name of Nash or Alnasl.
• δ Sagittarii (Kaus Medius, Kaus Media or Kaus Southernis), giant orange star and magnitude 2.72, fourth in brightness within Sagittarii.
• ε Sagittarii (Kaus Australis), the brightest of the constellation with magnitude 1.85, a giant white-azulada star at 145 light years.
• ج Sagittarii (Askella or Ascella), the third most brilliant constellation with magnitude 2,63, is a binary star with both white stars.
• Cristian Sagittarii, red giant of magnitude 3,11 with a tenuous companion to 3,6 seconds of arc.
• ι Sagittarii, orange giant of magnitude 4,13.
• λ Sagittarii (Kaus Borealis), an orange giant of magnitude 2,81 to 77 light years away.
• μ Sagittarii (Polis), white-azulada supergigante and distant eclipsing binary about 3600 light years.
• Sagittarii, an orange giant of magnitude 3.77.
• π Sagittarii (Albaldah), star system triple to 440 light years.
• ρ1 Sagittarii, Delta Scuti variable of magnitude 3.93.
• σ Sagittarii (Nunki), with magnitude 2,02, is the second most brilliant star despite having the Bayer denomination Sigma. His name is of Babylonian origin.
• τ Sagittarii, orange giant of magnitude 3.32.
• υ Sagittarii, a binary star of magnitude 4,58; it is a prototype of the very poor stars in hydrogen (“HdB stars”).
• φ Sagittarii, white-azulada star of magnitude 3,27.
• Terebellum, asterism made up of four visually very close stars; ω Sagittarii and 62 Sagittarii are part of it.
• 9 Sagittarii, star of spectral type O of magnitude 5.93.
• U Sagittarii, X Sagittarii, Y Sagittarii and V350 Sagittarii, the four strained variables; the second of them is the brightest, with a brightness ranging between magnitude 4,24 and 4.84 over a period of 7,013 days.
• RS Sagittarii, averaging binary of magnitude 6.03.
• RY Sagittarii, variable star R Coronae Borealis, the second most brilliant in his class.
• KW Sagittarii, although of visual magnitude 9,35, is a supergitant with a size of 1460 times the solar radius, one of the greatest known stars.
• VX Sagittarii, another red supergigant also among the greatest known stars.
• Star Gun (V4647 Sagittarii), a hypergittarii star 2 million times brighter than the Sun, located near the center of the Milky Way.
• V505 Sagittarii, complex star system dominated by an eclipsing binary.
• V3903 Sagittarii, massive binary star composed of two O-type hot stars.
• V3961 Sagittarii, variable Alpha2 Canum Venaticorum with an exceptionally long period of 2300 days.

S2 orbits and other stars of the cluster-S around Sagittarius A*.

• WR 104 (V5097 Sagittarii), star of Wolf-Rayet in which a spiral of gas and dust has been observed.
• HD 164270 (V4072 Sagittarii), star of Wolf-Rayet with a luminosity almost 80,000 times higher than that of the Sun.
• HD 165185, yellow dwarf of magnitude 5.95.
• HD 169830, metal-rich yellow dwarf that houses two extra-solar planets.
• HD 170657, orange dwarf of magnitude 6.82.
• HD 172051, yellow dwarf analogous to the Sun; it is a priority goal in the search for earthly planets that can shelter life.
• HD 179949, yellow star somewhat greater than the Sun with an extrasolate planet.
• Ross 154 (V1216 Sagittarii), small red dwarf distant 9.68 light years from the solar system.
• Gliese 783 (J. Herschel 5173) and Gliese 784; star systems at 19.7 and 20.2 light years respectively.
• OGLE BW3 V38, binary whose orbit is one of the smallest known.
• SWEEPS J175902.00−291323.7, distant red dwarf with a planet whose orbital period is only 10 hours.
• Sakurai object (V4334 Sagittarii), central star of a faint planetary nebula.
• LBV 1806-20, although visually only 35 magnitude, is considered the brightest star of the Milky Way.
• WR 102ka or Star of the Nebula Peonía, one of the most luminous stars in our galaxy; it is practically invisible to being surrounded by dust.
• S2, the name of the star closest to the supermassive black hole in the galactic center.

Deep Sky Objects

M55 globular cluster, 17 600 light years from the solar system.

Sagittarius Triphyte Nebula.

Viewed from Earth, the Milky Way presents its densest part through Sagittarius, which is where the center of the galaxy is. That is why there are many clusters and nebulae.

• M18 open cluster, poor and scattered. He's an estimated 32 million years old.
• The large volume of Sagittarius (M22), of magnitude 5,1, an easy to see object with prismatics. 10 000 light years is one of the closest.
• M23, open cluster near the solar system; it is 2 150 light years.
• M55 globular cluster, formed by about 100,000 stars.
• The Star Cloud of Sagittarius (M24).
• Nebula de la Laguna (M8), surrounding the cluster of stars NGC 6530. In a dark night you can see the north of the most fertile part of the Milky Way.
• Triff Nebula (M20), with the three dark clouds that divide it.
• Omega Nebula (M17), also called the Swan or the Shoemaker, can be clearly seen with exclusive help of binoculars.
• Red Spider Nebula (NGC 6537), a planetary nebula that houses one of the hottest stars known.
• NGC 6818 and NGC 6578, planetary nebulae; the first is about 6,000 light years away.
• Remains of supernova G1.9+0.3, SNR G000.9+00.1, G8.7-0.1, SNR G007.7-03.7, SNR G011.2−00.3, SNR G359.1-00.5 and W28; the first is the remnant of the last known supernova of our galaxy, and SNR G007-03.7 may be the rest of SN 386.
• Glybular cluster M69 and nearby M70, both located near the galactic center.
• M75 globular cluster, of magnitude 9,18 that is 67 500 light years away.
• M54 globular cluster, which is not part of the Milky Way and was the first extragalactic cluster discovered. It's one of the most dense globular clusters.
• Less prominent, but also remarkable, globular clusters NGC 6624, NGC 6638, NGC 6642, NGC 6652 and NGC 6723.
• The Sagittarius A radius, where Sagittarius A is located, which is associated with the supermassive black hole in the center of the galaxy.
• Elliptical Enana Galaxy of Sagittarius (SagDEG), small satellite galaxy of the Milky Way that forms part of the Local Group.
• Barnard Galaxy (NGC 6822), prototype of primitive galaxy. Distant 1.6 million light years, it has a large number of HII regions and emission nebulae formed mainly by ionized hydrogen, as well as a blue arm of young stars extending to the upper right area.
• Irregular Enana Galaxy of Sagittarius (SagDIG), a distant 3.4 million light years of the Milky Way.
• Cumulos Quintuple and Arches, the most dense and massive clusters of the Milky Way, which contain the most massive and luminous stars known.

Mythology

Sagittarius, Sagittarius or Archer.

There has been much controversy among ancient mythologists as to whether or not this constellation represented a Centaur. Eratosthenes and Hyginus were inclined towards the opinion that he was not a centaur, stating that no centaur had ever used a bow. They defended that it was a satyr named Croto, who lived in the company of the Muses on Mount Helicon. Croto invented the art of shooting arrows. He has traditionally been associated with the figure of Chiron.

Signal from outside

Sagittarius appears to be the only source of a possible extraterrestrial signal previously detected. The incident occurred on August 15, 1977 when radio telescopes at Ohio State University received a signal 30 times the normally detected background radiation. Professor Jerry R. Ehman, amazed at such a sign, wrote WOW! next to the paper since it was the first message captured from space by intelligent life. Although it was only the only message sent from space without us, the word, now known as the Wow! Signal, has never been used again.