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Constellations:
Sagittarius the Archer

Gen: Sagittarii
Abb: Sgr
Dec: -11.8° to –45.4°
RA:  17h 41m to 20h 25m

The most visible part of Sagittarius at our latitude is the " Teapot" asterism. This is the top of the constellation, with the rest of it to the south and east. It is hard to make out the lower part of the constellation, due to its lack of bright stars and all the haze on the horizon. Sagittarius rises in the East in June, it reaches its highest point around 10 PM in late July and drops below the horizon in September.

Sagittarius is Latin for "archer," and since ancient times the constellation has been represented as a creature wielding a bow and arrow. In many illustrations Sagittarius appears as a fierce beast, aiming his arrow at the heart of Scorpius, the legendary scorpion. The mythology for this constellation has been in dispute since its beginning. Some ancient records talk about the Teapot as being the human torso of Chiron, the wisest of the centaurs, holding his bow high in the sky. In this account, Chiron identified the constellation to help guide Jason and the Argonauts on the quest for the Golden Fleece. The dispute comes in because Chiron is more commonly referred to as being the constellation Centaurus, with Sagittarius being a more typical, unruly centaur.

Eratosthenes wrote that Sagittarius is the Satyr Crotus, son of Pan and Eupheme. Crotus is said to have invented archery and is shown in many pictures with a drawn bow. The ancient Egyptians also saw it as a similar beast, but to them it had the head of a lion. In the ancient Indian zodiac, the constellation was pictured as a horse traveling across the sky.

Exploring in Sagittarius

We will start this month at the tip of the teapot spout or, more correctly, the tip of the arrow. This star 10 Gamma (g) Sgr is called Alnasl (pronounced AL-na-zel) and means "The Point" (of an arrow). Just to the NNW of Alnasl there are two globular clusters NGC 6528 and NGC 6522. They will both fit into the view through a low power eyepiece with NGC 6522 being to the west and a little brighter than NGC 6528. From here hop down to the bottom of the bow (western base of the teapot) 20 Epsilon (e) Sgr or Kaus Australis (koss oss-TRAY-lis), the brightest star in Sagittarius.

About ¾° northeast of Kaus Australis is RV Sgr a Mira-type variable that changes from magnitude 7.2 to 14.8 during a period of 317 days. Continue another degree in the same direction to the class 5 globular cluster NGC 6637 (M69). M69 appears nearly round with a few streamers of stars around its edges. From here you get to take a break, because if you set the timer on your watch for 12 min. and turn off any clock drives you have, NGC 6681 (M70) should be drifting into your eyepiece just about the time that your timer goes off. M70 looks like it has a swarm of stars around the central core of the cluster.

If you take another 12 min, you should have a planetary nebula drift into view. This will be PK 003-14.1 a type II planetary at magnitude 10.9 with a central star that is magnitude 13.9 -- if you want to test your eyes and your scope, here is a good place to do it.

Two degrees north of the planetary nebula you will find NGC 6715 (M54), a class 3 globular cluster that appears almost round. This cluster is larger than M69 and M70 in both apparent and actual size. In this cluster, over 80 RR Lyrae type variables have been detected by astronomers with radio telescopes.

Note: The chart included here provides only a rough guide to the constellation. To follow the detailed description, refer to chart 22 in Tirion’s Sky Atlas 2000 (or equivalent).

The next Messier object in our journey will be M55 (NGC 6809). To find this cluster first move from M54 to the bright star at the bottom of the teapot handle, Zeta (z) Sgr. From here follow a trail of stars 7° just to the south of east and you will find M55. M55 is a class 11 open cluster with a magnitude of 6.3 and was first discovered by De Lacaille in 1751 from the Cape of Good Hope.

Time to shift to your Telrad and move to the top of the teapot (also known as the northern tip of the bow) Kaus Borealis (22 Lambda (l) Sgr). About ¾° northwest of l Sgr is the globular cluster NGC 6626 (M28). This is a nice cluster with a magnitude of 6.99 and shows many stars around its tight core. From here move 3° northeast to NGC 6656 (M22); you can’t miss this one even in binoculars it is a beautiful cluster. The cluster appears like someone splattered white paint on a black wall with thousands of tiny stars all around the core. The actual discoverer of this cluster was in doubt for a very long time, but the credit is now given to an obscure German amateur astronomer named M.J. Abraham Ihle. Ihle noted the cluster while observing Saturn, 99 years before Messier added it to his list.

Step back and look at the steam coming out of the teapot. This part of the sky holds so many splendors, all in a rather small area of the sky. I find that I really enjoy looking at this whole area with binoculars before I settle my eye into the restricted view through a telescope. So grab a chair and your binoculars, and let your eyes drift through the Sagittarius arm of our Milky Way Galaxy before we go on.

The first object above the teapots spout consists of an open cluster, NGC 6530, the star 9 Sgr, a large emission nebula, and three dark nebulae (B 88, B 89, and B 296) all of these close togethr form what we see as the Lagoon Nebula (NGC 6523; M8). The star 9 Sgr is considered to be the source of the radiation that causes the gas cloud to glow, while the dark nebula gives it the appearance of an island with a deep lagoon. Use an OIII filter to really bring out the structure in this area.

Moving up the steam cloud we come to the Trifid Nebula (NGC 6514; M20). M20 is also made up of an emission nebula, a reflection nebula, and the dark nebula B 85. The latter criss-crosses the area and gives the Trifid Nebula its segmented appearance. I find that the OIII filter makes the area too dark in my 10" scope and kills the reflection nebula around the cluster completely. The Trifid Nebula was added to Messier’s list in 1764 and was given its name by Sir John Herschel some time later. If you use a low power eyepiece you can view M20 and, about ¾° northeast, NGC 6531 (M21). M21is an open cluster with around 70 stars and shines at magnitude 5.9.

Return to the Lagoon then move 6° northeast. You will pass Mu (m) Sgr about half way to our next stop, the "Small Sagittarius Star Cloud," more commonly known as M24. This area has NGC 6603 embedded in it but I am not sure if I really see it or not. What I can see is in the northwestern edge of the cloud, there are two dark holes that are the dark nebulae B 93 and B 94. From the southern end of M24 move 4° east to find NGC 6494 (M23). M23 is a large 220 million-year-old cluster containing about 150 stars. Try to time your evening so at this point you can break for something to eat, and in about 35 minutes the open cluster IC 4725 (M25) will be in your eyepiece. M25 is 9° to the east of M23 and is magnitude 4.6. Near the center of the cluster look for a box of four bright stars.

Go back to M24 and look for a pair of stars (sixth and seventh magnitude) at the northeastern tip that point the way to NGC 6613 (M18), about 1° away to the north. This is a small open cluster of about 20 stars. In this cluster, try to find two triangles, one being much brighter than the other.

One degree farther north and we come to a fantastic emission nebula, NGC 6618 (M17). This nebula is known by at least three different names; the Omega Nebula, the Swan Nebula, or the Horseshoe Nebula. Regardless of what name you use it is great to look at, and with an OIII filter you can follow the nebulosity all the way around to form the W shape. When Messier first looked at it he thought it looked like a spindle. This cloud contains enough material to make 800 Sun-sized stars. Also embedded in the cloud is the cluster NGC 6618, containing about 35 stars that provide the radiation source for its glow.

Just above the Swan you will find the Eagle Nebula (NGC 6611; M16) in Serpens Cauda. This nebula needs a very dark area and a large scope to look its best. In my 10" scope it looks good, but it’s not even close to the view through a 20"+ scope. With its nebulosity enveloping its hot young stars, and the black nebula cutting into it forming the shape of an Eagle it is quite a sight to see.

This ends a nightlong tour of Sagittarius. There are many other objects to find in this area that can be found on a good star chart. I hope this gives you a good start.

So until next month,

Happy Star-Hopping...Eric Reichenbach

Copyrighted © 2000 by Stockton Astronomical Society
Lasted Updated: 11/20/2000
http://astro.sci.uop.edu/~sas/Newsletter/CON_Sagittarius.html