It's Over Your Head (2006)

060531.doc Ursa Major Page 1 of 5

From Celestial North, this is IT’S OVER YOUR HEAD, a look at what’s up in the sky over Puget Sound.

When you go outside and look up at the stars, what is the first constellation you typically look for? For many people, it is easiest to spot the Big Dipper. This constellation, also known as Ursa Major, is found in the northern night sky and often helps you get oriented for viewing other constellations or objects. The two stars forming the outside edge of its scoop or bowl determine a line that leads to

the North Star, Polaris, which is located almost exactly over our planet's axis of

rotation, in a location we call "Celestial North". The Big Dipper also has some

fascinating history surrounding it.

The constellation we envision today as a "Big Dipper" looked like different

things to several ancient cultures. Our familiar notion of the stars as being

something like a water dipper comes from Africa, where it was known as "the

drinking gourd." To the ancient Romans, the familiar seven stars looked like a giant bear in the sky, which they called Ursa Major. They believed that the princess Callisto fell in love with the King of the Gods, Zeus. When Zeus's wife

Hera discovered this, she turned Callisto into a bear and cast her into the heavens.

The Greeks also adopted the idea of a bear for the constellation. The

Greek word for bear is arktos, and since Ursa Major is seen over the northern regions of the globe, we still use the term "Arctic" regions for north ... and of course "Antarctic" for south.

The constellation also looked like a bear to Native American cultures, but instead of being the tail, the three stars that we see today as the handle were thought by these ancient peoples to be three hunters following the bear.

Ancient Germanic people saw the Big Dipper as a horse-drawn carriage, with the four stars that make up the "bowl" being the cart. The French called it

"La Casserole," because to them it looked like a casserole dish. Other farming cultures saw the shape as a simple wooden plow, which is why the Big Dipper is sometimes still called the "Plough" today.

Some cultures had much more elaborate pictures of what was up in the sky.

The ancient Chinese, for example, saw the stars as forming dividing lines between the Emperor in the Sky and his celestial bureaucrats below him. The

Egyptians probably had the most complicated idea of all--they saw a man lying down, followed by a walking hippopotamus with a crocodile on its back! But, despite all of these unique ways to look at this simple group of stars, the interpretation that has persisted is generally that of the African water dipper; hence the name "Big Dipper."

While Ursa Major looks like merely another constellation to us on Earth, something very interesting is happening with this particular group of stars. A constellation is made up of several stars, which typically are not physically related to each other. However, the stars in Ursa Major are actually moving together through space, and so are several stars around them. In fact, these stars are all part of one giant star cluster called the Ursa Major Moving Group, or

Collinder 285.

What other night sky objects are in or near the Big Dipper? Well, there are several Messier objects in the vicinity of Ursa Major that are very interesting to look at. One of them is the Owl Nebula, M97, one of the largest planetary nebula known. It has two round, dark regions that look like an owl's eyes. It is difficult to see, even on a clear night with a good telescope.

There is also a very interesting multiple star system in the handle of the

"Dipper," occurring around a star named Mizar. Two bright stars, Mizar and

Alcor, appear to be locked together in the gravitational game of tug-of-war between two stars know as a "binary" star system. However, in actuality, the connection between Mizar and Alcor is an optical illusion. Alcor is actually farther away, and is not a part of the Mizar multiple star system. The stars that are a part of the system are Mizar A and Mizar B, each of which is also a binary system. There are actually four stars, but the two stars that accompany Mizar A and B are so close to their companion stars that they can only be detected using spectroscopy.

Another Messier object in Ursa Major is the Pinwheel Galaxy, M101. It can be seen with binoculars. It is about 25 million light years away, and with a telescope, you can faintly make out its spiral arms.

Finally, there are also galaxies M81 and M82. They are only 12 million light years away, which by galactic standards would make them close neighbors to us.

They are the brightest members of a small group of galaxies, and are easily spotted with binoculars. M81 is a spiral galaxy, while M82 is an irregular galaxy with a burst of star formation at its active center.

There are many lovely constellations in the night sky. We each have our favorites, and like old friends, we tend to seek them out first. They help us feel at home while settling in to search for new things to look at.

We’re on the web at CelestialNorth.org. Until next time, this is ______and ______, with a reminder that the night is large and full of wonders.

REFERENCES:

1. http://www.windows.ucar.edu/tour/link=/the_universe/Constellations/circumpolar

/ursa_major.html&edu=high

2. http://www.dustbunny.com/afk/constellations/bigdipper/

3. http://en.wikipedia.org/wiki/Big_Dipper

4. www.american-buddha.com/pers.voyage.carl.htm

5. http://antwrp.gsfc.nasa.gov/apod/ap060317.html

6. http://www.astropix.com/HTML/C_SPRING/BIGDIP.HTM