<p>There is no function that converts <span class="math-container">$G$</span> to <span class="math-container">$V$</span> because the conversion depends on stellar temperature/colour.</p>
<p>For Gaia EDR3 data the most useful transform for you is
<span class="math-container">$$G-V=-0.02704 +0.01424(G_{bp}-G_{rp})-0.2156(G_{bp}-G_{rp})^2 + 0.01426(G_{bp}-G_{rp})^3 $$</span>
from <a href="https://gea.esac.esa.int/archive/documentation/GEDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html" rel="nofollow noreferrer">Table 5.7</a> of the EDR3 data guide.</p>
<p>Most sources with <span class="math-container">$G$</span> data will also have a <span class="math-container">$G_{bp}-G_{rp}$</span> colour.</p>
<p>Note that the DR2 and EDR3 photometric systems are slightly different.</p>
<p>For Gaia DR3 the transform is the same as for EDR3, from <a href="https://gea.esac.esa.int/archive/documentation/GDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html" rel="nofollow noreferrer">Table 5.9</a> of the DR3 documentation.</p>

CC BY-SA 4.0
2022-02-18T19:05:02.860Z
Thank you, that's it! I tested the equation on the entries having $G$, $V$, $G_{BP}$, and $G_{RP}$. I got 164 entries with a deviation < 0.1m; 60 entries <0.5m; 15 entries <1m; 9 entries > 1m deviation.
CR131873
CC BY-SA 4.0
2022-02-17T21:18:29.340Z
2022-06-13T11:38:36.257Z
2022-06-13T11:38:36.257Z
ProfRob
<p>To convert from G to V magnitude, use the following formula:
<span class="math-container">$$G-V=-0.0176 -0.00686(G_{bp}-G_{rp})-0.1732(G_{bp}-G_{rp})^2$$</span></p>
<p>From <a href="https://gea.esac.esa.int/archive/documentation/GDR2/Data_processing/chap_cu5pho/sec_cu5pho_calibr/ssec_cu5pho_PhotTransf.html#Ch5.T8" rel="nofollow noreferrer">Table 5.8</a> of the Gaia DR2 data release documentation.</p>

CC BY-SA 4.0
2022-02-17T11:42:51.710Z
Please add some context to your answer including defining the variables used and if possible a reference for the source of this expression.  Please also consider formatting you equation with [MathJax](https://math.meta.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-reference).
GrapefruitIsAwesome
CC BY-SA 4.0
2022-02-17T14:55:25.833Z
I'm Guessing $ Gbp $ and $ Grp $ should really be $ G_{bp} $ and $ G_{rp} $ ?
Greg Miller
CC BY-SA 4.0
2022-02-17T09:23:20.730Z
2022-02-17T20:18:38.283Z
2022-02-17T20:18:38.283Z
ShouldDoAsFirstApprox
<p><strong>Question:</strong> How can I convert the Gaia-magnitude <em>G</em> (and <em>G_ESTIMATE</em>) into a <em>V</em>-magnitude? The solution probably can be found in the Gaia Data Release Documentation for Gaia's EDR3 (&quot;Early Data Release 3&quot;), specifically section 5.5.1, <a href="https://gea.esac.esa.int/archive/documentation/GEDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html" rel="noreferrer">Relationships with other photometric systems</a>, but I have to admit, that I can't make heads or tails of it.</p>
<p><strong>Related Question:</strong> The question <em><a href="https://astronomy.stackexchange.com/questions/44670/relationships-between-g-magnitude-johnson-v-magnitude-and-spectral-type-of-sta">Relationships between G magnitude, Johnson V magnitude, and spectral type of stars?</a></em> is related in that it wants to explore the relationship between <em>G</em> and Johnson-<em>V</em>. However, my separate question is not a &quot;duplicate&quot;, because the other question has the relation between the magnitudes and spectral type as its focus (perhaps erroneously, shouldn't the relation be between a colour index and spectral type?).</p>
<p><strong>Background:</strong> I am exploring the solar neighbourhood based on the paper <em>The 10 parsec sample in the Gaia era</em> (see the <a href="https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/650/A201" rel="noreferrer">CDS</a>-entry). Among other things I want to know the objects' respective brightnesses and what is visible to the naked eye (or a specific instrument). For this I want to use the &quot;V&quot;-filter (column &quot;V&quot; in the catalogue to the paper). A bit more than 50% of the non-planetary objects in the catalogue have a V-mag listed. Most of the remaining non-planetary objects (stars, low-mass stars, white dwarfs, brown dwarfs) have either a Gaia-magnitude <em>G</em> or an estimated Gaia-magnitude <em>G_ESTIMATE</em>. Therefore I need a way to convert <em>G</em> to <em>V</em>. The conversion probably not straight-forward since <span class="math-container">$G$</span> is a band covering wavelengths 330–1050 nm, and <span class="math-container">$V$</span> seems to cover only 470-700 nm.</p>
<p><strong>Looking for:</strong> I need an equation I can use in a spreadsheet program to convert <em>G</em>/<em>G_ESTIMATE</em> to <em>V</em>. Is there one, or does conversion require more information than is given in the 10 parsec-catalog?
Thank you for your help!</p>

<magnitude><gaia>
How can I convert a GAIA-magnitude "G" to Johnson "V"?
CR131873
# How can I convert a GAIA-magnitude "G" to Johnson "V"?

(Asked by: username_0 on 2022-02-13)

**Question:** How can I convert the Gaia-magnitude *G* (and *G\_ESTIMATE*) into a *V*-magnitude? The solution probably can be found in the Gaia Data Release Documentation for Gaia's EDR3 ("Early Data Release 3"), specifically section 5.5.1, [Relationships with other photometric systems](https://gea.esac.esa.int/archive/documentation/GEDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html), but I have to admit, that I can't make heads or tails of it.

**Related Question:** The question *[Relationships between G magnitude, Johnson V magnitude, and spectral type of stars?](https://astronomy.stackexchange.com/questions/44670/relationships-between-g-magnitude-johnson-v-magnitude-and-spectral-type-of-sta)* is related in that it wants to explore the relationship between *G* and Johnson-*V*. However, my separate question is not a "duplicate", because the other question has the relation between the magnitudes and spectral type as its focus (perhaps erroneously, shouldn't the relation be between a colour index and spectral type?).

**Background:** I am exploring the solar neighbourhood based on the paper *The 10 parsec sample in the Gaia era* (see the [CDS](https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/650/A201)-entry). Among other things I want to know the objects' respective brightnesses and what is visible to the naked eye (or a specific instrument). For this I want to use the "V"-filter (column "V" in the catalogue to the paper). A bit more than 50% of the non-planetary objects in the catalogue have a V-mag listed. Most of the remaining non-planetary objects (stars, low-mass stars, white dwarfs, brown dwarfs) have either a Gaia-magnitude *G* or an estimated Gaia-magnitude *G\_ESTIMATE*. Therefore I need a way to convert *G* to *V*. The conversion probably not straight-forward since $G$ is a band covering wavelengths 330–1050 nm, and $V$ seems to cover only 470-700 nm.

**Looking for:** I need an equation I can use in a spreadsheet program to convert *G*/*G\_ESTIMATE* to *V*. Is there one, or does conversion require more information than is given in the 10 parsec-catalog?
Thank you for your help!

**Tags:** magnitude, gaia


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## 2 answers

### Answer by username_1 on 2022-02-17. Score: 4 | Accepted answer

There is no function that converts $G$ to $V$ because the conversion depends on stellar temperature/colour.

For Gaia EDR3 data the most useful transform for you is
$$G-V=-0.02704 +0.01424(G\_{bp}-G\_{rp})-0.2156(G\_{bp}-G\_{rp})^2 + 0.01426(G\_{bp}-G\_{rp})^3 $$
from [Table 5.7](https://gea.esac.esa.int/archive/documentation/GEDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html) of the EDR3 data guide.

Most sources with $G$ data will also have a $G\_{bp}-G\_{rp}$ colour.

Note that the DR2 and EDR3 photometric systems are slightly different.

For Gaia DR3 the transform is the same as for EDR3, from [Table 5.9](https://gea.esac.esa.int/archive/documentation/GDR3/Data_processing/chap_cu5pho/cu5pho_sec_photSystem/cu5pho_ssec_photRelations.html) of the DR3 documentation.

> **Comments:**
> **username_0**: Thank you, that's it! I tested the equation on the entries having $G$, $V$, $G_{BP}$, and $G_{RP}$. I got 164 entries with a deviation < 0.1m; 60 entries <0.5m; 15 entries <1m; 9 entries > 1m deviation.  

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### Answer by username_2 on 2022-02-17. Score: 2

To convert from G to V magnitude, use the following formula:
$$G-V=-0.0176 -0.00686(G\_{bp}-G\_{rp})-0.1732(G\_{bp}-G\_{rp})^2$$

From [Table 5.8](https://gea.esac.esa.int/archive/documentation/GDR2/Data_processing/chap_cu5pho/sec_cu5pho_calibr/ssec_cu5pho_PhotTransf.html#Ch5.T8) of the Gaia DR2 data release documentation.

> **Comments:**
> **username_3**: Please add some context to your answer including defining the variables used and if possible a reference for the source of this expression.  Please also consider formatting you equation with [MathJax](https://math.meta.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-reference).  
> **username_4**: I'm Guessing $ Gbp $ and $ Grp $ should really be $ G_{bp} $ and $ G_{rp} $ ?  

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