<p>In order to be in hydrostatic equilibrium, high surface gravity atmospheres have higher pressures at the same sort of temperatures. That means the density of atoms and ions is higher.</p>

<p>If the particle density is higher then there is more chance of interactions between the particles. In particular, collisions between atoms perturb their energy levels and reduce the lifetimes of atoms in particular states. The natural frequency width is proportional to the reciprocal of the radiative lifetime. If this lifetime is truncated due to frequent collisional (de)excitation, then the lifetime is shortened and the frequency width increased. This is known as <a href="http://www.fas.harvard.edu/~scidemos/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html" rel="noreferrer">collisional broadening</a>.</p>

CC BY-SA 3.0
2017-05-03T07:24:56.847Z
2017-05-03T07:24:56.847Z
ProfRob
<p>It's always made intuitive sense to me that a higher surface gravity leads to broader spectral lines from a star, but yesterday I realized that I don't have a physical explanation for why this is so.  Why is this so?</p>

CC BY-SA 3.0
2017-05-03T06:11:21.977Z
Duplicate of https://physics.stackexchange.com/questions/257375/different-width-of-spectral-lines-for-different-groups-of-stars  ?
DJohnM
<star><spectra>
Why does higher surface gravity broaden spectral lines?
NeutronStar
# Why does higher surface gravity broaden spectral lines?

(Asked by: username_0 on 2017-05-02)

It's always made intuitive sense to me that a higher surface gravity leads to broader spectral lines from a star, but yesterday I realized that I don't have a physical explanation for why this is so. Why is this so?

**Tags:** star, spectra

> **Comments:**
> **username_1**: Duplicate of https://physics.stackexchange.com/questions/257375/different-width-of-spectral-lines-for-different-groups-of-stars  ?  

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## 1 answers

### Answer by username_2 on 2017-05-03. Score: 11

In order to be in hydrostatic equilibrium, high surface gravity atmospheres have higher pressures at the same sort of temperatures. That means the density of atoms and ions is higher.

If the particle density is higher then there is more chance of interactions between the particles. In particular, collisions between atoms perturb their energy levels and reduce the lifetimes of atoms in particular states. The natural frequency width is proportional to the reciprocal of the radiative lifetime. If this lifetime is truncated due to frequent collisional (de)excitation, then the lifetime is shortened and the frequency width increased. This is known as [collisional broadening](http://www.fas.harvard.edu/~scidemos/QuantumRelativity/CollisionalBroadening/CollisionalBroadening.html).


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