<p>There are no exact boundaries in temperature, luminosity, surface gravity etc. for spectral classes because the classification system works in a different way - it is fundamentally an <em>empirical</em> system, with classification based only on the appearance of features in the spectra.</p>
<p>The Yerkes or Morgan-Keenan (MK) system is based <em>only</em> on a set of standard stars and their spectra, and the type of any other given star (that is not one of those standards) is based only on where that star’s spectral lines fit into the two-dimensional matrix of standards. The introduction of <a href="http://articles.adsabs.harvard.edu//full/1973ARA%26A..11...29M/0000029.000.html" rel="nofollow noreferrer">this Annual Reviews article</a> by Morgan and Keenan spells out the principle.</p>
<p>Of course those lines reflect physical effects
Iike temperature or luminosity (really primarily surface gravity), but those physical properties are not the basis of the classification.  And there are other factors that can change the underlying properties while preserving the appearance of the spectral lines.  For example in the intro of <a href="https://iopscience.iop.org/article/10.1086/319956/fulltext/" rel="nofollow noreferrer">this paper</a>, Gray discusses how stars of the same luminosity class can have different luminosities due to rotation.</p>
<p>Divorcing the classification from the physical properties is an interesting philosophical approach, but one that has stood the test of time even as our understanding of stars and their physical properties has improved in the past 70 years.  If you have time, I encourage you to read the first couple of pages of the review article linked above - the quote from Dimitri Mihalas (who was a leading theorist on stellar atmospheres) is especially interesting, I think.</p>

CC BY-SA 4.0
2020-08-15T14:11:00.983Z
2020-08-15T14:11:00.983Z
Eric Jensen
<p>Usually, the spectral classification is done analyzing the spectra of the star, we measure the depths of several key lines and compare them, each spectral type has his own characteristic relative intensities etc.
Is not a perfect boundary and some times the stars bounce from type to type depending on who classified it and what data is available (is not the same low-res than high-res spectra) for detailed information check <a href="http://adsabs.harvard.edu/abs/2014ApJS..211...10S" rel="nofollow noreferrer">Sota et al. 14</a>  and similar papers.</p>

CC BY-SA 4.0
2020-08-18T16:34:45.000Z
I used the first author name and year of the paper, Sota et al.14 and the link to the doi of the paper in ADS, but as you wish I'll put the full URL
Emberck
CC BY-SA 4.0
2020-08-18T02:37:23.680Z
2020-08-18T16:49:01.960Z
2020-08-18T16:49:01.960Z
Emberck
<p>How can I determine spectral type of star, if I know all its another properties? For Example, Rigel A has spectral type <code>B8Ia</code>.</p>
<p>Rigel has temperature 12 500 K. According to <a href="https://en.wikipedia.org/wiki/Stellar_classification#Harvard_spectral_classification" rel="noreferrer">Harvard spectral classification</a>, spectral class of this star is <code>B</code> (10 000-30 000 K) and subclass is <code>8</code> (12 000-14 000 K), so <code>B8</code>.</p>
<p>Are there any exact boundaries for <a href="https://en.wikipedia.org/wiki/Stellar_classification#Yerkes_spectral_classification" rel="noreferrer">Yerkes spectral classification</a> as well? How can I tell if a star is super giant (<code>I</code>), bright giant (<code>II</code>) or giant (<code>III</code>)? And then, how can I tell if it is bright (<code>a</code>), normal (<code>ab</code>) or faint (<code>b</code>)?</p>

<star><spectral-type>
Determine spectral type of star from its properties
Michal
# Determine spectral type of star from its properties

(Asked by: username_0 on 2020-08-11)

How can I determine spectral type of star, if I know all its another properties? For Example, Rigel A has spectral type `B8Ia`.

Rigel has temperature 12 500 K. According to [Harvard spectral classification](https://en.wikipedia.org/wiki/Stellar_classification#Harvard_spectral_classification), spectral class of this star is `B` (10 000-30 000 K) and subclass is `8` (12 000-14 000 K), so `B8`.

Are there any exact boundaries for [Yerkes spectral classification](https://en.wikipedia.org/wiki/Stellar_classification#Yerkes_spectral_classification) as well? How can I tell if a star is super giant (`I`), bright giant (`II`) or giant (`III`)? And then, how can I tell if it is bright (`a`), normal (`ab`) or faint (`b`)?

**Tags:** star, spectral-type


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## 2 answers

### Answer by username_1 on 2020-08-15. Score: 3 | Accepted answer

There are no exact boundaries in temperature, luminosity, surface gravity etc. for spectral classes because the classification system works in a different way - it is fundamentally an *empirical* system, with classification based only on the appearance of features in the spectra.

The Yerkes or Morgan-Keenan (MK) system is based *only* on a set of standard stars and their spectra, and the type of any other given star (that is not one of those standards) is based only on where that star’s spectral lines fit into the two-dimensional matrix of standards. The introduction of [this Annual Reviews article](http://articles.adsabs.harvard.edu//full/1973ARA%26A..11...29M/0000029.000.html) by Morgan and Keenan spells out the principle.

Of course those lines reflect physical effects
Iike temperature or luminosity (really primarily surface gravity), but those physical properties are not the basis of the classification. And there are other factors that can change the underlying properties while preserving the appearance of the spectral lines. For example in the intro of [this paper](https://iopscience.iop.org/article/10.1086/319956/fulltext/), Gray discusses how stars of the same luminosity class can have different luminosities due to rotation.

Divorcing the classification from the physical properties is an interesting philosophical approach, but one that has stood the test of time even as our understanding of stars and their physical properties has improved in the past 70 years. If you have time, I encourage you to read the first couple of pages of the review article linked above - the quote from Dimitri Mihalas (who was a leading theorist on stellar atmospheres) is especially interesting, I think.


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### Answer by username_2 on 2020-08-18. Score: 1

Usually, the spectral classification is done analyzing the spectra of the star, we measure the depths of several key lines and compare them, each spectral type has his own characteristic relative intensities etc.
Is not a perfect boundary and some times the stars bounce from type to type depending on who classified it and what data is available (is not the same low-res than high-res spectra) for detailed information check [Sota et al. 14](http://adsabs.harvard.edu/abs/2014ApJS..211...10S) and similar papers.

> **Comments:**
> **username_2**: I used the first author name and year of the paper, Sota et al.14 and the link to the doi of the paper in ADS, but as you wish I'll put the full URL  

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