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by ortusdux
1045 days ago
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The chart is H vs. Albedo, and is awkwardly formatted.
H is the brightness observed on earth. Albedo is the inherent reflectivity of the object. What the chart is saying is that if we observe an object that is 19.5 H bright, it could either be highly reflective (0.30 albedo) and 310m across, very dim (0.05 albedo) and 760m across, or somewhere in between. As I understand it, we can only calculate the albedo if we know the size and apparent brightness. I'll note that the linked NEOscan page has an H of 19.9, while JPL's CNEOS has an H of 19.2. |
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H is basically the same as M, an absolute magnitude, but for solar system objects. Difference being that for solar system objects H, as you've noted, is an absolute magnitude for an object 1 AU from Earth and 1 AU from Sun in a triangle where phase angle is zero meaning it's a straight line instead of a triangle and M is an absolute magnitude for an object (usually self illuminating) outside of solar system set at 10 parsecs from Earth. A star with M = 1 and a solar system object with H = 1 would roughly be the same brightness. It's a log scale, lower the number the brighter it is, so if you put that star at the same spot as the object it'd be a difference of about 26 where each unit is roughly 2.5 times brighter. What's interesting is that at zero it used to (roughly) be Vega (star). Both M and H are band-dependent, I guess it's V-band (visual or green-ish) if it's not mentioned. If you have B (blue) and V band, the difference between the two tells you a (visual) color which can be also interpreted as temperature, the lower the number the bluer it is and higher means red.
I'm all out of trivia for the night. I'm sure there are (amateur and not) astronomers and astrophotographers here that know this stuff way more about. All I knew is that without albedo you couldn't get to the size with absolute magnitude alone, we could if we had both absolute and apparent. What I didn't know if that apparently there's a rough table of correlation between H and albedo values probably based on most rocks we've seen in space around us.