<p><span class="math-container">$\eta$</span> Cassiopeiae A has an estimated mass of 0.972 M<span class="math-container">$_\odot$</span>, an estimated temperature of 5973 K, and a B-V color index of ~0.58<a href="https://en.wikipedia.org/wiki/Eta_Cassiopeiae" rel="nofollow noreferrer">[1]</a>. In addition to the spectrum of the star, we look at these and other properties when attempting to classify main sequence stars. You can see a table <a href="https://en.wikipedia.org/wiki/G-type_main-sequence_star" rel="nofollow noreferrer">[here]</a> which shows the bulk properties of each spectral type that we can use in addition to spectrum to help classify the star. The mass estimate places it into the G5V category, while the temperature and B-V place it definitively into the G0V category<a href="https://en.wikipedia.org/wiki/G-type_main-sequence_star" rel="nofollow noreferrer">[2]</a>. Hence, I would go with G0V. The mass is a little on the light side for the G0V category, but the temperature and color index and right on for the characteristic values for G0V.</p>

CC BY-SA 4.0
2021-01-06T14:41:15.757Z
@antispinwards this is a great point. I updated my answer to emphasize that the bulk properties are a secondary classifier to the observed spectrum of the star, and linked to a table showing the relationship between spectral type and various properties. In addition, I think the confusion in OPs question is coming from classification ambiguities in the bulk properties, not the observed spectrum.
Joseph Farah
CC BY-SA 4.0
2021-01-06T14:46:12.227Z
@uhoh G0V and F9V are very close classifications, but I would choose G0V because F-type stars typically have masses greater than 1 M$_\odot$. antispinwards correctly points out that really the observed spectrum is the primary classifier of a star's spectral type; but we can have conversations like this because other properties of the star also slot nicely into the same categories. When you have a bunch of ways to classify stars, you can get ambiguities like in OP's question if some bulk properties point to one classification and other properties point to another.
Joseph Farah
CC BY-SA 4.0
2021-01-06T05:14:53.757Z
This is all over my head, but "...I would go with..." suggest that neither is conclusively wrong, and to the question "Eta Cassiopeiae star type; G0V? F9V? Both?" if you had to choose, would "Both, but with a preference" be your choice? Or do you think that G5V is wrong? What about the OP's suggestion of F9V, is that wrong?
uhoh
CC BY-SA 4.0
2021-01-06T04:17:17.037Z
2021-01-06T14:38:11.100Z
2021-01-06T14:38:11.100Z
Joseph Farah
<p>The spectral type of a star is determined by looking at its spectrum. Sometimes authors will use other, approximate, relationships between spectral type and colour or mass, or they will look at the spectrum compared with standard templates in different wavelength regions.</p>
<p>These are all possible reasons why different sources might suggest slightly different spectral types. I would doubt there is any strong reason to prefer one classification over another when they differ by just one subtype.</p>

CC BY-SA 4.0
2021-01-06T08:05:06.310Z
2021-01-06T08:05:06.310Z
ProfRob
<p>For a story I am looking into the star system of Eta Cassiopeiae, which is a binary with a main sequence star called Achird (very similar to the sun) and a K7V star.</p>
<p>The issue I am running into is the discrepancies of what the star type is; some say it is a G0V and some say it is a F9V.</p>
<p>I've looked at a dozen sources so I won't link anything, but I am wondering if anyone might know the most reliable and or most current source to go off of.</p>

CC BY-SA 4.0
2021-01-05T22:46:17.397Z
Technically, they're almost the same. Wikipedia uses G0V with respect to Eta Cas A
WarpPrime
CC BY-SA 4.0
2021-01-05T23:26:32.343Z
I've even seen it called up as a G3V. The most common information I have found puts it at slightly less massive than the sun, slightly larger and about 1.25 times the luminosity. And a very close temperature to the sun.
Markitect
<star><binary-star><star-systems><star-catalogues><stellar-classification>
Eta Cassiopeiae star type; G0V? F9V? Both?
Markitect
# Eta Cassiopeiae star type; G0V? F9V? Both?

(Asked by: username_0 on 2021-01-05)

For a story I am looking into the star system of Eta Cassiopeiae, which is a binary with a main sequence star called Achird (very similar to the sun) and a K7V star.

The issue I am running into is the discrepancies of what the star type is; some say it is a G0V and some say it is a F9V.

I've looked at a dozen sources so I won't link anything, but I am wondering if anyone might know the most reliable and or most current source to go off of.

**Tags:** star, binary-star, star-systems, star-catalogues, stellar-classification

> **Comments:**
> **username_1**: Technically, they're almost the same. Wikipedia uses G0V with respect to Eta Cas A  
> **username_0**: I've even seen it called up as a G3V. The most common information I have found puts it at slightly less massive than the sun, slightly larger and about 1.25 times the luminosity. And a very close temperature to the sun.  

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## 2 answers

### Answer by username_2 on 2021-01-06. Score: 2 | Accepted answer

$\eta$ Cassiopeiae A has an estimated mass of 0.972 M$\_\odot$, an estimated temperature of 5973 K, and a B-V color index of ~0.58[[1]](https://en.wikipedia.org/wiki/Eta_Cassiopeiae). In addition to the spectrum of the star, we look at these and other properties when attempting to classify main sequence stars. You can see a table [[here]](https://en.wikipedia.org/wiki/G-type_main-sequence_star) which shows the bulk properties of each spectral type that we can use in addition to spectrum to help classify the star. The mass estimate places it into the G5V category, while the temperature and B-V place it definitively into the G0V category[[2]](https://en.wikipedia.org/wiki/G-type_main-sequence_star). Hence, I would go with G0V. The mass is a little on the light side for the G0V category, but the temperature and color index and right on for the characteristic values for G0V.

> **Comments:**
> **username_3**: This is all over my head, but "...I would go with..." suggest that neither is conclusively wrong, and to the question "Eta Cassiopeiae star type; G0V? F9V? Both?" if you had to choose, would "Both, but with a preference" be your choice? Or do you think that G5V is wrong? What about the OP's suggestion of F9V, is that wrong?  
> **username_2**: @antispinwards this is a great point. I updated my answer to emphasize that the bulk properties are a secondary classifier to the observed spectrum of the star, and linked to a table showing the relationship between spectral type and various properties. In addition, I think the confusion in OPs question is coming from classification ambiguities in the bulk properties, not the observed spectrum.  
> **username_2**: @username_3 G0V and F9V are very close classifications, but I would choose G0V because F-type stars typically have masses greater than 1 M$_\odot$. antispinwards correctly points out that really the observed spectrum is the primary classifier of a star's spectral type; but we can have conversations like this because other properties of the star also slot nicely into the same categories. When you have a bunch of ways to classify stars, you can get ambiguities like in OP's question if some bulk properties point to one classification and other properties point to another.  

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### Answer by username_4 on 2021-01-06. Score: 1

The spectral type of a star is determined by looking at its spectrum. Sometimes authors will use other, approximate, relationships between spectral type and colour or mass, or they will look at the spectrum compared with standard templates in different wavelength regions.

These are all possible reasons why different sources might suggest slightly different spectral types. I would doubt there is any strong reason to prefer one classification over another when they differ by just one subtype.


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