Sagitta is a dim but distinctive constellation in the northern sky. Its name is Latin for ‘arrow,’ and it should not be confused with the larger constellation Sagittarius, the archer. Although Sagitta is an ancient constellation, it has no star brighter than 3rd magnitude and has the third-smallest area of all constellations (only Equuleus and Crux are smaller). It was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations defined by the International Astronomical Union. Located to the north of the equator, Sagitta can be seen from every location on Earth except within the Antarctic circle.
Covering 79.9 square degrees and hence 0.194% of the sky, Sagitta ranks 86th of the 88 modern constellations by area. Its position in the Northern Celestial Hemisphere means that the whole constellation is visible to observers north of 69°S. It is bordered by Vulpecula to the north, Hercules to the west, Aquila to the south, and Delphinus to the east. In the equatorial coordinate system, the right ascension coordinates of the constellation lie between 18h 57.2m and 20h 20.5m, while the declination coordinates are between 16.08° and 21.64°.
Sagitta flying near the feet of Vulpecula, from the Atlas Coelestis of John Flamsteed (1729). The constellation’s brightest star, Gamma Sagittae, lies where the right forepaw of Vulpecula touches the arrow’s shaft.
[http://www.ianridpath.com/startales/sagitta.htm]
Sagitta’s shape is reminiscent of an arrow, and many cultures have interpreted it thus, among them the Persians, Hebrews, Greeks and Romans.
In ancient Greece, Sagitta was regarded as the weapon that Hercules used to kill the eagle (Aquila) of Jove that perpetually gnawed Prometheus’ liver. The Arrow is located beyond the north border of Aquila, the Eagle. According to R.H. Allen, the Arrow could be the one shot by Hercules towards the adjacent Stymphalian birds (6th labor) who had claws, beaks and wings of iron, and who lived on human flesh in the marshes of Arcadia- Aquila the Eagle, Cygnus the Swan, and Lyra (the Vulture)- and still lying between them, whence the title Herculea (although Allen cites no reference to support this assertion). Eratosthenes claimed it as the arrow with which Apollo exterminated the Cyclopes.
The Arabs called it as-Sahm, a name that was transferred Sham and now refers to α Sge only.
Johann Bayer gave designations to eight stars, labelling them Alpha to Theta. 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. He also depicted Delta and Zeta as the spike, Alpha, Beta and Epsilon as the fins of the arrow.
Constellations of Aquila and Sagitta
[http://www.davidmalin.com/fujii/source/Aql.html]
[https://bestdoubles.wordpress.com/2010/07/27/sagitta-sightings-epsilon-h-n-84-and-zeta/]
Gamma Sagittae is the brightest star in Sagitta constellation. It is an orange giant with the stellar classification K5III. It has an apparent magnitude of 3.51 and is approximately 274 light years distant from the solar system. The star is about 640 times more luminous than the Sun and has a mass 2.5 times solar.
Delta Sagittae has the stellar classification of M2II+B6. It is a close multiple star system that has a red bright giant for the primary component. The secondary component is a white or blue-white main sequence star in a 3,725 day orbit. The system is approximately 448 light years distant from the Sun and has an apparent magnitude of 3.68. It is 2,800 times more luminous than the Sun.
Alpha Sagittae is the third brightest star in Sagitta. It is a yellow bright giant with the stellar classification of G1 II. It has an apparent magnitude of 4.39 and is approximately 620 light years distant from Earth. It is 340 times more luminous than the Sun and has a mass four times solar. The star’s radius is about 20 times that of the Sun. The star’s traditional name, Sham or Alsahm, is derived from the Arabic sahm, which means ‘the arrow.’
Beta Sagittae is a yellow giant star with the stellar classification G8IIIa. It has an apparent magnitude of 4.387 and is approximately 470 light years distant from the solar system. The star has a radius 10 times that of the Sun.
[http://www.constellation-guide.com/constellation-list/sagitta-constellation/]
Hubble Space Telescope image of nebula M1-67 around Wolf- Rayet star WR 124
WR 124 is a Wolf- Rayet star in the constellation of Sagitta surrounded by a ring nebula of expelled material, known as M1-67. It is one of the fastest runaway stars in the galaxy with a radial velocity around 200 km/s. It was discovered by Paul W. Merrill in 1938, identified as a high velocity Wolf-Rayet star. It is listed in the General Catalogue of Variable Stars as variable with a range of 0.08 magnitudes.
A recent study of WR 124 directly measured the expansion rate of the M1-67 nebula expelled from the star using WFPC2 camera images taken 11 years apart, and compared that rate to the expansion velocity measured by the Doppler shift of the nebular emission lines. This yields a direct geometric measurement of the distance to WR 124, something that has only been done for one other WR star (Gamma Velorum), and which should be less subject to error than other methods of distance measurement. The distance calculated from the nebular expansion rate is 3.35kpc, and the resulting luminosity is 150,000 times that of the sun (L☉).
Assuming a distance of 3.35kpc, WR 124 is estimated to have a current mass of 9 M☉, with a probable initial mass around 25 M☉. It has blown away a large portion of its mass through the intense stellar winds of the Wolf-Rayet phase, and possibly also earlier hypergiant and luminous blue variable phases. The temperature of around 36,000K means that most of its energy is emitted at ultraviolet wavelengths, the visual absolute magnitude is -5.3, and WR 124 is ten times the radius of the sun.
WR 124 is surrounded by the intensely hot nebula M1-67 formed from the star’s extreme stellar wind. The nebula M1-67 is expanding at a rate of over 150,000 km/h (100,000 mph) and is nearly 6 light-years across, leading to the dynamical age of 20,000 years. M1-67 has little internal structure, though large clumps of material have been detected, some of which have 30 times the mass of Earth and stretch out up to 150 billion km (90 billion miles). If placed in the Solar System, one of these clumps would span the distance from the Sun to Saturn. WR 124 can be seen as a glowing body in the center of a gigantic fireball.
[https://en.wikipedia.org/wiki/WR_124]
Messier 71 is a very loose globular cluster mistaken for quite some time for a dense open cluster. It lies at a distance of about 13,000 light-years from Earth and was first discovered by the French astronomer Philippe Loys de Chéseaux in the year 1745 or 1746:
Messier 71: an unusual globular cluster
This spectacular NASA/ESA Hubble Space Telescope image shows a bright scattering of stars in the small constellation of Sagitta (the Arrow). This is the center of the globular cluster Messier 71, a great ball of ancient stars on the edge of our galaxy around 13,000 light-years from Earth. M71 is around 27 light-years across.
Globular clusters are like galactic suburbs, pockets of stars that exist on the edge of major galaxies. These clusters are tightly bound together by their gravitational attraction, hence their spherical shape and their name: globulus means ‘little sphere’ in Latin.
Around 150 such globular clusters are known to exist around our Milky Way, each one of them containing several hundred thousand stars.
Messier 71 has been known for a long time, having been first spotted in the mid eighteenth century by Swiss astronomer Jean-Philippe de Cheseaux. Cheseaux discovered a number of nebulae in his career, and also spent much time studying religion: one posthumously published work attempted to derive the exact date of Christ’s crucifixion from astronomical events noted in the Bible.
Despite being a familiar object, Messier 71’s precise nature was disputed until recently. Was it simply an open cluster, a loosely bound group of stars? This was for many years the dominant view. But in the 1970s, astronomers came to the view that it is in fact a relatively sparse globular cluster.
The stars in Messier 71, as is usual in such clusters, are relatively old, at around 9 to 10 billion years, and consequently are low in elements other than hydrogen and helium.
This picture was created from images taken with the Wide Field Channel of the Advanced Camera for Surveys on Hubble. It is a combination of images taken through yellow (F606W- colored blue) and near-infrared (F814W- colored red) filters. The exposure times were 304s and 324s respectively. The field of view is about 3.4 arcminutes across.
[https://www.spacetelescope.org/images/potw1018a/]
The Necklace Nebula is a planetary nebula in Sagitta. It is approximately 15,000 light years distant from the solar system. This Nebula was created when a giant star came too close to its binary companion and exploded, producing the nebula. It was discovered in 2005:
[http://www.constellation-guide.com/constellation-list/sagitta-constellation/]
Hubble Offers a Dazzling ‘Necklace’
A giant cosmic necklace glows brightly in this NASA Hubble Space Telescope image.
The object, aptly named the Necklace Nebula, is a recently discovered planetary nebula, the glowing remains of an ordinary, Sun-like star. The nebula consists of a bright ring, measuring 12 trillion miles wide, dotted with dense, bright knots of gas that resemble diamonds in a necklace.
A pair of stars orbiting close together produced the nebula, also called PN G054.2-03.4. About 10,000 years ago one of the aging stars ballooned to the point where it engulfed its companion star. The smaller star continued orbiting inside its larger companion, increasing the giant’s rotation rate.
The bloated companion star spun so fast that a large part of its gaseous envelope expanded into space. Due to centrifugal force, most of the gas escaped along the star’s equator, producing a ring. The embedded bright knots are dense gas clumps in the ring.
The pair is so close, only a few million miles apart, they appear as one bright dot in the center. The stars are furiously whirling around each other, completing an orbit in a little more than a day.
The Necklace Nebula is located 15,000 light-years away in the constellation Sagitta. In this composite image, taken on July 2, Hubble’s Wide Field Camera 3 captured the glow of hydrogen (blue), oxygen (green), and nitrogen (red).
[http://www.nasa.gov/mission_pages/hubble/science/necklace-nebula.html]
G54.1+0.3, is a pulsar, the dusty remains of a collapsed star:
G54.1+0.3: Ashes to Ashes, Dust to Dust
A new composite image of Chandra and Spitzer data shows G54.1+0.3, the dusty remains of a collapsed star. X-rays from Chandra reveal a wind of high-energy particles from the pulsar at the center of the image. The Spitzer data shows an infrared shell around the pulsar that is made of gas and dust that condensed from the supernova.
The dust is flying past and engulfing a nearby family of stars. Scientists think the stars in the image are part of a stellar cluster in which the supernova exploded. The material ejected in the explosion is now blowing past these stars at high velocities.
The composite image of G54.1+0.3 shows X-rays from Chandra in blue, and data from Spitzer in green (shorter wavelength infrared) and red-yellow (longer wavelength infrared). The white source near the center of the image is a dense, rapidly rotating neutron star, or ‘pulsar,’ left behind after a core-collapse supernova explosion. The pulsar generates a wind of high-energy particles- seen in the Chandra data- that expands into the surrounding environment, illuminating the material ejected in the supernova explosion.
The infrared shell that surrounds the pulsar wind is made up of gas and dust that condensed out of debris from the supernova. As the cold dust expands into the surroundings, it is heated and lit up by the stars in the cluster so that it is observable in the infrared. The dust closest to the stars is the hottest and is seen to glow in yellow in the image. Some of the dust is also being heated by the expanding pulsar wind as it overtakes the material in the shell.
The unique environment into which this supernova exploded makes it possible for astronomers to observe the condensed dust from the supernova that is usually too cold to emit in the infrared. Without the presence of the stellar cluster, it would not be possible to observe this dust until it becomes energized and heated by a shock wave from the supernova. However, the very action of such shock heating would destroy many of the smaller dust particles. In G54.1+0.3, astronomers are observing pristine dust before any such destruction.
[http://chandra.harvard.edu/photo/2010/g541/index.html]
[https://en.wikipedia.org/wiki/Sagitta]
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