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Monday, August 11, 2008

Constellation Showcase: Sagittarius

Welcome to this month's Constellation Showcase! This monthly segment details a currently viewable constellation, typically somewhere along the ecliptic. This month's Constellation Showcase feature will be Sagittarius. Sagittarius contains many great deep sky objects. It is also very easy to find! Just look to the South and find the set of bright stars in the shape of a teapot. Sagittarius is best positioned for viewing in August around 10-11 PM in the low Southern sky. Its teapot shape, along with its relatively bright stars, makes it easily recognizable. The "spout" of the teapot dips into the Milky Way, as if it were pouring it out. The center of our galaxy is also in the direction of Sagittarius. Because the center of the galaxy is so thick with dust, most of the features in Sagittarius are inside our own galaxy. This is why Sagittarius' features are mostly clusters and nebulae.

Although it is most commonly recognized as a teapot, Sagittarius is in fact named after the Greek mythological centaur (a half-man, half-horse creature). Sagittarius was an archer that in constellation mythology fought Scorpius, a nearby constellation.


M22, in Sagittarius, is a globular cluster featuring half a million stars. These half a million stars are condensed into a ball about 50 light years in diameter, and 10,000 light years away from Earth. There is actually a cloud of dust between this M22 and Earth, so its apparent brightness is reduced. The strange thing is that M22 also houses a planetary nebula inside it. This nebula inside M22 is not going to be visible in your backyard telescope, however.

M28 is also an interesting target. M28 consists of much less stars, about a hundred thousand, and is more sparse, at a diameter of 65 light years. Although it won't appear as large as M22, it is interesting that M28 lies about half-way from Earth to the galactic center.

M55 is one of the more well known nebulae in Sagittarius. M55 is very large, covering about 19' of the sky, which is about 2/3 the apparent visual size of the moon. In actuality, though, since the cluster is about 17,300 light years away, it is much larger. It has a diameter of about 100 light years. Because it is relatively sparse, it makes a great target for smaller telescopes, since it is much easier to resolve individual stars. This cluster will look more "grainy" than most globular clusters.


The Lagoon Nebula, also known as M8, lies just North of the "handle" of the teapot, and makes a great target for telescopes. M8 is actually a double target, as it contains an open cluster, NGC 6530. M8 is a cloud of hydrogen gas located about 5,000 light years away. M8 will be visible even in medium light pollution, although like most deep sky objects, will look much better from a dark sky site. The open cluster NGC 6530 visible with M8 is a cluster of 24 stars, and is considered to be one of the youngest clusters near Earth.

The Trifid Nebula, M20, is another notable nebula in Sagittarius. M20, like M8, is a cloud of ionized hydrogen gas. John Herschel was the first to call this object the Trifid Nebula, perhaps because of its three-lobed appearance. It is actually possible to photograph M20 and M8 in the same field of view with a very wide lens. Some astronomers disagree on how far the Trifid Nebula is, however, with estimates ranging from 2,200 light years to 9,000 light years. In the picture to the right, note the stellar jet protruding from a pillar of gas inside the Trifid Nebula.

M17 is another nebula in Sagittarius, and goes by many names. It is sometimes known as the Swan Nebula, the Horseshoe Nebula, or in the Southern hemisphere, the Lobster Nebula. With its apparent magnitude of 6.0, M17 can be seen with the naked eye under perfect sky conditions. Through a telescope or binoculars, it is a great target, and may show a slight pink hue in larger telescopes. There is also a small cluster of about 35 stars inside this nebula, whose light fuels the emission nebula's color and intensity.

Other Objects

The most interesting object in Sagittarius, however, may be one that you can't see with your telescope. Nor can you see it with any kind of optical light detector. This object is Sagittarius A* (A-Star) and is strongly suspected to be a supermassive black hole in the center of our galaxy. At the Max Planck Institute for Extraterrestrial Physics, astronomers and physicists have studied the orbit of S2, a nearby star. S2's orbit suggests that it orbits an object with a mass of 3.7 million solar masses. This enormous mass is confined into about 6.25 light-hours, a size slightly larger than our solar system's diameter of 5.51 light-hours. This incredible mass, along with radio emissions that show features of a black hole, point to the theory that Sagittarius A* is a supermassive black hole in the center of our galaxy.

While this tour of Sagittarius contains some beautiful deep sky objects, it is in no way "complete" There are many, many objects in Sagittarius, thanks mainly to the fact that it is in the direction of the galactic center. I hope you enjoyed this month's installment of the Constellation Showcase, and that you will join us next month, when we will showcase another constellation!

To receive more articles like this, subscribe via RSS Feed or e-mail!

Clear skies!

Next Constellation Showcase - Cygnus


Clowncar said...

What an informative post! I just bought my first serious telescope (8" dobsonian) and found your post very helpful while exploring all the cool stuff around Sagittarius. Thanks!

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