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Street lights as standard candles: A student activity for understanding astronomical distance measurements

Astronomers measure cosmic distances to objects beyond our own galaxy using standard candles: objects of known intrinsic brightness, whose apparent brightnesses in the sky are then taken as an indication of their distances from the observer. In this activity, we use street lights and a digital camera to explore the method of standard candles as well as some of its limitations and possible sources of error. 1. ASTRONOMICAL DISTANCE MEASUREMENTS The astronomer’s typical role is that of a distant observer, trying to reconstruct an object’s physical properties from afar. The key to inferring such properties is knowledge of the object’s distance. After all, bright objects can look dim when seen from afar, and large objects can look small. Only when an object’s distance to us is known can we relate observational data to physical properties: The object’s angular diameter to its physical size, or the amount of light we receive from an object to its intrinsic brightness. Unfortunately, estimating cosmic distances is a difficult task. As no single method can cover the whole gamut of astronomical distances, astronomers have constructed a cosmic distance ladder that proceeds step by step, method by method, from distances within our solar system to the largest cosmic distances of billions of light-years (Grijs 2011; Webb 1999). The fundamental concepts of this distance ladder are part of the curriculum of introductory astronomy courses (Sec. III.C. in Coble et al. 2013) and commonly considered basic astronomical knowledge – essential for understanding how astronomers know what they know.