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Could someone tell me how distance is calculated from the diagram.
Thank you! (Unsigned comment from User:JML)
I'm not experienced in the field, but I'd guess that since the colour and apparent magnitude are known, one can assume that the absolute magnitude is on the main sequence, and use the ratio of apparent to absolute magnitude to determine the distance. --Stoive 18:40, 23 September 2006 (UTC)
From page 66 of Astronomy by Dinah Moche ("the star lady") we see that brightness, color, and heat do not determine the distance. Rather, astronomers measure the difference in the viewing angle from one side of Earth's orbit to the opposite side measurement (parallax). They then calculate the star's distance in parsecs for each second of arc. The equation is: Distance = 1/parallax. For example, the measured parallax for Alpha Centauri is 0.74 seconds and the distance from Earth is 1.35pc (a parsec is 3.26lightyears). Each second of parallax adds a parsec (pc) or 31 trillion km (19 trillion miles.) Thanks, Star Lady. You should have a Wikipedia page, having written 18 books, and recognized. — Charles Edwin Shipp (talk) 20:02, 20 March 2014 (UTC)
I read a little further in Dr Moche's book, Astronomy, a Self-Teaching Guide, Seventh Edition: "For a distant star whose parallax cannot be measured but whose absolute magnitude [luminosity, radiating into space], as from consideration of its spectrum, the distance modulus can be used to calculate distance. m - M = 5log(D/10) where D is the distance in parsecs. Can you even imagine the vastness of creation? (m is the apparent magnitude.) — Charles Edwin Shipp (talk) 01:04, 21 March 2014 (UTC)
Vertical axis mislabeled in the two open clusters' HRD figure
I feel sheepish for noticing this only now (I have used the figure multiple times, for several years, and I really like it) but the vertical axis of the M67/NGC188 HRD figure, http://en.wikipedia.org/wiki/File:Open_cluster_HR_diagram_ages.gif ,is mislabeled: larger absolute magnitudes go toward the bottom of the graph, so the arrow should point down, not up, in that graph. The originator of the image has retired from Wikipedia, so it's unlikely we can prevail upon him to fix it. Can someone generate a corrected version? BSVulturis (talk) 01:59, 16 August 2013 (UTC)