<p>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.</p>

CC BY-SA 4.0
2018-05-16T15:45:58.573Z
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.)
Daniel Griscom
CC BY-SA 4.0
2018-05-16T18:16:10.967Z
Mind if I edit your answer to more clearly include the info in my comment? (Or, edit it yourself if you like.)
Daniel Griscom
CC BY-SA 4.0
2018-05-16T21:21:50.590Z
A confirming source: https://arxiv.org/pdf/1008.0815.pdf cites Gbp and Grp as "Gaia magnitudes".
Daniel Griscom
CC BY-SA 4.0
2018-05-16T15:07:05.123Z
2018-05-18T07:03:33.160Z
2018-05-18T07:03:33.160Z
ProfRob
<p>Magnitude decreases when it is brighter. So G_BP value goes larger, G_RP goes smaller when the star gets redder and less blue.</p>

CC BY-SA 4.0
2022-03-23T15:31:11.420Z
Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers [in the help center](/help/how-to-answer).
Community
CC BY-SA 4.0
2022-03-23T13:20:18.260Z
2022-03-23T13:20:18.260Z
Jerry Zhang
<p>As an enthusiastic amateur scientist, I'm plowing my way through a cool <a href="http://sci.esa.int/gaia/" rel="nofollow noreferrer">Gaia</a> <a href="https://www.cosmos.esa.int/web/gaia/dr2" rel="nofollow noreferrer">DR2</a>-based paper, <a href="https://arxiv.org/pdf/1805.05849.pdf" rel="nofollow noreferrer">"An Empirical Measurement of the Initial-Final Mass Relation with Gaia White Dwarfs"</a>. I'm doing all right, but have hit a roadblock: understanding the meaning of the stellar color value G<sub>BP</sub> - G<sub>RP</sub>.</p>

<p>G<sub>BP</sub> is the blue photometry value, and G<sub>RP</sub> is the red photometry value, so G<sub>BP</sub> - G<sub>RP</sub> 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:</p>

<p><a href="https://i.sstatic.net/yvmaD.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/yvmaD.png" alt="Gaia graph; absolute magnitude versus GBP - GRP"></a></p>

<p>It's a graph of white dwarf ("WD") color versus absolute magnitude. On the left, the absolute magnitude M<sub>G</sub> increases (downwards) as the star grows dimmer, and across the bottom the color G<sub>BP</sub> - G<sub>RP</sub> increases (rightwards) as the star grows redder. Hence the slope of the curve; as a WD cools, it gets smoothly dimmer and redder.</p>

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

<p>Should G<sub>BP</sub> - G<sub>RP</sub> get larger as a star cools and gets redder? Where am I confused? (I'm assuming the professionals aren't all confused...)</p>

<temperature><white-dwarf><gaia><absolute-magnitude>
Gaia Gbp - Grp: why does it get larger as star gets redder?
Daniel Griscom
# Gaia Gbp - Grp: why does it get larger as star gets redder?

(Asked by: username_0 on 2018-05-16)

As an enthusiastic amateur scientist, I'm plowing my way through a cool [Gaia](http://sci.esa.int/gaia/) [DR2](https://www.cosmos.esa.int/web/gaia/dr2)-based paper, ["An Empirical Measurement of the Initial-Final Mass Relation with Gaia White Dwarfs"](https://arxiv.org/pdf/1805.05849.pdf). 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](https://i.sstatic.net/yvmaD.png)](https://i.sstatic.net/yvmaD.png)

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...)

**Tags:** temperature, white-dwarf, gaia, absolute-magnitude

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## 2 answers

### Answer by username_1 on 2018-05-16. Score: 3 | Accepted answer

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.

> **Comments:**
> **username_0**: 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.)  
> **username_0**: Mind if I edit your answer to more clearly include the info in my comment? (Or, edit it yourself if you like.)  
> **username_0**: A confirming source: https://arxiv.org/pdf/1008.0815.pdf cites Gbp and Grp as "Gaia magnitudes".  

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### Answer by username_2 on 2022-03-23. Score: 0

Magnitude decreases when it is brighter. So G\_BP value goes larger, G\_RP goes smaller when the star gets redder and less blue.

> **Comments:**
> **username_3**: Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers [in the help center](/help/how-to-answer).  

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