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As an enthusiastic amateur scientist, I'm plowing my way through a cool Gaia DR2-based paper, "An Empirical Measurement of the Initial-Final Mass Relation with Gaia White Dwarfs". I'm doing all right, but have hit a roadblock: understanding the meaning of the stellar color value GBP - GRP.

GBP is the blue photometry value, and GRP is the red photometry value, so GBP - GRP is the difference between the two, giving a number broadly encoding the star's color. Here's a graph from the paper that uses this value:

Gaia graph; absolute magnitude versus GBP - GRP

It's a graph of white dwarf ("WD") color versus absolute magnitude. On the left, the absolute magnitude MG increases (downwards) as the star grows dimmer, and across the bottom the color GBP - GRP increases (rightwards) as the star grows redder. Hence the slope of the curve; as a WD cools, it gets smoothly dimmer and redder.

But wait! as a star gets redder, the the "blue" brightness should become relatively smaller than the "red" brightness, right? So, the GBP - GRP value should get smaller and smaller, right? But that's the opposite of what this graph (and other graphs in the paper) apparently shows.

Should GBP - GRP get larger as a star cools and gets redder? Where am I confused? (I'm assuming the professionals aren't all confused...)

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But you demonstrate that you know the answer to this question! Fainter objects have larger magnitudes. So as the white dwarf becomes cooler and redder, its blue magnitude $(G_{BP})$ grows by more than its red magnitude $(G_{RP})$ and the colour-index $(G_{BP} - G_{RP})$ becomes larger. This is very similar to the commonly used $B-V$ colour, which is larger for cooler stars.

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  • $\begingroup$ So, the "G" is a magnitude (larger <=> dimmer), not a brightness (larger <=> brighter)? That would explain it. (That would also explain why the difference is a useful near-zero value; it corresponds to a ratio of the brightness.) $\endgroup$ – Daniel Griscom May 16 '18 at 15:45
  • $\begingroup$ Mind if I edit your answer to more clearly include the info in my comment? (Or, edit it yourself if you like.) $\endgroup$ – Daniel Griscom May 16 '18 at 18:16
  • $\begingroup$ A confirming source: arxiv.org/pdf/1008.0815.pdf cites Gbp and Grp as "Gaia magnitudes". $\endgroup$ – Daniel Griscom May 16 '18 at 21:21

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