|MadSci Network: Astronomy|
Hi -- I'd have to see an example of your chart to see what data are being shown, in case the chart actually plots something like stellar magnitudes and calls it parallaxes. In any case, parallaxes are always positive quantities as you correctly pointed out. But as with any quantity, there are errors of measurement. When stars are farther off than 50 - 100 pc, the parallax is nearly zero, and an error of measurement can place the actual value slightly less than zero. If your chart grabbed parallaxes from a catalog without paying attention to what the errors are, it is entirely possible that there would be negative parallaxes shown.
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