# Gaia Gbp - Grp: why does it get larger as star gets redder?

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:

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...)

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.

• 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.) Commented May 16, 2018 at 15:45
• Mind if I edit your answer to more clearly include the info in my comment? (Or, edit it yourself if you like.) Commented May 16, 2018 at 18:16
• A confirming source: arxiv.org/pdf/1008.0815.pdf cites Gbp and Grp as "Gaia magnitudes". Commented May 16, 2018 at 21:21

Magnitude decreases when it is brighter. So G_BP value goes larger, G_RP goes smaller when the star gets redder and less blue.