Sagitta

History

The ancient Greeks called Sagitta Oistos 'the arrow', and it was one of the 48 constellations described by Ptolemy. Sagitta is located beyond the north border of Aquila, the Eagle. An amateur naturalist, polymath Richard Hinckley Allen proposed that the constellation could represent the arrow shot by Hercules towards the adjacent Stymphalian birds (which feature in Hercules' sixth labour) who had claws, beaks, and wings of iron, and who lived on human flesh in the marshes of Arcadia—denoted in the sky by the constellations Aquila the Eagle, Cygnus 'the Swan', and Lyra 'the Vulture'—and still lying between them, whence the title Herculea. Greek scholar Eratosthenes claimed it as the arrow with which Apollo exterminated the Cyclopes. The Romans named it Sagitta. In Arabic, it became al-sahm 'arrow', though this name became Sham and was transferred to Alpha Sagittae only. The Greek name has also been mistranslated as hó istos 'the loom' and thus in Arabic al-nawl. It was also called al-'anaza 'pike/javelin'.

Characteristics

The four brightest stars make up an arrow-shaped asterism located due north of the bright star Altair. Covering 79.9 square degrees and hence 0.194% of the sky, Sagitta ranks 86th of the 88 modern constellations by area. Only Equuleus and Crux are smaller. Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 69°S.{{efn|1=While parts of the constellation technically rise above the horizon to observers between the 69°S and 73°S, stars within a few degrees of the horizon are to all intents and purposes unobservable. The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of twelve segments (illustrated in infobox). In the equatorial coordinate system, the right ascension coordinates of these borders lie between and , while the declination coordinates are between 16.08° and 21.64°.

Notable features

Stars

Celestial cartographer Johann Bayer gave Bayer designations to eight stars, labelling them Alpha to Theta. English astronomer John Flamsteed added the letters x, mistaken as Chi (χ), y and z to 13, 14, and 15 Sagittae in his Catalogus Britannicus. All three were dropped by later astronomers John Bevis and Francis Baily.

Bright stars

Ptolemy saw the constellation's brightest star Gamma Sagittae as marking the arrow's head, while Bayer saw Gamma, Eta, and Theta as depicting the arrow's shaft. and magnitude 3.47. It lies at a distance of from Earth. With around 90% of the Sun's mass,{{cite journal

Delta Sagittae is the second-brightest star in the constellation and is a binary. Delta and Zeta depicted the spike according to Bayer. The Delta Sagittae system is composed of a red supergiant of spectral type M2 II Zeta Sagittae is a triple star system,{{citation | doi-access=free | arxiv=0806.2878 | s2cid=14878976 | postscript=. }} approximately from Earth. The primary and secondary are A-type stars.{{citation

In his Uranometria, Bayer depicted Alpha, Beta, and Epsilon Sagittae as the fins of the arrow. Also known as Sham, Alpha is a yellow bright giant star of spectral class G1 II with an apparent magnitude of 4.38, which lies at a distance of from Earth. Four times as massive as the Sun, it has swollen and brightened to 21 times the Sun's radius and 340 times its luminosity. Also of magnitude 4.38, Beta is a G-type giant located distant from Earth. Estimated to be around 129 million years old, it is 4.33 times as massive as the Sun, and has expanded to roughly 27 times its radius. Epsilon Sagittae is a double star whose component stars can be seen in a small telescope. With an apparent magnitude of 5.77, that has swollen to its radius. It is distant. The visual companion of magnitude 8.35 is 87.4 arcseconds distant, but is an unrelated blue supergiant around distant from Earth.

Eta Sagittae is an orange giant of spectral class K2 III{{citation

Variable stars

Variable stars are popular targets for amateur astronomers, their observations providing valuable contributions to understanding star behaviour. R Sagittae is a member of the rare RV Tauri variable class of star. It ranges in magnitude from 8.2 to 10.4. It is around distant. It has a radius times that of the Sun, and is as luminous, yet most likely is less massive than the Sun. An aging star, it has moved on from the asymptotic giant branch of stellar evolution and is on its way to becoming a planetary nebula. FG Sagittae is a "born again" star, a highly luminous star around distant from Earth. It reignited fusion of a helium shell shortly before becoming a white dwarf, and has expanded first to a blue supergiant and then to a K-class supergiant in less than 100 years. It is surrounded by a faint (visual magnitude 23) planetary nebula, Henize 1–5, that formed when FG Sagittae first left the asymptotic giant branch.

S Sagittae is a classical Cepheid that varies from magnitude 5.24 to 6.04 every 8.38 days. It is a yellow-white supergiant that pulsates between spectral types F6 Ib and G5 Ib. Around 6 or 7 times as massive and 3,500 times as luminous as the Sun, it is located around from Earth. HD 183143 is a remote highly luminous star around away, that has been classified as a blue hypergiant. Infrared bands of ionised buckminsterfullerene molecules have also been found in its spectrum. WR 124 is a Wolf–Rayet star moving at great speed surrounded by a nebula of ejected gas.

U Sagittae is an eclipsing binary that varies between magnitudes 6.6 and 9.2 over 3.4 days, making it a suitable target for enthusiasts with small telescopes. The system is distant. Near U Sagittae is X Sagittae, a semiregular variable ranging between magnitudes 7.9 and 8.4 over 196 days.

Located near 18 Sagittae is V Sagittae, the prototype of the V Sagittae variables, cataclysmic variables that are also super soft X-ray sources. It is expected to become a luminous red nova when the two stars merge around the year 2083, and briefly become the most luminous star in the Milky Way and one of the brightest stars in Earth's sky. WZ Sagittae is another cataclysmic variable, composed of a white dwarf that has about 85% the mass of the Sun, and low-mass star companion that has been calculated to be a brown dwarf of spectral class L2 that is only 8% as massive as the Sun. Normally a faint object dimmer than magnitude 15, it flared up in 1913, 1946 and 1978 to be visible in binoculars. It is a massive neutron star that is ablating its brown dwarf-sized companion which causes the pulsar's radio signals to attenuate as they pass through the outflowing material.

Stars with exoplanets

HD 231701 is a yellow-white main sequence star hotter and larger than the Sun, with a Jupiter-like planet that was discovered in 2007 by the radial velocity technique. The planet orbits at a distance of from the star with a period of 141.6 days. It has a mass of at least 1.13 Jupiter masses.{{cite journal

HAT-P-34 is a star times as massive as the Sun with times its radius and times its luminosity. With an apparent magnitude of 10.4, it is distant. A planet times as massive as Jupiter was discovered transiting it in 2012. With a period of 5.45 days and a distance of from its star, it has an estimated surface temperature of .

15 Sagittae is a solar analog—a star similar to the Sun, with times its mass, times its radius and times its luminosity. It has an apparent magnitude of 5.80.{{citation

Deep-sky objects

The band of the Milky Way and the Great Rift within it pass through Sagitta, with Alpha, Beta and Epsilon Sagittae marking the Rift's border. Located between Beta and Gamma Sagittae is Messier 71, it was first discovered by the French astronomer Philippe Loys de Chéseaux in the year 1745 or 1746. The loose globular cluster has a mass of around and a luminosity of approximately 19,000 .

There are two notable planetary nebulae in Sagitta: NGC 6886 is composed of a hot central post-AGB star that has 55% of the Sun's mass yet times its luminosity, with a surface temperature of , and surrounding nebula estimated to have been expanding for between 1,280 and 1,600 years, The nebula was discovered by Ralph Copeland in 1884. The Necklace Nebula—originally a close binary, one component of which swallowed the other as it expanded to become a giant star. The smaller star remained in orbit inside the larger, whose rotation speed increased greatly, resulting in it flinging its outer layers off into space, forming a ring with knots of bright gas formed from clumps of stellar material. It was discovered in 2005 and is around 2 light-years wide. It has a size of .{{cite journal | display-authors=1

Notes

References

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