Sagittarius constellation lies in the southern sky. It is one of the constellations of the zodiac. The constellation represents the archer in Greek mythology and is usually depicted as a centaur holding a bow and arrow.
Sagittarius’ location is easy to find because the constellation lies on the Milky Way. Its symbol is ♐. Like other zodiac constellations, Sagittarius was originally catalogued by the Greek astronomer Ptolemy in the 2nd century.
The constellation contains the luminous Pistol Star, the galactic centre, the radio source Sagittarius A, and a number of very famous deep sky objects, including the Sagittarius Dwarf Elliptical Galaxy, the Sagittarius Dwarf Irregular Galaxy, Barnard’s Galaxy, the Bubble Nebula, and as many as 15 Messier objects, among them the Sagittarius Star Cloud (Messier 24), the Omega Nebula (Messier 17), Messier 18, the Lagoon Nebula (Messier 8), and the Trifid Nebula (Messier 20).
FACTS, LOCATION & MAP
Sagittarius is the 15th largest constellation in the sky. It occupies an area of 867 square degrees.
It is located in the fourth quadrant of the southern hemisphere (SQ4) and can be seen at latitudes between +55° and -90°.
Sagittarius contains 15 Messier objects: Messier 8 (M8, NGC 6523, Lagoon Nebula), Messier 17 (M17, NGC 6618 Omega, Swan, Horseshoe or Lobster Nebula), Messier 18 (M18, NGC 6613), Messier 20 (M20, NGC 6514, Trifid Nebula), Messier 21 (M21, NGC 6531), Messier 22 (M22, NGC 6656, Sagittarius Cluster), Messier 23 (M23, NGC 6494), Messier 24 (M24, NGC 6603, Sagittarius Star Cloud), Messier 25 (M25, IC 4725), Messier 28 (M28, NGC 6626), Messier 54 (M54, NGC 6715), Messier 55 (M55, NGC 6809), Messier 69 (M69, NGC 6637), Messier 70 (M70, NGC 6681) and Messier 75 (M75, NGC 6864). The constellation also has 22 stars with confirmed planets.
The brightest star in the constellation is Kaus Australis, Epsilon Sagittarii, with an apparent visual magnitude of 1.79.
There are no meteor showers associated with the constellation.
In Greek mythology, Sagittarius represents a centaur, a half human, half horse creature with the torso of a man and the body and four legs of a horse. The centaur is depicted as aiming an arrow toward the heart of the neighbouring constellation Scorpio, represented by the red supergiant star Antares. Sometimes Sagittarius is wrongly identified as the centaur Chiron, represented by the constellation Centaurus.
Sagittarius constellation has its roots in Sumerian mythology. Eratosthenes associated it with Crotus, a mythical creature with two feet and a satyr’s tail, who was the nurse to the nine Muses, daughters of Zeus. He argued that the constellation really represented a satyr and not a centaur. According to the Roman author Hyginus, Crotus was the son of Pan and the archer the constellation was named after. Crotus invented archery and lived on Mount Helicon. Because he was close to the Muses, they were the ones who asked Zeus to place him in the sky.
In Babylonian mythology, Sagittarius is associated with the centaur-like god Nergal, and depicted with two heads – one human and one panther – and also wings, and the stinger of a scorpion positioned above a horse’s tail.
MAJOR STARS IN SAGITTARIUS
Kaus Australis – ε Sagittarii (Epsilon Sagittarii)
Epsilon Sagittarii is a binary star approximately 143 light years distant. It is a blue class B giant with an apparent magnitude of 1.79 and a luminosity 375 times that of the Sun. Kaus Australis is the brightest star in the constellation Sagittarius and the 36th brightest star in the sky. It has a faint 14th magnitude companion 32 arcseconds away.
The star’s traditional name, Kaus Australis, comes from the Arabic word for “bow” (qaws) and the Latin word for “southern” (australis). The star marks the base of the archer’s bow. Together with the stars Delta (Kaus Media) and Lambda Sagittarii (Kaus Borealis), Epsilon Sagittarii represents the archer’s bow.
Nunki – σ Sagittarii (Sigma Sagittarii)
Sigma Sagittarii is the second brightest star in the constellation Sagittarius. It is a hydrogen fusing dwarf star that belongs to the spectral type B2.5 V. It has an apparent visual magnitude of 2.1. The star has a luminosity 3,300 times that of the Sun and about seven solar masses. It is a very fast rotator, spinning with a speed of more than 200 kilometres per second, which is about 100 times faster than the Sun. The star lies approximately 228 light years from Earth.
Sigma Sagittarii is sometimes also known as Nunki. This is the star’s modern name, which is either Babylonian or Assyrian in origin. The significance of the name is unknown, except that it is a proper name. It was recovered by archaeologists and made public by Richard Hinckley Allen in his book Star names, their lore and meaning.
Nunki has a faint, magnitude 9.5 companion about 5.2 arcminutes away. Because Nunki is located so close to the eclipic, it can sometimes be occulted by the Moon and, however rarely, by planets. It was last occulted by a planet on November 17, 1981, when Venus passed in front of it.
Sigma Sagittarii is also notable for being the brightest star that can be occulted by an exterior planet. This, however, only applies to Mars and it happens extremely rarely. The last time it did was on September 3, 423.
Kaus Media – δ Sagittarii (Delta Sagittarii)
Delta Sagittarii is a multiple star system approximately 306 light years distant in the constellation Sagittarius. It has an apparent magnitude of 2.72 and belongs to the spectral type K3III. Delta Sagittarii has a radius 62 times solar, a mass about five times solar, and is 1180 times more luminous than the Sun. The star’s traditional name, Kaus Media, means “the middle bow.” The primary star in the Delta Sagittarii system has three dim companions: Delta Sagittarii B, a 14th magnitude star 26 arcseconds away, Delta Sagittarii C, a 15th magnitude star 40 arcseconds away, and Delta Sagittarii D, a 13th magnitude star 58 arcseconds away.
The Delta Sagittarii system is known in fiction from William R. Forstchen’s 1969 novel Into the Sea of Stars. It is the destination of a crew of women who travel onboard the Colonial Unit 122 to the Delta Sagittarii star system carrying a supply of sperm purged of the Y chromosome.
Kaus Borealis – λ Sagittarii (Lambda Sagittarii)
Lambda Sagittarii is an orange giant star, belonging to the spectral class K1+IIIb, with an apparent magnitude of 2.82. It is approximately 77.3 light years distant. It has a radius 11 times solar and is 52 times more luminous than the Sun. Lambda Sagittarii is what astronomers sometimes refer to as a clump star: one undergoing the final stages of its existence, but stable nevertheless and fusing helium into carbon and oxygen in its core.
The star’s traditional name, Kaus Borealis, means “the northern bow.” It marks the top of the centaur’s bow. Since it lies very close to the ecliptic, Lambda Sagittarii is occasionally occulted by the Moon and, more rarely, by planets. The last time this happened was on November 19, 1984, when Venus passed in front of the star, eclipsing it. Before that, Mercury occulted the star on December 5, 1865.
Rukbat – α Sagittarii (Alpha Sagittarii)
Alpha Sagittarii is a blue dwarf belonging to the spectral class B8V. It has an apparent visual magnitude of 3.97 and is approximately 170 light years distant. The star is believed to have a debris disk, like Vega, the brightest star in the constellation Lyra. What is atypical for a B8V class star is that Alpha Sagittarii is emitting an excess flux of X-rays, possibly because its companion star is still in the pre-main sequence stage.
Alpha Sagittarii shares its traditional name, Rukbat, with the star Delta Cassiopeiae. The name is derived from the Arabic word rukbah, which means “knee.”
In fiction, Rukbat is probably best known from Anne McCaffrey’s Dragonriders of Pern series of novels and stories. Pern, the planet on which the action takes place, orbits Rukbat, which is described as a class G yellow star in the novels.
Arkab – β Sagittarii (Beta Sagittarii)
Beta Sagittarii is a designation shared by two star systems, Beta-1 Sagittarii and Beta-2 Sagittarii, which lie 0.36° apart in the sky. The system is also known by its traditional name, Arkab, from the Arabic carqūb, which means “hamstring.”
Beta-1 Sagittarii, or Arkab Prior (prior because it leads Beta-2 across the sky), is a double star approximately 378 light years distant. It belongs to the spectral type B9V. The primary component, Arkab Prior A, is a B9 type main sequence dwarf with an apparent magnitude of 3.96, while the companion star, Arkab Prior B, is a class A3 dwarf with an apparent magnitude of 7.4. The stars are 28 arcseconds apart.
Beta-2 Sagittarii, or Arkab Posterior (because it trails after Beta-1), is a giant star belonging to the spectral type F2III. It has an apparent visual magnitude of 4.27 and is approximately 137 light years distant.
Ascella – ζ Sagittarii (Zeta Sagittarii)
Zeta Sagittarii is another binary star in Sagittarius. It is the third brightest star in the constellation, after Epsilon and Sigma Sagittarii. Its traditional name, Ascella, means “armpit” in Latin. The star is approximately 89.1 light years distant from Earth.
Zeta Sagittarii consists of a class A2 giant star with an apparent visual magnitude of 3.26 and an A4 type subgiant star with an apparent magnitude of 3.37. The binary system has a combined apparent visual magnitude of 2.60. The two stars are separated by 13.4 astronomical units (AU). Ascella also has a dim companion 75 arcseconds away. It is a 10th magnitude star.
φ Sagittarii (Phi Sagittarii)
Phi Sagittarii is a B8 class giant approximately 231 light years from Earth. It has an apparent magnitude of 3.17. It is one of the stars that form the Teapot asterism; it marks the junction of the handle and the lid of the Teapot.
Albaldah – π Sagittarii (Pi Sagittarii)
Pi Sagittarii is also sometimes known as Albaldah, from the Arabic bálda, which means “the town.” Egyptian astronomer Al Achsasi al Mouakket designated it Nir al Beldat in his star catalogue Calendarium in the mid-17th century. Nir al Beldat was later translated into Latin as Lucida Oppidi, which means “the brightest of the town.”
Pi Sagittarii is a triple star system approximately 440 light years distant, with an apparent magnitude of 2.88. It belongs to the spectral class F2II. The primary star has two companions; Pi Sagittarii B 0.1 arcseconds away and Pi Sagittarii C 0.4 arcseconds away.
Like several other stars in Sagittarius, Albaldah is near the ecliptic and can occasionally be occulted by the Moon and planets. The next occultation by a planet (Venus) will take place in our lifetime: on February 17, 2035.
Alnasl (Nushaba) – γ Sagittarii (Gamma Sagittarii)
Gamma Sagittarii is a K type (K0III) giant star with an apparent visual magnitude of 2.98. It is approximately 96.1 light years distant. The Gamma designation used to be shared by two star systems in Sagittarius, Gamma-1 and Gamma-2, which were separated by 0.86° in the sky.
Gamma Sagittarii is also known by its traditional names, Alnasl and Nushaba. Alnasl comes from the Arabic al-naşl which means “arrowhead,” and Nushaba is derived from Zujj al-Nashshaba which means the same thing.
τ Sagittarii (Tau Sagittarii)
Tau Sagittarii is an orange giant star belonging to the spectral type K1 or K2, approximately 120 light years distant. It has an apparent magnitude of 3.32. It is one of the stars that mark the handle of the Teapot asterism. It is located between Zeta and Sigma Sagittarii. It is a suspected binary star, even though a companion has never been confirmed.
Tau Sagittarii is well known to astronomers as the closest visible star to the origin of the Wow! signal, the first and only radio signal ever received that indicated the possibility of extraterrestial intelligence. The signal was received by SETI researcher Dr. Jerry R. Ehman on August 15, 1977 at the Ohio State University. It lasted for full 72 seconds, but has never been detected since. When Ehman discovered it, he printed his findings out, circled the signal and wrote “Wow!” next to it, which is how the signal got its name.
Sephdar (Ira Furoris) – η Sagittarii (Eta Sagittarii)
Eta Sagittarii is another multiple star system in Sagittarius. It is approximately 149 light years distant. The origin of Eta Sagittarii’s proper name Sephdar is unknown. Before being assigned to Sagittarius, the star system was known as Beta Telescopii. Around the year 6300, the system will have moved from Sagittarius to the Corona Australis constellation.
The primary component in the Eta Sagittarii system is a type M3.5 red giant classified as an irregular variable star: The star’s apparent magnitude varies from 3.08 to 3.12.
The brightest companion star is an F class dwarf with an apparent visual magnitude of 7.77. It lies 3.6 arcseconds from the primary star.
Another companion star is only 13th magnitude and lies 33 arcseconds away from the primary, while the faintest star in the system is 10th magnitude and lies 93 arcseconds away from the brightest star.
The Pistol Star is one of the most luminous stars known. It is a blue variable star that is about four million times as luminous as our Sun and 120-200 times as massive. It has about a third of the luminosity of the Eta Carinae binary system in the constellation Carina and is believed to radiate as much energy in 20 seconds as the Sun does in a year.
Located in the galactic center region, the Pistol Star lies approximately 25,000 light years from our solar system. Without the interstellar dust in the way, despite its distance it would be a fourth magnitude star, visible to the naked eye.
The star was named after the shape of the nebula it illuminates, the Pistol Nebula.
KW Sagittarii is one of the largest known stars. It is a red supergiant located about 10,000 light years from Earth, with an apparent visual magnitude varying between 8.5 and 11.The star’s diameter is 1,460 times the diameter of the Sun.
Polis – μ Sagittarii (Mu Sagittarii)
Mu Sagittarii is a multiple star system in Sagittarius, with individual components designated Polis A through Polis E. Its traditional name, Polis, comes from the Coptic Egyptian word for “foal.”
Polis is about 3,912 light years distant from Earth. The primary component in the star system is a class B giant 23 times as massive as the Sun, and 180,000 times more luminous. It is an eclipsing binary star, with a B8 supergiant star for a primary component and a B2 type giant as the companion star. The apparent magnitude of Polis A ranges from 3.84 to 3.96.
ρ Sagittarii (Rho Sagittarii)
Rho Sagittarii is a binary star with a subgiant belonging to the spectral class F0 as the primary component and a K0 type giant as the companion, separated by 0.46° from the primary. The subgiant has an apparent visual magnitude of 3.93 and is approximately 122 light years distant from the solar system. The companion star has an apparent magnitude of 5.84 and is located about 359 light years from Earth.
υ Sagittarii (Upsilon Sagittarii)
Upsilon Sagittarii is a spectroscopic binary star. It is one of only four star systems known to be hydrogen-deficient, which makes it difficult to classify U Sgr. Most likely, the primary component is an A type supergiant. It is classified as an irregular variable star, with an apparent magnitude varying from 4.51 to 4.65 with a period of about 20 days.
The companion star is more massive than the primary, but so faint that optical telescopes cannot detect it. The star is probably a B or O type main sequence dwarf that has accreted much of the primary star’s mass. The Upsilon Sagittarii system is about 1,672 light years distant and has an orbital period of 137.939 days.
Ross 154 (V1216 Sagittarii)
Ross 154 is a red dwarf only 9.68 light years away from the solar system. It is one of the closest stars to the Sun and the nearest star in Sagittarius constellation. It is located only 5.41 light years from Barnard’s Star in the neighbouring constellation Ophiuchus. The star is a relatively young one, with an estimated age of less than a billion years. It is a known X-ray source. It will make its closest approach to the Sun in about 150,000 years, when it comes within 6.13 light years of our solar system.
Ross 154 was originally catalogued by the American astronomer and physicist Frank Elmore Ross in 1925. It is classified as a UV Ceti-type flare star, one undergoing sudden dramatic increases in brightness for a few minutes. (Flare stars are typically dim red dwarfs or less massive brown dwarfs.)
Some of the brightest stars in Sagittarius – Delta, Epsilon, Gamma-2, Lambda, Zeta, Phi, Tau and Sigma Sagittarii – form an asterism known as the Teapot. Sigma and Tau Sagittarii mark the handle, Delta, Epsilon, Zeta and Phi Sagittarii form the body of the Teapot, Lambda Sagittarii marks the point of the lid and Gamma-2 Sagittarii marks the tip of the spout.
The Terebellum is a quadrilateral formed by four fourth magnitude stars in Sagittarius, all within two degrees of each other: Omega Sagittarii, 59 Sagittarii, 60 Sagittarii and 62 Sagittarii.
Omega Sagittarii is a G-type subgiant marking the northeast corner of the Terebellum, about 78 light years distant from Earth. 59 Sagittarii (sometimes also known as b Sagittarii) is a K-type bright giant at the southeast corner, about 1,200 light years distant. 60 Sagittarii (or A Sagittarii) is a G-type giant at the northwest corner of the asterism, approximately 340 light years away. 62 Sagittarii (or c Sagittarii) is an M-type giant at the southwest corner, about 450 light years from Earth. The star is classified as an irregular variable.
The stars are located at different distances from the solar system and are not gravitationally bound.
DEEP SKY OBJECTS IN SAGITTARIUS
Sagittarius A is a radio source located at the centre of the Milky Way, in the direction of the Sagittarius constellation. It is obscured from view by large cosmic dust clouds in the galaxy’s spiral arms.
Sagittarius A consists of the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a bright radio source located at the centre of the spiral, Sagittarius A*.
The supernova remnant Sagittarius A East is about 25 light years wide and appears to have originated in an explosion that occurred between 35,000 and 100,000 years ago.
Because it has an imposing size and energy, Sagittarius A East is believed to be a remnant of the explosion of a star that came close to the central black hole and was gravitationally compressed.
The spiral structure Sagittarius A West appears like a three-arm spiral and is sometimes called the Minispiral. It does not really have the structure of a spiral. It is made of clouds of dust and gas that circle Sagittarius A* and fall onto it at extremely high velocities, up to 1000 kilometres per second. The clouds have an ionized surface.
Sagittarius A* is the leading candidate for the location of the supermassive black hole believed to be at the centre of the Milky Way Galaxy. Stars orbit the object at speeds greater than those of any other stars found in the Milky Way.
Sagittarius B2 is a very large molecular cloud of dust and gas approximately 390 light years from the centre of the Milky Way. Spanning an area of 150 light years across, Sagittarius B is one of the largest molecular clouds in our galaxy and the single largest one in the vicinity of the galaxy’s core. It has a mass three million times that of the Sun.
Lagoon Nebula – Messier 8 (M8, NGC 6523)
The Lagoon Nebula is a large interstellar cloud classified as an emission nebula.
It is one of the several notable H II regions in Sagittarius. It has an apparent magnitude of 6.0 and is approximately 4,100 light years distant.
Messier 8 is one of only two nebulae that are star forming regions and can be seen by naked eye.
It was first discovered by the French astronomer Guillaume Le Gentil in 1747.
The Hourglass Nebula contains several Herbig-Haro objects, small patches of nebulosity indicating that there are newly born stars in the vicinity, and providing direct evidence of star forming activity in the region.
Omega Nebula – Messier 17 (M17, NGC 6618, Swan, Horseshoe or Lobster Nebula)
The Omega Nebula is an emission nebula in Sagittarius.
It goes by many different names: Omega Nebula, Horseshoe Nebula, Lobster Nebula, Swan Nebula, Checkmark Nebula, and Sharpless 45.
The nebula is an H II region originally discovered by the Swiss astronomer Jean-Philippe Loys De Chéseaux in 1745.
Messier included it in his catalogue in 1764.
The Omega Nebula has an apparent magnitude of 6.0 and is between 5,000 and 6,000 light years distant.
It is about 15 light years in diameter.
The nebula contains an open cluster of 35 hot young stars which illuminate its gases.
Messier 18 (M18, NGC 6613)
Messier 18 is an open star cluster in Sagittarius.
It was originally discovered by Charles Messier in 1764.
The cluster is nine light years in radius, has an apparent visual magnitude of 7.5, and is approximately 4,900 light years distant.
It can be found between the Sagittarius Star Cloud (Messier 24) and the Omega Nebula (Messier 17).
The cluster is believed to be about 32 million years old.
Trifid Nebula – Messier 20 (M20, NGC 6514)
The Trifid Nebula is a bright, colourful emission/reflection nebula in Sagittarius.
It is in fact a combination of an emission nebula (the lower part), a reflection nebula (the upper part) and an open cluster.
It is an H II region, containing a stellar nursely full of embryonic stars.
It is about 28 arcminutes across in size.
The nebula’s name means ‘divided into three lobes’.
It can be observed through a small telescope and is a popular deep sky object among amateur astronomers.
The Trifid Nebula has an apparent visual magnitude of 6.3 and is approximately 5,200 light years distant.
Messier 21 (M21, NGC 6531)
Messier 21 is another open cluster in Sagittarius.
It is a relatively young cluster, only 4.6 million years old, and it contains roughly 57 stars.
The cluster was first observed by Charles Messier in June 1764. He later included it in his catalogue.
Messier 21 has an apparent magnitude of 6.5 and is approximately 4,250 light years distant.
Sagittarius Cluster – Messier 22 (M22, NGC 6656)
The Sagittarius Cluster is one of the brightest globular star clusters in the sky.
It is elliptical in shape and about 32 arcminutes across in size. It has an apparent visual magnitude of 5.1 and is approximately 10,600 light years distant. It is one of the nearest globular star clusters to Earth.
Messier 22 was originally discovered by the German amateur astronomer Johann Abraham Ihle in 1665. Messier included the cluster in his catalogue in June 1764.
The Sagittarius Cluster is located near the galactic bulge, the central group of stars in the Milky Way.
It is also notable for being one of only four known globular clusters that contain a planetary nebula. (The other three are Messier 15 in the constellation Pegasus, Palomar 6 in Ophiuchus and NGC 6441 in Scorpius.)
The planetary nebula in the Sagittarius Cluster, designated GJJC1 is believed to be only 6,000 years old and has a blue star at the centre.
Messier 23 (M23, NGC 6494)
Messier 23 is an open star cluster, discovered by Charles Messier in June 1764.
The cluster has an apparent visual magnitude of 6.9 and is about 2,150 light years distant. It is 15-20 light years in radius and contains about 150 stars. The brightest one has a magnitude of 9.2.
The cluster is believed to be about 220 million years old.
Sagittarius Star Cloud (Delle Caustiche) – Messier 24 (M24, NGC 6603, IC 4715)
The Sagittarius Star Cloud (Messier 24) is, as the name says, a star cloud in the constellation Sagittarius.
It is the most dense concentration of stars that can be seen using binoculars; about a thousand stars are visible within a single field of view.
The cloud is about 600 light years wide and approximately 10,000 light years distant. It was discovered by Charles Messier in 1764. He described it as a ‘large nebulosity containing many stars.’
The stars and clusters in the Sagittarius Star Cloud are part of the Sagittarius or Sagittarius-Carina spiral arms of the Milky Way.
The Carina-Sagittarius Arm is particularly notable for containing a large number of H II regions, giant molecular clouds and young stars.
Messier 25 (M25, IC 4725)
Messier 25 is an open cluster with an apparent magnitude of 4.6, about 2,000 light years from Earth. The cluster is about 19 light years across. Its estimated age is 90 million years. It was discovered by Philippe Loys De Chéseaux in 1745 and added to Messier’s catalogue in 1764.
Messier 28 (M28, NGC 6626)
Messier 28 is a globular cluster located near Lambda Sagittarii (Kaus Borealis). It is between 18,000 and 19,000 light years distant and has an apparent magnitude of 7.66.
The cluster contains 18 RR Lyrae type variable stars. These are pulsating stars belonging to the spectral class A (and occasionally F), usually with a mass of about half the Sun’s, used as standard candles to measure galactic distances.
Messier 28 was the first globular star cluster in which a milisecond pulsar (a pulsar with a rotational period between 1 and 10 miliseconds) was discovered in 1986.
Messier 54 (M54, NGC 6715)
Messier 54 is a dense globular cluster with an apparent magnitude of 8.37, approximately 87,400 light years distant and about 150 light years across.
The cluster was discovered by Charles Messier in 1778, who later included it in his catalogue.
Messier 54 is believed to belong to the Sagittarius Dwarf Elliptical Galaxy. It lies close to the star Zeta Sagittarii.
Messier 55 (M55, NGC 6809)
Messier 55 is another globular cluster in Sagittarius.
It was discovered by the French astronomer Nicolas Louis de Lacaille in 1751 and included by Messier in his catalogue in 1778.
The cluster is a relatively large and bright one, with an apparent visual magnitude of 7.42.
It is located approximately 17,300 light years from Earth.
Messier 69 (M69, NGC 6637)
Messier 69 is a globular cluster.
It was discovered by Charles Messier on August 31, 1780, along with another globular cluster, Messier 70.
Messier 69 has a radius of 42 light years and an apparent magnitude of 8.31. It contains very few variable stars.
M69 is only 1,800 light years away from M70 and located near the galactic centre. It is approximately 29,700 light years distant from Earth.
Messier 70 (M70, NGC 6681)
Messier 70 is a globular cluster in Sagittarius, located close to the galactic centre.
Charles Messier discovered it in 1780 and subsequently included it in his catalogue.
The cluster is about 34 light years in radius, has an apparent visual magnitude of 9.06 and is approximately 29,300 light years distant.
Messier 75 (M75, NGC 6864)
Messier 75 is a globular star cluster about 67,500 light years from Earth. It was discovered by the French astronomer Pierre Méchain in 1780.
The cluster is some 67 light years in radius and has an apparent magnitude of 9.18. It is a densely populated cluster, classified as class I.
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy (Sag DEG) is an elliptical galaxy shaped like a loop. Sometimes it is also called the Sagittarius I Dwarf or Sagittarius Dwarf Spheroidal.
It is a satellite galaxy of the Milky Way and headed for a collision with our galaxy. Sag DEG’s ellipse is already extended around our galaxy, and the main cluster will pass through the galactic disc of the Milky Way in the next million years.
The Sagittarius Dwarf is believed to already have orbited the Milky Way about 10 times in the last billion years or so, and it still appears to have coherence as an elongated ellipse despite being torn apart by enormous tidal forces as a result of the interaction.
The Sagittarius Dwarf Elliptical Galaxy has an apparent visual magnitude of 4.5 and is approximately 65,000 light years distant. It is located about 50,000 light years from the centre of the Milky Way.
The galaxy is about 10,000 light years in diameter and it consists of four globular star clusters, the main one of which was discovered in 1994. At the time, Sag DEG was the nearest known neighbouring galaxy of the Milky Way. (In 2003, the Canis Major Dwarf Galaxy was discovered and recognized as the nearest neighbour.)
The Sagittarius Dwarf Galaxy is an old galaxy, composed mainly of Population II stars (old, metal-poor stars). The globular cluster Messier 54 lies at the core of the galaxy.
NGC 6565 is a planetary nebula in Sagittarius.
Sagittarius Dwarf Irregular Galaxy
The Sagittarius Dwarf Irregular Galaxy (Sag DIG) is a dwarf galaxy located in Sagittarius, approximately 3.39 million light years from the solar system.
It has an apparent magnitude of 15.5.
The galaxy was discovered on a photographic plate taken for the ESO Atlas in June 1977.
The Sagittarius Dwarf Irregular Galaxy contains mainly intermediate-age stars as a result of a prolonged period of star formation. It is one of the most metal-poor galaxies known.
NGC 6578 is another planetary nebula.
NGC 6522 is a globular cluster, possibly the oldest one in the Milky Way Galaxy. The estimated age of the cluster is more than 12 Gigayears.
The cluster was first discovered by the German-born British astronomer William Herschel in June 1784.
It lies in an area of the sky known as Baade’s Window. The region has relatively low amounts of dust and allows a view of the galactic centre.
Barnard’s Galaxy – NGC 6822 (IC 4895, Caldwell 57)
NGC 6822 is a barred irregular galaxy that belongs to the Local Group of galaxies. It has a similar structure to the Small Magellanic Cloud, the dwarf galaxy in the constellation Tucana.
NGC 6822 has an apparent visual magnitude of 9.3 and is approximately 1.63 million light years distant.
The galaxy was named after the American astronomer E.E. Barnard, who discovered it in 1881 using a six-inch refractor telescope.
Edwin Hubble identified 15 variable stars in Barnard’s Galaxy, 11 of which were Cepheids, luminous variable stars that have a strong relationship between luminosity and pulsation period, which makes them excellent standard candles (objects with a known luminosity) for determining galactic and extragalactic distance scales.
Hubble established a distance of over 700,000 light years using the Cepheids’ period-luminosity relationship, and this was the first system located beyond the Magellanic Clouds to have its distance accurately determined.
NGC 6723 is a globular star cluster in Sagittarius.
The Bubble Nebula (Hubble 1925 I)
The Bubble Nebula is an emission nebula located in Barnard’s Galaxy. It contains areas of massive H II emission, large clouds of partly ionized gas which show evidence of recent star forming activity.
Little Gem Nebula – NGC 6818
NGC 6818 is also a planetary nebula in Sagittarius.
It was first discovered by William Herschel in 1787.
The nebula’s inner elongated shape is believed to be the result of a fast wind of material travelling away from the hot central star.