<p>With an absolute magnitude of zero and a colour (any colour) of zero, your stars are around spectral type A0 (by definition). The only thing that can confuse this is reddening/extinction.</p>

<p>If there is appreciable reddening/extinction, then your stars could be more luminous and hotter (O/B stars), but there isn't any way to tell from your data.</p>

CC BY-SA 4.0
2018-05-13T19:03:19.553Z
2018-05-13T19:03:19.553Z
ProfRob
<p>Recently I took pictures of the NGC 884 cluster in the B and I bands and selected some 30-what stars from them. I've reduced the data to a colour-magnitude diagram (absolute magnitude in the I band $M_I$ vs. (B-I) colour index). I am now asked to designate where the stars I've selected from this cluster are located on the Hertzsprung-Russell diagram and the approximate age of the stars.</p>

<p>My problem is the following: any HR diagram that I can find is either expressed in effective temperature instead of colour index, or is in the colour index (B-V). Also I can only seem to find empirical formulas for the effective temperature that use the (B-V) colour index, not the (B-I) one.</p>

<p>Does anyone know where to find an HR diagram that uses the (B-I) colour index?</p>

<p><a href="https://i.sstatic.net/izyMF.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/izyMF.png" alt="absolute mag vs. (B-I) colour index"></a></p>

<amateur-observing><data-analysis><hr-diagram>
Locating a star cluster on a Hertzsprung-Russell diagram with other color index
sivlomme
# Locating a star cluster on a Hertzsprung-Russell diagram with other color index

(Asked by: username_0 on 2018-05-12)

Recently I took pictures of the NGC 884 cluster in the B and I bands and selected some 30-what stars from them. I've reduced the data to a colour-magnitude diagram (absolute magnitude in the I band $M\_I$ vs. (B-I) colour index). I am now asked to designate where the stars I've selected from this cluster are located on the Hertzsprung-Russell diagram and the approximate age of the stars.

My problem is the following: any HR diagram that I can find is either expressed in effective temperature instead of colour index, or is in the colour index (B-V). Also I can only seem to find empirical formulas for the effective temperature that use the (B-V) colour index, not the (B-I) one.

Does anyone know where to find an HR diagram that uses the (B-I) colour index?

[![absolute mag vs. (B-I) colour index](https://i.sstatic.net/izyMF.png)](https://i.sstatic.net/izyMF.png)

**Tags:** amateur-observing, data-analysis, hr-diagram

---

## 1 answers

### Answer by username_1 on 2018-05-13. Score: 2

With an absolute magnitude of zero and a colour (any colour) of zero, your stars are around spectral type A0 (by definition). The only thing that can confuse this is reddening/extinction.

If there is appreciable reddening/extinction, then your stars could be more luminous and hotter (O/B stars), but there isn't any way to tell from your data.


---

