<div class="pull-left"><img src="https://upload.wikimedia.org/wikipedia/commons/1/1c/Solar_eclipse_1999_4_NR.jpg" /> <center><a href="https://lucnix.be/">By Luc Viatour</a>,<a href="https://en.wikipedia.org/wiki/Stellar_atmosphere"> wikicommons</a>, <a href="https://creativecommons.org/licenses/by-sa/3.0/"> CC BY-SA 3.0</a></center></div>
<p>What is the stellar atmosphere? Well, in simple words the atmosphere of the stars. But is there a boundary or how do we define that this is stellar atmosphere? The answer is there is no specific boundary which we call the stellar atmosphere. Space between the stellar interior and the interstellar medium is called stellar atmosphere. In stellar astrophysics, we don't really make difference between the interior and the atmosphere as the material around is in the plasma state and the same physical law is used to describe them. Usually, stellar atmosphere is the area where the radiation comes from or where our observed spectra are formed.</p>

<h1>Photosphere</h1>
<p>Almost all the visible part of the spectra comes from the deepest and densest part of the atmosphere which we know as photosphere. I really like the name photosphere as it means the sphere of light. It is kinda cute to me. Photosphere is the place where most of the spectra are formed as for example the observed continuum and the absorption lines as well. 
<div class="pull-right"><img src="https://upload.wikimedia.org/wikipedia/commons/6/6a/Spectral_lines_en.PNG" /> <center><a href="https://commons.wikimedia.org/wiki/File:Spectral_lines_en.PNG">wikicommons</a><a href="https://commons.wikimedia.org/wiki/File:Spectral_lines_en.PNG"> CCO</a></center></div>
Usually, the photosphere is thin for most of the star. As the thickness of star's photosphere is inversely proportional to the attraction force of the star or gravity. For example, the thickness of sun's photosphere is only a few hundreds of kilometers. On the other hand, the thickness of the atmosphere for Giants or supergiants are a few hundred or thousand times more. After photosphere, there exists the chromosphere which is much hotter and then the corona. From there we get the emission lines (linear spectrum). Spectra below the photosphere is totally unknown to me</p>

<h1> How to model the atmosphere</h1>
<div class="pull-left"><img src="https://upload.wikimedia.org/wikipedia/en/d/d1/Model_atmospheres_Bengt_Gustafsson.jpg" /><center><a href="https://www.iau.org/administration/membership/individual/1858/">By Bengt Gustafsson</a>,<a href="https://en.wikipedia.org/wiki/Model_photosphere#/media/File:Model_atmospheres_Bengt_Gustafsson.jpg">wikcommons</a> , <a href="https://creativecommons.org/licenses/by-sa/3.0/">  CC BY-SA 3.0</a></center></div>
<p>When we look at the spectra we see different kinds for different stars. The question is why? Well, it is because the spectra of a star depend on the different physical process going on the star's atmosphere. So, many different kinds of phenomenon take place all the time in the stellar atmosphere.</p>
<p>Before we start to model stellar atmosphere, we have to first fulfill the conservation laws ( of mass, energy, and impulse). Beside this conservation laws, we have to give attention to the physical phenomenon which takes place in the atmosphere and form a set of equations.  But these equations which take into account all the relative or mutually related physical processes between different kinds of particles can be a very complicated process. So, we always have to take some approximation so that we can make a model which can describe the stellar atmosphere. Now, let us start with the geometrical approximation.</p>

<h1> Geometry of atmosphere</h1>
<p>Usually, the atmosphere is thought of homogenous or plane-parallel or spherically symmetrical.  Plane-parallel geometry is considered when the radius of the star is bigger than the thickness of the atmosphere. But stars like giant or supergiants have thicker atmosphere rather than their radius. In that case, we use spherically symmetric geometry. The homogenous approximation is much easier and less efficient approximation we can consider. It is an approximation which is much more one dimensional. The Solar atmosphere has shown us that it is nonhomogenous. To some stars, nonhomogeneity can be seen because of the hydrodynamic phenomenon in the convective zone. So, only stars without strong convective zones can be counted as homogenous.</p>
<p>Another approximation can be the stationary approximation. In this case, we neglect time-dependent phenomenon. For example- time-dependent magnetic field or when the shell is increasing. We have to consider that the distribution function of the particles, gas or radiation is totally independent of time</p>
<p>We can think of also that everything is static. We have to neglect the motion in small dimension as well as in bigger dimension, for example, granulation, supergranulation or the differential rotation. We have to think that the atmosphere is in  hydrodynamic equilibrium and pressure from the gas and radiation is in equilibrium with gravity.</p>
<p> We know that there is no source of energy in the atmosphere. It is the medium through which energy is just transmitted outside. The amount of energy radiated is always in equilibrium with the amount transmitted from the source. It can be considered that the energy is done by radiation process. We have to neglect the other processes by which energy can be transmitted. </p> 

<h1>Spectra as a source of information</h1>
The energy which is emitted by a star is produced in the core by the thermonuclear reaction. In higher temperature, through fusion reaction, lighter core become heavier and release an enormous amount of energy through high energy gamma radiation and neutrino. Neutrinos can leave the star without any interaction but it takes photons millions of years to come to the star's atmosphere as it interacts with other particles in stellar plasma and it has so lower mean free path.

<img src="https://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Spectrum.png">
<center>solar spectra as an example | <a href="http://berkeleyearth.org/team/robert-rohde/">By Robert Rohde</a><a href="https://commons.wikimedia.org/wiki/File:Solar_Spectrum.png">wikicommons</a><a href="https://creativecommons.org/licenses/by-sa/3.0/deed.en"> CC BY-SA 3.0</a></center>

<p>The spectra of deep and hot interior of star follow the Planck distribution as it is isolated from the cold interstellar gas by the outer layers of the star.  As we go far and far from the center, because of interactions and different physical laws the spectral distribution changes. As because of those interaction photons are absorbed and reemitted until it makes its way to the atmosphere. Through interaction with the atmospheric gas, radiation gets the observed distribution. This observed distribution gives information about the atmosphere of the star and the physical process taking place there. When this radiation passes through the interstellar medium, spectra changes but with reduction processes we can know about the original spectra of the stellar atmosphere. I will talk another day about these reduction processes. Thats all for today. Have a nice time until the next time and keep steeming on.
<center><h1>References</h1></center>
https://steemitimages.com/DQmUPhem6oHxktMCRRLjuN3MYaM97b5sGBfjPNQdUXPM55x/steemdivider.jpg

<1> Dimitri Mihalas, Stellar Atmosphere<br>
<2>Erika Bohm Vitense, Stellar Astrophysics<br>
<3> <a href="http://www-star.st-and.ac.uk/~pw31/education.html">Peter woitke, Stellar Physics</a><br>
<4><a href="https://en.wikipedia.org/wiki/Stellar_structure"> wikipedia(stellar structure)</a>

<center>https://steemitimages.com/0x0/https://steemitimages.com/DQmeJgsbM5K3pUC8kPBToDKRxE2gijUXvgvX6oUiBgaaiyk/atom.gif</center>