## Harvard Spectral Classification Scheme
This is known as the **Harvard Spectral Classification Scheme**. Each spectral classification ([[#O]], [[#B]], [[#A]], [[#F]], [[#G]], [[#K]], [[#M]]) can be subdivided using a numeric digit with $0$ being hottest and $9$ being coolest.
> [!note]
> The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as:
> - $D$ for white dwarfs
> - $S$ and $C$ for carbon stars
> - $L$, $T$, and $Y$ for red and brown dwarf stars
> - [Many others...](https://en.wikipedia.org/wiki/Stellar_classification#Extended_spectral_types)
![[spectralClassification.png|align:center|600]]
![[spectralClassification2.jpeg|align:center|600]]
| <nobr>Class</nobr> | <nobr>Example</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})
lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
|:------------------:| -------------------- |:--------------------------------------:|:-------------------------:|:----------------------------------:|:------------------------------------:|:----------------------------------------:|:--------------------:|:-----------------------------------------:|:-----------------------------:|
| [[#O]] | | $\ge 33000$ | blue | $\ge 16$ | $\ge 6.6$ | $\ge 30000$ | Weak | $\ce{H}$, $\ce{HeI}$, $\ce{HeII}$ | $0.00003 \%$ |
| [[#B]] | | $10000 - 33000$ | bluish white | $2.1 - 16$ | $1.8 - 6.6$ | $25 - 30,000$ | Medium | $\ce{H}$, $\ce{HeI}$ | $0.12 \%$ |
| [[#A]] | | $7300 - 10000$ | white | $1.4 - 2.1$ | $1.4 - 1.8$ | $5 - 25$ | Strong | $\ce{H}$, $\ce{CaII}$ | $0.61 \%$ |
| [[#F]] | | $6000 - 7300$ | yellowish white | $1.04 - 1.4$ | $1.15 - 1.4$ | $1.5 - 5$ | Medium | $\ce{H}$, metals ($\ce{CaII}$, $\ce{Fe}$) | $3.0 \%$ |
| [[#G]] | | $5300 - 6000$ | yellow | $0.8 - 1.04$ | $0.96 - 1.15$ | $0.6 - 1.5$ | Weak | $\ce{H}$, metals, some molecules | $7.6 \%$ |
| [[#K]] | | $3900 - 5300$ | light orange | $0.45 - 0.8$ | $0.7 - 0.96$ | $0.08 - 0.6$ | Very weak | metals, some molecules | $12 \%$ |
| [[#M]] | | $2300 - 3900$ | orangish red | $0.08 - 0.45$ | $\le 0.7$ | $\le 0.08$ | Very weak | metals, some molecules | $76 \%$ |
| [[#L]] | | $1300-2300$ | | $\sim 0.05 - 0.45$ | | | | metals, some molecules | |
| [[#T]] | | $500-1300$ | | $\sim 0.05 - 0.45$ | | | | methane | |
| [[#Y]] | | lt; 500$ | | $\sim 0.05 - 0.45$ | | | | Unknown, but distinct | |
^spectral-type-table
![[Mnemonics#^b814bb]]
### O
| <nobr>Class</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
| :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: |
| [[#O]] | $\ge 33000$ | blue | $\ge 16$ | $\ge 6.6$ | $\ge 30000$ | Weak | $\ce{H}$, $\ce{HeI}$, $\ce{HeII}$ | $0.00003 \%$ |
**Characteristics:**
- Very hot and extremely luminous
- Burn hydrogen fuel quickly
- First stars to leave the main sequence
- Dominant emission in the ultraviolet range
- Very rare ($0.00003\%$) in solar neighborhood
- Strong spectral lines for
- $\ce{H}$ (Balmer Lines)
- $\ce{He I}$ & $\ce{He II}$
- prominent ionized ($\ce{Si IV}$, $\ce{O III}$, $\ce{N III}$, $\ce{C III}$)
- Do not retain extensive atmospheres/corona due to the extreme velocity of their stellar wind $(\sim 2000 \; {\rm km/s})$
- High metallicities
### B
| <nobr>Class</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
| :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: |
| [[#B]] | $10000 - 33000$ | bluish white | $2.1 - 16$ | $1.8 - 6.6$ | $25 - 30,000$ | Medium | $\ce{H}$, $\ce{HeI}$ | $0.12 \%$ |
**Characteristics:**
- Very hot and luminous
- Energy generation from the [[Carbon-Nitrogen-Oxygen Cycle|CNO cycle]]
- Very temperature sensitive process causing a convection zone in the core.
- Steady mixing of hydrogen and helium fuel
- Strong spectral lines
- $\ce{H}$ (medium strength)
- $\ce{He I}$
- Do not retain extensive atmospheres/corona due to the extreme velocity of their stellar wind $(\sim 3000 \; {\rm km/s})$
### A
| <nobr>Class</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
| :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: |
| [[#A]] | $7300 - 10000$ | white | $1.4 - 2.1$ | $1.4 - 1.8$ | $5 - 25$ | Strong | $\ce{H}$, $\ce{CaII}$ | $0.61 \%$ |
**Characteristics:**
- More common stars to the naked eye
- Strong spectral lines
- $\ce{H}$ (medium strength)
- $\ce{Ca II}$
- ionized metals ($\ce{Fe II}$, $\ce{Mg II}$, $\ce{Si II}$)
- Do not have convective zones and are not expected to harbor magnetic dynamos
- Do not have strong stellar winds and lack a means to generate X-ray emissions
### F
| <nobr>Class</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
| :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: |
| [[#F]] | $6000 - 7300$ | yellowish white | $1.04 - 1.4$ | $1.15 - 1.4$ | $1.5 - 5$ | Medium | $\ce{H}$, metals ($\ce{CaII}$, $\ce{Fe}$) | $3.0 \%$ |
**Characteristics:**
- Strong spectral lines
- $\ce{H}$ (weak strength)
- Ca II (H and K lines)
- Neutral metals ($\ce{Fe I}$, $\ce{Cr I}$)
- Ionized metal lines by late F
- Have a life-cycle similar to [[#G]]-type stars
- hydrogen-fusing >>> helium-fusing >>> carbon-fusing >>> planetary nebula >>> hot white dwarf
- White dwarf stable for $\sim 2 - 4 \times 10^{9} \; {\rm yr}$ (for [[#G]] stars: $\sim 10^{10} \; {\rm yr}$)
### G
| <nobr>Class</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
| :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: |
| [[#G]] | $5300 - 6000$ | yellow | $0.8 - 1.04$ | $0.96 - 1.15$ | $0.6 - 1.5$ | Weak | $\ce{H}$, metals, some molecules | $7.6 \%$ |
**Characteristics:**
- Strong spectral lines
- $\ce{H}$ (weaker strength)
- Neutral metals ($\ce{Mn I}$, $\ce{Fe I}$, $\ce{Si I}$)
- molecular bands of titanium oxide
- The [[sun]] is a G2 star
### K
| <nobr>Class</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
| :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: | :--: |
| [[#K]] | $3900 - 5300$ | light orange | $0.45 - 0.8$ | $0.7 - 0.96$ | $0.08 - 0.6$ | Very weak | metals, some molecules | $12 \%$ |
**Characteristics:**
- Strong spectral lines
- $\ce{H}$ (extremely weak, if present)
- Ca II (H and K lines)
- Neutral metals
- Ionized metals
- $\ce{CN}$ molecules
### M
| <nobr>Class</nobr> | <nobr>$T_{\rm eff}$ $({\rm K})lt;/nobr> | <nobr>Chromaticity</nobr> | <nobr>MS Mass $(M_{\odot})lt;/nobr> | <nobr>MS Radius $(R_{\odot})lt;/nobr> | <nobr>MS Luminosity $(L_{\odot})lt;/nobr> | <nobr>H Lines</nobr> | <nobr>Spectral Features</nobr> | <nobr>$\%$ of MS stars</nobr> |
| :----------------: | :------------------------------------: | :-----------------------: | :--------------------------------: | :----------------------------------: | :--------------------------------------: | :------------------: | :----------------------------: | :---------------------------: |
| [[#M]] | $2300 - 3900$ | orangish red | $0.08 - 0.45$ | $\le 0.7$ | $\le 0.08$ | Very weak | metals, some molecules | $76 \%$ |
**Characteristics:**
- The most common spectral type
- Such low luminosities that none are bright enough to be seen with the unaided eye
- Strong spectral lines
- $\ce{H}$ (absent)
- All neutral metals
- oxide molecules (visible spectrum)
## L
**Characteristics:**
- A more recent addition to the classification scheme
- Used to describe [[Stellar Classes#Red Dwarf]] stars
- Strong spectral lines
- metal hydrides
- alkali metals
## T
**Characteristics:**
- A more recent addition to the classification scheme
- Used to describe [[Stellar Classes#Brown Dwarf]] stars
- emission peaks in the [[Electromagnetic Spectrum|infrared]]
- Strong spectral lines
- Methane
## Y
**Characteristics:**
- A more recent addition to the classification scheme
- Used to describe cooler [[Stellar Classes#Brown Dwarf]] stars
- Only $\sim 17$ classified objects
- Strong spectral lines
- no well-defined spectral sequence yet and no prototypes
- display absorption around $1.55 \; {\rm \mu m}$ (maybe ammonia?)