Whitby & District Astronomical Society

September presents an excellent opportunity to observe Sagittarius, the constellation richest in deep sky objects visible from the UK. Unfortunately, together with Scorpius, it is also the most southerly zodiac constellation along the ecliptic, which can limit visibility. However, with a favourable southern horizon, clear skies, and stable atmospheric conditions, observing Sagittarius remains highly rewarding.

In this two-part 'In Focus' feature (the second part will be available on the WDAS website during September), we will examine the ‘Archer’ in greater detail.  

Latin for "Archer", Sagittarius is often depicted as a centaur drawing a bow and is one of the 48 original constellations catalogued by 2nd-century astronomer Ptolemy.  Chiefly visible between latitudes +55° and -90°, surprisingly Sagittarius only ranks 15th largest of the 88 modern constellations, spanning some 867 square degrees, especially when given part of the constellation remains below the horizon at UK latitudes.  Sagittarius borders Scorpius and Ophiuchus to the west, Capricornus and Microscopium to the east, with borders also touching Aquila, Corona Australis, Indus, Serpens Cauda, Scutum and Telescopium. 

There is some debate about who Sagittarius represents from mythology.  One of the 12 signs of the zodiac, the group is depicted as an archer.  He is most commonly associated with Chiron, the wise centaur who trained many Greek heroes, Achilles and Asclepius amongst them. However, Chiron was not regarded as a great archer and is also associated with another constellation – Centaurus.   

Thus, an alternative Greek mythological tale linked to Sagittarius sees the Archer named as Crotus, a satyr and a son of the god Pan, who was believed to have invented the hunting bow. Crotus was a companion of the Muses of Mount Helicon; thus, it was the Muses who requested he should be placed in the heavens as Sagittarius.  

The Stars
The constellation contains 17 named stars, seven of which are brighter than magnitude 3.0, but none are ranked first magnitude.  The Archer also contains three stars located within 10 parsecs (32.6 light years) of Earth. The large asterism known as the 'teapot' largely rests on the southern horizon when viewed from the south of the UK. It is formed by the stars Kaus Media, Kaus Australis, Ascella and Phi Sagittarii (the body of the pot). Kaus Borealis marks the point of the lid, Alnasl is the tip of the spout and Nunki the handle. 

Kaus Australis – ε Sagittarii (Epsilon Sagittarii) brightest in the constellation with an apparent magnitude of 1.79 and a luminosity 400 x that of the Sun. It has a spectral class of B and lies 143 light years distant. The star’s traditional name, Kaus Australis, comes from the Arabic word for “bow” (qaws). 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) is the second brightest star in the constellation with an apparent magnitude of 2.1 and is over 3000 x more luminous than the Sun 7 solar masses.  It has a spectral class of B2.5 V and spins 100 times faster than the Sun. The star lies approximately 228 light years from Earth.                                                                             
Kaus Media – δ Sagittarii (Delta Sagittarii) is a multiple star system approximately 306 light years distant.  The main component has 5 solar masses and is over 1000 times more luminous. It has an apparent magnitude of 2.72 and belongs to the spectral type K3III. The star’s traditional name, Kaus Media, means “the middle bow.”

Kaus Borealis – λ 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 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. Lambda Sagittarii marks the handle of the Teapot asterism and points to the famous interstellar cloud, the Lagoon Nebula.

Ascella – ζ Sagittarii (Zeta Sagittarii) is the third brightest star in the constellation after Epsilon and Sigma Sagittarii. It is another binary star consisting of a class A2 giant star with an apparent magnitude of 3.26 and an A4 type subgiant star with an apparent magnitude of 3.37 separated by just 13.4 astronomical units (AU). The stars are approximately 89.1 light years distant from Earth and have a combined apparent magnitude of 2.60. Its traditional name, Ascella, means “armpit” in Latin.

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.

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. It is also sometimes known as Albaldah. which means “the town. 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) is a K type (K0III) giant star with an apparent magnitude of 2.98. It is approximately 96.1 light years distant. Gamma Sagittarii is also known by its traditional names, Alnasl and Nushaba the “arrowhead”.

τ 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 located between Zeta and Sigma Sagittarii. Tau Sagittarii is well known to astronomers as the closest visible star to the origin of the Wow! signal, the only radio signal ever received that indicated the possibility of extraterrestrial intelligence. The signal was received on August 15, 1977 and lasted 72 seconds, but has never been detected since. When discovered the finding was circled with the word “Wow!” next to it.

Deep Sky Objects 

Sagittarius is rich in DSOs with the Milky Way at its densest near Sagittarius.  There are in total 36 DSOs brighter than mag +10 (within range of 80mm telescopes) alone.  It is in the direction of the Sagittarius/Scorpius border that the Galactic Centre lies. 

The constellation contains two star clouds. The Large Sagittarius Star Cloud is the brightest visible region of the Milky Way. It is a visible portion of the central bulge of our galaxy seen around the thick dust of the Great Rift and is the innermost galactic structure that can be observed in visible wavelengths. It has several embedded clusters and superimposed dark nebulae. The Small Sagittarius Star Cloud, is also known as Messier 24,

Not surprisingly Sagittarius contains a plethora of Messier objects - 15 in total!  
Messier numbers M18, M21, M23, and M25 are all open star clusters.  Messier numbers M22, M28, M54. M55, M69, M70 and M75 are all globular clusters.  M8, M17, and M20 are nebula and M24 is a star cloud.  

We shall look in more detail at these objects in part 2 of the 'Archer' article. See Website, here's a teaser image.