Building the Big Dipper, by John Cowens

An easy-to-make 3-D model of this star formation will help to put its familiar shape in perspective

The shape of the Big Dipper has fascinated people for centuries. According to Native American legends, the bowl of the Big Dipper is part of a giant bear and the stars that make up the handle are three warriors chasing it. In other cultures, the Big Dipper was thought to be a wagon or cart, a plow, a bull's thigh or (to the Chinese) the government. The Big Dipper was also an important part of the Underground Railroad. Songs that mentioned a "drinking gourd" gave escape instructions to slaves.

The real dipper deal
Contrary to what most of us think, the Big Dipper is not a constellation. It's an asterism – a small formation of stars that's part of a constellation. In this case, the Big Dipper is the hind quarter of Ursa Major, the Big Bear. The seven main stars of the Big Dipper range from 59-107 light years away and are different sizes and ages.

The Big Dipper includes more than seven stars. When we view the Big Dipper on a dark, clear night, the seven main stars are easy to see. However, a telescope reveals a faint star near the second star from the "handle's" end. Some people can even see it without binoculars or a telescope. If you have a telescope, more objects can be found in Ursa Major such as a double star, four spiral galaxies, an irregularly-shaped galaxy and a planetary nebula.

When viewed from directly below, all the beads in your model of the Big Dipper will appear to be the same distance away.

A different point of view
Presently, from Earth, the Big Dipper looks like a pan with a bent handle. Now that you know the seven main stars are at different distances from us, what would the Big Dipper look like from different angles? Building a three-dimensional model of the Big Dipper will allow us to see different views of this star formation.

Materials:

• chart of the Big Dipper
• transparent tape
• one sheet of blue or black foam board 55 cm x 38 cm
• seven 1-cm wide white beads (one for each star of the Big Dipper)
• one 1/2-cm wide white bead (to represent Alcor, the faint star in the handle)
• one long sewing needle
• meter stick
• eight 120-cm strands of black thread

Making the model

1. Draw the stars of the Big Dipper onto the foam board so that the drawing's length is 25 cm from the end of the handle to the farthest point of the pan. Place each star's distance from Earth on the drawing, as shown below.

2. Cut pieces of black thread about 120 cm long. You will need one piece of thread for each star marked on the constellation chart.

3. Place a bead on each thread by first passing the thread through it, then around and through it again. By stringing the bead this way, you will be able to slide the bead along the length of the string so it doesn't slip down the thread.

4. Using a sewing needle, punch holes through the foam board at each star's location. Use the needle to pull a piece of thread (with a bead strung on it) through each hole, leaving about 2 cm at the back (tape down the excess thread). When finished, you should have eight long threads hanging from the front side.

5. Trim the threads to 110 cm each. To make a three-dimensional model of the asterism, slide the beads down the strings until they're the correct scale distance from Earth. Use the star chart to find the distance of each star.

   The star at the end of the handle, Alkaid, is 107 light years from Earth. Slide Alkaid away from the foam board 2 cm. This star will be your base from which to subtract the other star's distances. The second star from on the handle of the Big Dipper, Mizar, is 59 light years from Earth. Subtract: 107 – 59 = 48. Slide Mizar 48 cm lower than Alkaid's location. Repeat the procedure for the other "stars." Once you have positioned each of the beads, trim the threads.

6. Have a partner hold the foam board 2-3 meters directly above you so that it's easy to view the Big Dipper's shape. Notice how each star appears to be the same distance from you.

7. Observe your 3-D model of the Big Dipper from various angles and sketch what you see.

Extensions

• Research the size, distance and temperature of each star in the Big Dipper. Is the closest star the brightest star? Is there a star that's farther from Earth, but brighter than a closer star?

• Research another constellation or asterism and make a 3-D model of it.

John Cowens teaches science at Fleming Middle School in Grants Pass, OR.