Black Holes to Blackboards: Drifting Through Andromeda

Mercury, July/August 1997 Table of Contents

Jeffrey F. Lockwood
Saguaro High School

Where are we in the universe? A new set of exercises is allowing students to answer for themselves that age-old question.

A color image of the Andromeda galaxy always takes a little of my breath away: its golden yellow center, its bluish gossamer arms, its small, burr-like, fuzzy elliptical companion, M32. I want to live there, in the same way that I feel an internal tug when a deserted beach on Maui fills my television screen. I can see the travel brochure now... "A long drive, but well worth the billions of wonders which will amaze your body and mind." (At 75 mph, the "drive" will take, well, a very, very long time.)

Imagine: You are standing in a vast forest of tall trees; thousands of square miles of them surround you. If you couldn't climb any of them (very thorny bark?), how could you map the boundaries of the forest and determine your place within it? Now imagine you are on a tiny planet circling a star in the midst of 200 billion other stars in a system called the Milky Way. How do you uncover the secrets of an object we are hidden in the middle of?

The beautiful images of Andromeda are a start; they give us a sense of what good form is for galaxies like our own. But they are not a complete solution. Astronomers have charted our galaxy by mapping the positions of its most conspicuous members -- galactic clusters of stars, gaseous nebulae, Hi and Hii star-forming regions, and ultra-bright 'O' and 'B' stars -- and plotting the relative velocities of nearby stars, gradually filling in a picture of where we are and where we are going. Meanwhile, radio astronomers have peered to the very center of the Galaxy and the hungry black hole there; the long radio wavelengths cut right through the dense underbrush of the Galactic forest.

But how can our students discover the nature of their home galaxy for themselves? In many cases they can do exactly what astronomers have done, and activities exist which attempt to lead them through the process. Project STAR has an exercise, "Locating the Solar System in the Milky Way Galaxy," in which students plot the positions of clusters, constellations, nebulae, pulsars, quasars, and other galaxies on a flat projection of the sky. By so doing, they can locate the approximate center of the Milky Way in the constellation Sagittarius.

Students can also figure out which objects might lie far outside the confines of their galaxy. If they draw their plots on transparent plastic sheets, the teacher can create a planar model of the Galaxy by overlaying the sheets. As the teacher superimposes the sheets one at a time, students see that nebulae, galactic clusters, and pulsars occur only along the plane of the Milky Way -- unlike quasars and other galaxies. The plot of globular clusters demonstrates that we Earthlings are not the center of the Milky Way. Sorry.

Other labs exist which ask students to plot the positions of globular clusters on polar-coordinate graph paper. From this, students can find the direction and the actual distance to the Galactic center. In these exercises, however, students don't ever do anything but labor-intensive plotting.

The time-honored practice of showing slides or videodisc images of galaxies can be made more interactive by constructing red and blue filter cards. Tape red and blue clear plastic over square holes cut in a 3Ç5 index card, and have students look through the filters one at a time to examine color slides of spiral galaxies. Why do the spiral arms disappear when you look at them through the red filter? (The arms are mostly hot, bluish, newly hatched stars.) Why does the central bulge of the galaxy almost vanish when you look through the blue filter? (The bulge consists mostly of cool, reddish, old nursing-home stars.)

A slide of M87, a giant elliptical galaxy, can spark a discussion about globular clusters. A thousand or so globulars orbit M87, looking like tiny bees around a celestial hive. They are obviously uniformly distributed around this elliptical, so shouldn't we expect them to be uniformly distributed around spirals as well?

I am now developing a series of activities for the ASP in which students examine clusters, describe their shapes, and use their distribution to locate the center of the Galaxy and its spiral arms. I also plan a galaxy-classification exercise and a tour of Messier objects using the RealSky CD-ROM set. If you would like to test these materials this fall, please email me.

Scale modeling of galaxies always reveals the stark contrast between the spacing of stars in our galaxy and the spacing of galaxies in our universe. Galaxies collide with great frequency, yet while they collide, none of the stars in them hit one another. How can this be? If you have your students draw the Galaxy on a 9-inch paper plate and then calculate where our nearest spiral neighbor, Andromeda, would be at this scale, they will discover that it is only 16 feet away. Indeed, all of the 24 galaxies in the Local Group can be represented at various locations within a normal-size classroom. But if you hold up a soccer ball (again, 9 inches in diameter) to represent the Sun and ask your students where the nearest star, Proxima Centauri, would be at this scale, they will find that it is a whopping 48 miles away! No wonder galaxies jostle all the time, but not stars. There are as many galaxies in the observable universe as stars in the Galaxy, but in proportionately much less space.

As our students' minds wrestle with fascinating abstractions such as the vastness of time and space, it is good to introduce a modicum of form and structure to ground them in reality. Understanding their place in the Milky Way and modeling it in 3-D gives students a foothold in the concrete as they attempt to grasp the intangible. Galactic mapping is a chance for students to practice their description, observation, and classifying skills, while learning about their home galaxy. Traveling to Andromeda for two-week vacation may never be possible, but I will travel there in my mind each time the slide hits the screen.

JEFFREY F. LOCKWOOD is a high-school and college astronomy and physics teacher at Sahuaro High School and Pima Community College in Tucson, Ariz. He is an advisor to national educational projects such as Hands-on Astrophysics, AASTRA, RBSE (Research-Based Science Education), and the Image Processing for Teachers Program, and is on the ASP Board of Directors. His email address is iplockwood@aol.com.

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