October 03, 2007


I really enjoyed writing the blog entry a while back about Scorpius, even though it took a fair bit of research. I would like to continue with a series of posts about different constellations relying on many of the same sources that helped me before. So today, I will talk about the constellation Sagittarius.

At one of the two intersections of the ecliptic plane and the plane of the Milky Way galaxy, Sagittarius always has something interesting to look at, including frequent occultations of objects by planets. The three brightest stars in Sagittarius are Rukbat (knee of the archer), Arkab (achilles tendon) and Al Nasl (the point of the arrow). The bow of Sagittarius is aimed toward Antares, the heart of the Scorpion.

In most mythologies, Sagittarius is an archer and/or a centaur. In the Babylonian mythos, Sagittarius was the god of war. In the Chinese tradition, this constellation is the tiger. There are a couple of asterisms here, the most famous of which is the teapot, a collection of eight bright stars visible from within cities. There is also the Milk Dipper, which is just the teapot minus the rightmost three stars in the teapot (the spout). The Dipper is metaphorically dipping into the Milky Way.

Many Messier objects lie in this direction, including two of the best: the three-part Trifid Nebula (M 20) and the spectacular Lagoon Nebula (M 8). Shown here as part of the same field of view, the distances to these two objects are close enough together that it is probably not a coincidence. They are likely two bright regions on the front edge of a much larger cloud complex in that region of the sky.

Also nearby, just above the teapot, is M 22, perhaps the closest (10,000 light years) globular cluster to us and the third brightest in the sky behind the two I mentioned previously in Scorpius (47 Tuc and Omega Cen). Burnham describes this by quoting Tolkien: "It was as if a globe had been filled with moonlight and hung before them in a net woven of the glint of frosty stars."

Other objects of note in this constellation include Upsilon Sagittarii, a very odd double-star system with a strange spectrum (seems to indicate a star with lots of metals but very little hydrogen compared to other stars). It is a binary system, and one of the two stars is an A-class star (part of the OBAFGKM sequence). As a learned astronomer once told me in graduate school, there is no such thing as a "normal" A-star. They all are peculiar in some way, it seems.

Also present, of course, is Sgr A, the object at the galactic center. Thought to be a supermassive black hole with surrounding material emitting lots of radio energy, this is well-studied despite the incredible amount of obscuring dust along the line of sight to it. Because of all of this obscuring material, it is hard to get information on the galactic center region in shorter wavelengths like visible light. Just south of the galactic center, though, there is a little pathway that seems to cut through the intervening material, and this is called Baade's Window, after the astronomer who discovered it. The location is shown in this helpful image.

Also very difficult to see in this direction is the Sagittarius Dwarf galaxy, the 2nd closest companion galaxy to our own (even closer than the Magellanic Clouds!). As you might imagine, this is very difficult to observe through all the muck of the disk of the Milky Way! Of course, one man's trash is another man's treasure.

On a dark night, few things are prettier in the sky that the band of the Milky Way galaxy gracefully arcing over your head. Stretching this evening from low in the Southwestern sky in Sagittarius to Northeast through Cassiopeia, you can see it very well in good sky conditions, well outside of cities. Burnham quoted an anoymous Chinese poet about the Milky Way: "Look now upon the River of Heaven, Sky-Road of the Immortals, White with the star-frost of a billion years..."

Posted by Observer at October 3, 2007 03:06 PM

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Sagittarius is one of my favorite constellations and I can spend hours looking at the many objects in and around it. When I was back in Australia this year, Dad and I scanned the region with his 10" Dob - simply brilliant stuff.

Posted by: Phil on October 5, 2007 07:49 AM