Thursday, May 21, 2009

Three Leaps of the Gazelle

Between two of the boldest naked-eye star patterns in the spring sky lies a third more demure pattern that’s one of my personal favorites. It’s quite easy to spot when you know where to look, so let’s go stargazing, shall we?

1) About an hour after your local sunset time, face west. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west— close enough for our purposes.

2) Tilt your head back and look at the zenith, the point in the sky that’s directly overhead. A little to the north (right), you should spy the distinctive seven-star pattern of the Big Dipper. The Big Dipper is an asterism, a recognizable star pattern, and it lies in the constellation of Ursa Major the Big Bear.

Star maps created with Your Sky

3) Now look a little south (left) and west from the zenith. Look for the Sickle, the curved asterism that marks the head of Leo the Lion. The Sickle looks a bit like its namesake, the old-fashioned farm implement, or like a backwards question mark. Spot it? Great.

4) Look in the space between the curved top of the Sickle and the bottom of the bowl of the Dipper for three pairs of stars widely spaced from one another. Each pair is around the same distance from the other pairs, and the spacing between the two stars of each pair is around the same. In addition, the stars are all around the same magnitude of brightness. This similarity of pattern helps the little pairs to stand out from the stars around them.

These form the asterism known as Three Leaps of the Gazelle. The three star pairs all lie within the constellation of Ursa Major the Big Bear, and they mark three of the bear’s paws.

Ancient Arabic star lore relates that the gazelle was startled by the lash of the lion’s tail when it sprang from “the Pond,” what we know as the Coma Star Cluster in the constellation Coma Berenices (Berenice's Hair). You can spot the Pond naked eye from a dark site; it looks like a large, bright patch just off Leo's tail.

5) Starting at the Pond, the first set of hoofprints you come to are the stars Alula Australis and Alula Borealis. Alula (uh-LOO-luh) is from the Arabic for first leap; Australis and Borealis are Latin for southern and northern, referring to the respective position of each star.

The middle set of hoofprints are the stars Tania Australis and Tania Borealis. Tania (TAH-nih-yuh) is from the Arabic for second leap.

The final set of hoofprints are the ones farthest from the Pond. The northernmost star is Talitha (TAH-lith-uh), from the Arabic for third leap. Its companion has no traditional name, so we know it simply as Kappa, its star catalog designation.

Can you imagine the swift gazelle leaving behind those three pairs of watery hoofprints as it leapt between twin perils of lion and bear?

Astronomy Essential: The Milky Way galaxy belongs to a galaxy group.

The Milky Way, our home galaxy, is a member of a group of galaxies called the Local Group. It contains around 30 galaxies. The two that are closest to the Milky Way are called the Large Magellanic Cloud and the Small Magellanic Cloud. They are much smaller than the Milky Way and can be seen with the naked eye from the Southern Hemisphere.

Another well-known member of our Local Group is the Andromeda Galaxy, which can be seen naked eye from the Northern Hemisphere. The Andromeda Galaxy is the closest large galaxy to our Milky Way; it lies around two and a half million light years from Earth.

Galaxies throughout the universe tend to cluster into groups. The members of these galaxy groups interact gravitationally and sometimes collide.