Distances to Stars A little experiment to try: 1. Hold a pen/pencil about 1.5 feet from your face. Alternate opening and closing each eye, and notice how the pen appears to move with respect to background objects. This apparent motion is called “parallax”. 2. Now bring the pencil in at about 0.5 feet from your face. When you alternate opening and closing each eye, does the pen appear to move more or less than before? 3. Now move the pencil out to about 3 feet from your face. When you alternate opening and closing each eye, does the pen appear to move more or less than before? 4. Now have a partner take the pencil and walk away from you as you alternate opening and closing each eye until you can no longer detect any parallax. How far did your partner get? Questions: a) How does parallax vary with distance of the pen from your face? b) Is this method useful for finding distances to very far away things? Why or why not? c) To measure a parallax, you need a baseline—the distance between observation points. In the activity, the baseline was the distance between your eyes. In order to measure the distances to very distant stars, what baseline would you use?

Unfortunately, other stars are really far away… Earth-based telescopes can only measure distances in this way out to 50 parsecs Space-based telescopes can measure distances out to 500 parsecs Nearest star (Proxima Centauri) is 1.3 parsecs away Our galaxy (Milky Way) is 30,000 parsecs in diameter