hoose a variable star and find a reference chart for it via the BAA (British Astronomical Association, ) or AAVSO (American Association). Both organisations have charts for variable stars which identify non-varying comparison stars to use when estimating the variable's brightness. Pick two stars, one brighter (A) and one dimmer (B) than the variable (V). If the difference in brightness between A and V equals that between V and B, V is mid-way between A and B, and you'd write that as ‘A(1) V(1)B’; the brighter comparison is always presented first. If the difference between A and V were three times larger than that between V and B this would be written as ‘A(3)V(1) B’ and so on. You're mentally dividing the difference in brightness between A and B into a number of parts, in this example 4, and placing the variable along that scale. If A's magnitude was +3.7 and the real difference between A and B was 0.4 magnitudes, each ‘A(3)V(1) B’ division represents 0.4 ÷ 4, or 0.1 magnitudes. In this example, V is 0.1 magnitudes brighter than B and 0.3 magnitudes dimmer than A, making it mag. +3.7 — 0.3, or +3.4. The magnitude difference between A and B should be less than 0.6.
