How many times more distant is HD 49980? distance to Sirius d1 = 2.64 pc distance to HD 49980 d2 = 500 pc Ratio of distances is distance to HD 49980 500 pc ---------------------- = ------- = 189 distance to Sirius 2.64 pc So HD 49980 is about 189 times farther away than Sirius. If we were to pick up Sirius and move it so that it was at the same distance as HD 49980, how many times fainter would it become? The apparent brightness of a source goes as the INVERSE SQUARE of its distance from the viewer, so 2 I(Sirius at new distance) ( current dist ) ------------------------- = ( ------------- ) I(Sirius at current dist) ( new distance ) 2 ( 2.64 pc ) = ( ----------- ) ( 500 pc ) = 0.000 027 9 Wow. Sirius would become much, much fainter. Its new apparent brightness would be about 1/35870 of its current brightness. Which star would look brighter if they were sitting side-by-side at the same distance? At current positions, brightness of Sirius = 832 x (brightness of HD 49980) If we move Sirius to the same distance as HD 49980, it fades a lot. new brightness of Sirius = (current brightness) * 0.000 027 9 = (832 x HD 49980) * 0.000 027 9 = 0.023 x HD 49980 So Sirius would look fainter than HD 49980 if the two stars were placed side-by-side.