<p>Spectral type is determined by (and is an indicator of) photospheric temperature and surface gravity. The classification of G2V applies to a star with a photospheric temperature of about 5800K and a surface gravity of about 100-1000 m/s<span class="math-container">$^{2}$</span>.</p>

<p>The Sun's temperature and gravity have not changed greatly since it first appeared on the "zero age main sequence" - it has got a little bigger and more luminous and a little bit hotter (by about 170 K- <a href="https://academic.oup.com/mnras/article/386/1/155/977315" rel="noreferrer">Schroder &amp; Smith 2008</a>) - so maybe it was a G3V-G4V star at the zero age main sequence (<a href="http://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt" rel="noreferrer">Mamajek 2018</a>).</p>

<p>However, there were much bigger changes during the 20 million years or so before the Sun reached the ZAMS. It was cooler and briefly much larger. This would be reflected in a spectral classification that was something like K5IV- M0IV; indicating a subgiant-like gravity and a temperature of about 4000K. As it aged, it would then shrink and become hotter, passing through the G5V-K5V spectral types before reaching the main sequence.</p>

<p>If you can synthesise these two diagrams in your head, you can see what I mean.</p>

<p><em>Pre main sequence evolution in the Hertzsprung-Russell diagram</em>
<a href="https://i.sstatic.net/cWQNt.png" rel="noreferrer"><img src="https://i.sstatic.net/cWQNt.png" alt="PMS evolution"></a></p>

<p><em>Spectral types on the Hertzsprung-Russell diagram</em>
<a href="https://i.sstatic.net/IZVtC.jpg" rel="noreferrer"><img src="https://i.sstatic.net/IZVtC.jpg" alt="Spectral type HR"></a></p>

CC BY-SA 4.0
2018-10-24T23:13:21.890Z
2018-10-24T23:13:21.890Z
ProfRob
<p>It is true that on the main sequence, where most stars spend most of their lifetimes, mass is the main determining factor of all stellar parameters, with metallicity and angular momentum distant second-most important factors.</p>

<p>However when a star is born, from the moment it becomes detectable, it moves through several stages of structural evolution, which surely do not classify as the stellar class that it later has on the main sequence. </p>

<p>The evolutionary stages of this pre-main-sequence (PMS) star can be described in the astronomer's favourite diagram - the <a href="https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram" rel="nofollow noreferrer">HR-diagram</a> - via the <a href="https://en.wikipedia.org/wiki/Hayashi_track" rel="nofollow noreferrer">Hayashi track</a>.
The Hayashi track is for any star of given mass the sequence of colour-luminosity values that it attains during its PMS evolution.</p>

<p>As colour gives the stellar class, there is an evolution in stellar class for all stars, even on the main sequence.</p>

<p>However </p>

<blockquote>
  <p>since its birth, the Sun has brightened significantly.  </p>
</blockquote>

<p>is incorrect. Once the sun arrived at the main sequence, it still brightened, yes, but that means there was an increase in Luminosity. As the <a href="https://en.wikipedia.org/wiki/Sun#Life_phases" rel="nofollow noreferrer">wiki page on the sun's evolution</a> clarifies in the figure seen below, the temperature however didn't change much during that time, therefore also not the stellar class.   </p>

<p><a href="https://i.sstatic.net/pB4Ac.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/pB4Ac.png" alt="enter image description here"></a></p>

<p><strong>Summarizing</strong>
During the pre-main-sequence of a star the stellar class evolves significantly, on the main sequence it doesn't.</p>

CC BY-SA 4.0
2018-10-24T23:20:37.953Z
2018-10-24T23:20:37.953Z
AtmosphericPrisonEscape
<p>Mass is one of the major factors that determine the spectral class of a star when it is born. </p>

<p>During a star's protostar stages, a spectral class determines the length of its protostar stage and other factors such as its lifetime. Even though a spectral class is not physically assigned to them, the life stages that they undergo are essentially determined by their spectral class at birth.</p>

<p>So even though protostars don't have a spectral class determined by scientists, the star that it ends up becoming in its main sequence is really what its spectral class is.</p>

<p>So in this case, the Sun in it's protostar stages still was on the G2V route - it was taking the same (or at least similar) life cycles as stars in the G2V spectral class.</p>

<p>Hope that helps!</p>

<hr>

<p>To the 2nd part:
Check out this website, especially from page 18: <a href="http://www.astro.umass.edu/~myun/teaching/a100_old/Astro100Mar25.pdf" rel="nofollow noreferrer">http://www.astro.umass.edu/~myun/teaching/a100_old/Astro100Mar25.pdf</a>. There's a lot of equations on there.</p>

<p>There's also more information here on the luminosity-mass relationship: <a href="https://www.e-education.psu.edu/astro801/content/l7_p3.html" rel="nofollow noreferrer">https://www.e-education.psu.edu/astro801/content/l7_p3.html</a></p>

<p>On a side note, looks like someone asked about the luminosity+temperature and mass relationship on here as well: <a href="https://physics.stackexchange.com/questions/12589/are-there-formulae-for-calculating-stellar-luminosity-and-effective-temperature">https://physics.stackexchange.com/questions/12589/are-there-formulae-for-calculating-stellar-luminosity-and-effective-temperature</a></p>

<p>If I find more information, I'll comment it or edit it onto my answer. Hope that helps!</p>

CC BY-SA 4.0
2018-10-24T22:43:41.177Z
Spectral type is measured from a spectrum. It is determined by temperature and to a lesser extent surface gravity. It is related to mass but not determined by it.
ProfRob
CC BY-SA 4.0
2018-10-24T23:39:58.840Z
Oh I didn't know that... I read from somewhere that the mass affects spectral class as well... Must have been incorrect... Looks like your answer seems to have it right - I will upvote it :)
MystaryPi
CC BY-SA 4.0
2018-10-24T23:43:45.540Z
Mass only affects spectral type insofar as it affects gravity which affects the spectrum.
ProfRob
CC BY-SA 4.0
2018-10-24T23:48:58.160Z
Oh... So it does affect spectral class, it's just not the main cause of it. Thank you so much for helping me understand :)
MystaryPi
CC BY-SA 4.0
2018-10-25T06:34:01.400Z
Two stars of different mass, but the same temperature and gravity would have the same spectral type. Other permutations of the same sentence would not.
ProfRob
CC BY-SA 4.0
2018-10-28T14:17:55.227Z
Also note that the above link about the M-L relationship gives the oft-seen yet completely wrong explanation "Since higher mass means a larger gravitational force, higher mass must also mean that higher pressure is required to maintain equilibrium. If you increase the pressure inside a star, the temperature will also increase. So, the cores of massive stars have significantly higher temperatures than the cores of Sun-like stars."  High mass MS stars have weak gravity and low pressure, so the explanation is wrong.  M-L has little to do with nuclear fusion (stars have that L prior to the MS).
Ken G
CC BY-SA 4.0
2018-10-24T22:24:37.467Z
2018-10-24T22:35:08.587Z
2018-10-24T22:35:08.587Z
MystaryPi
<p>The Sun currently has a stellar class of G2V. However, since its birth, the Sun has brightened significantly. What would the Sun's class have been when it was born ~4.6 gyr ago?</p>

CC BY-SA 4.0
2018-10-24T22:03:50.420Z
Since the spectral class is really determined at birth from its mass, its spectral type has always been G2V - the path that it takes throughout is lifetime is similar to the paths taken by other G2V stars... I don't think that the spectral class system really applies to protostars.
MystaryPi
CC BY-SA 4.0
2018-10-24T22:14:43.463Z
@MystaryPi Ok, interesting. So stellar classification is less an indication of surface temperature/luminosity, but rather determined strictly by mass?
Arkenstein XII
CC BY-SA 4.0
2018-10-24T22:16:40.143Z
Mass is one of the major factors that determine the spectral class of a star when they're born.
MystaryPi
CC BY-SA 4.0
2018-10-24T22:20:56.540Z
@MystaryPi Thanks for your answer. Do you know whether it is possible to determine a star's Surface Temperature and Luminosity if you know its Mass and Age?
Arkenstein XII
CC BY-SA 4.0
2018-10-24T22:26:00.623Z
I have posted my answer below. Hm.. I'm not sure about that, I'll do some research and perhaps edit my answer.
MystaryPi
CC BY-SA 4.0
2018-10-24T22:42:01.100Z
Mass does not determine spectral type, since for almost all stars the mass is unknown (other than by appealing to relationships between mass and spectral type!)
ProfRob
<the-sun><stellar-evolution><classification>
Sun's Stellar Class at Birth?
Arkenstein XII
# Sun's Stellar Class at Birth?

(Asked by: username_0 on 2018-10-24)

The Sun currently has a stellar class of G2V. However, since its birth, the Sun has brightened significantly. What would the Sun's class have been when it was born ~4.6 gyr ago?

**Tags:** the-sun, stellar-evolution, classification

> **Comments:**
> **username_1**: Since the spectral class is really determined at birth from its mass, its spectral type has always been G2V - the path that it takes throughout is lifetime is similar to the paths taken by other G2V stars... I don't think that the spectral class system really applies to protostars.  
> **username_0**: @username_1 Ok, interesting. So stellar classification is less an indication of surface temperature/luminosity, but rather determined strictly by mass?  
> **username_1**: Mass is one of the major factors that determine the spectral class of a star when they're born.  
> **username_0**: @username_1 Thanks for your answer. Do you know whether it is possible to determine a star's Surface Temperature and Luminosity if you know its Mass and Age?  
> **username_1**: I have posted my answer below. Hm.. I'm not sure about that, I'll do some research and perhaps edit my answer.  
> **username_2**: Mass does not determine spectral type, since for almost all stars the mass is unknown (other than by appealing to relationships between mass and spectral type!)  

---

## 3 answers

### Answer by username_2 on 2018-10-24. Score: 8

Spectral type is determined by (and is an indicator of) photospheric temperature and surface gravity. The classification of G2V applies to a star with a photospheric temperature of about 5800K and a surface gravity of about 100-1000 m/s$^{2}$.

The Sun's temperature and gravity have not changed greatly since it first appeared on the "zero age main sequence" - it has got a little bigger and more luminous and a little bit hotter (by about 170 K- [Schroder & Smith 2008](https://academic.oup.com/mnras/article/386/1/155/977315)) - so maybe it was a G3V-G4V star at the zero age main sequence ([Mamajek 2018](http://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt)).

However, there were much bigger changes during the 20 million years or so before the Sun reached the ZAMS. It was cooler and briefly much larger. This would be reflected in a spectral classification that was something like K5IV- M0IV; indicating a subgiant-like gravity and a temperature of about 4000K. As it aged, it would then shrink and become hotter, passing through the G5V-K5V spectral types before reaching the main sequence.

If you can synthesise these two diagrams in your head, you can see what I mean.

*Pre main sequence evolution in the Hertzsprung-Russell diagram*
[![PMS evolution](https://i.sstatic.net/cWQNt.png)](https://i.sstatic.net/cWQNt.png)

*Spectral types on the Hertzsprung-Russell diagram*
[![Spectral type HR](https://i.sstatic.net/IZVtC.jpg)](https://i.sstatic.net/IZVtC.jpg)


---

### Answer by username_3 on 2018-10-24. Score: 2

It is true that on the main sequence, where most stars spend most of their lifetimes, mass is the main determining factor of all stellar parameters, with metallicity and angular momentum distant second-most important factors.

However when a star is born, from the moment it becomes detectable, it moves through several stages of structural evolution, which surely do not classify as the stellar class that it later has on the main sequence.

The evolutionary stages of this pre-main-sequence (PMS) star can be described in the astronomer's favourite diagram - the [HR-diagram](https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram) - via the [Hayashi track](https://en.wikipedia.org/wiki/Hayashi_track).
The Hayashi track is for any star of given mass the sequence of colour-luminosity values that it attains during its PMS evolution.

As colour gives the stellar class, there is an evolution in stellar class for all stars, even on the main sequence.

However

> since its birth, the Sun has brightened significantly.

is incorrect. Once the sun arrived at the main sequence, it still brightened, yes, but that means there was an increase in Luminosity. As the [wiki page on the sun's evolution](https://en.wikipedia.org/wiki/Sun#Life_phases) clarifies in the figure seen below, the temperature however didn't change much during that time, therefore also not the stellar class.

[![enter image description here](https://i.sstatic.net/pB4Ac.png)](https://i.sstatic.net/pB4Ac.png)

**Summarizing**
During the pre-main-sequence of a star the stellar class evolves significantly, on the main sequence it doesn't.


---

### Answer by username_1 on 2018-10-24. Score: 1

Mass is one of the major factors that determine the spectral class of a star when it is born.

During a star's protostar stages, a spectral class determines the length of its protostar stage and other factors such as its lifetime. Even though a spectral class is not physically assigned to them, the life stages that they undergo are essentially determined by their spectral class at birth.

So even though protostars don't have a spectral class determined by scientists, the star that it ends up becoming in its main sequence is really what its spectral class is.

So in this case, the Sun in it's protostar stages still was on the G2V route - it was taking the same (or at least similar) life cycles as stars in the G2V spectral class.

Hope that helps!

---

To the 2nd part:
Check out this website, especially from page 18: <http://www.astro.umass.edu/~myun/teaching/a100_old/Astro100Mar25.pdf>. There's a lot of equations on there.

There's also more information here on the luminosity-mass relationship: <https://www.e-education.psu.edu/astro801/content/l7_p3.html>

On a side note, looks like someone asked about the luminosity+temperature and mass relationship on here as well: <https://physics.stackexchange.com/questions/12589/are-there-formulae-for-calculating-stellar-luminosity-and-effective-temperature>

If I find more information, I'll comment it or edit it onto my answer. Hope that helps!

> **Comments:**
> **username_2**: Spectral type is measured from a spectrum. It is determined by temperature and to a lesser extent surface gravity. It is related to mass but not determined by it.  
> **username_1**: Oh I didn't know that... I read from somewhere that the mass affects spectral class as well... Must have been incorrect... Looks like your answer seems to have it right - I will upvote it :)  
> **username_2**: Mass only affects spectral type insofar as it affects gravity which affects the spectrum.  
> **username_1**: Oh... So it does affect spectral class, it's just not the main cause of it. Thank you so much for helping me understand :)  
> **username_2**: Two stars of different mass, but the same temperature and gravity would have the same spectral type. Other permutations of the same sentence would not.  
> **username_4**: Also note that the above link about the M-L relationship gives the oft-seen yet completely wrong explanation "Since higher mass means a larger gravitational force, higher mass must also mean that higher pressure is required to maintain equilibrium. If you increase the pressure inside a star, the temperature will also increase. So, the cores of massive stars have significantly higher temperatures than the cores of Sun-like stars."  High mass MS stars have weak gravity and low pressure, so the explanation is wrong.  M-L has little to do with nuclear fusion (stars have that L prior to the MS).  

---

