17 Dec
2003
17 Dec
'03
6:10 p.m.
Ok, here it is from memory. Early plates were taken twice, once with a blue-sensitive emulsion, once with a yellow (or red, not sure after all these years) emulsion. Subtracting one value from the other yields a "color index" for the star, helping place it on the H-R diagram. When the visual values were written-down, typically only one exposure was consulted, the blue one. Some objects are MUCH brighter to the eye than they appear on a blue-sensitive plate. I may not have recalled it 100% accurately, but I think that's the gist of it. C. __________________________________ Do you Yahoo!? New Yahoo! Photos - easier uploading and sharing. http://photos.yahoo.com/