# My 3rd question - About Stellar Spectrum

#### ThomasJoe40

Hello, all

I have just read some foundamental concepts about EM spectrum in astronomy. One thing I don't quite understand is that "Stars with higher temperature or higher pressure shows a broadened line on its absorption spectrum". A breif explanation states that a higher temperature or pressure leads to a greater number of atomic collisions each time.

I am not absolutely convinced with the relation between atomic collision and broadening absorption line. An assuption I made is that the KE produced by collisions between atoms or ions does not absolutely fit the certain energy levels of an element (Hydrogen?). So, the broad of an absorption line is due to these random collisions. With higher the temperature or pressure, there should be a greater probabilities of collisions in each instance, which should leads to a broader line.

One last thing I can't figure out with is that the area of the absorption line (width times intensity) is always the same (same energy produced) Why???

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#### lomidrevo

There are three main mechanisms causing the broadening of spectral lines:
1. Natural Broadening (due to Heisenberg's uncertainty principle)
It seems that OP is mixing together mechanisms 2. and 3. in his post.

The source of Doppler Broadening is the fact that in thermal equilibrium the atoms in the stellar atmosphere are moving randomly with velocities described by Maxwell-Boltzmann distribution. The light emitted or absorbed by the atoms is affected by Doppler effect: blue-shifted or red-shifted depending on the atom's velocity relative to the observer. The higher is the temperature, the wider is the distribution of velocities of the atoms and thus the broader line observed.

On the other hand, in case of the Collisional Broadening, the width of the spectral line is proportional to the density of the gas. The denser is the stellar atmosphere, the more frequent are the collisions and the broader is the line. This mechanism also explains the MK luminosity classes: The more luminous giant stars have lower densities in their upper atmospheres and thus their spectral lines are narrower. On the other hand, main-sequence stars have denser atmospheres where collisions are more frequent and thus the spectral lines are broader.

#### lomidrevo

One last thing I can't figure out with is that the area of the absorption line (width times intensity) is always the same (same energy produced) Why???
This doesn't make sense to me. I don't think it is truth.

"My 3rd question - About Stellar Spectrum"

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