Hi all,

In my last blog I mentioned that you can generate spectra from "man-made" stellar models. In this blog I will talk a little bit more about my experience with these models.

Just to rewind a bit say you have gone to the bother of capturing a stellar spectra. So you have this spectra with a number of lines in it. So what? what does it mean its just a bunch of lines - not very exciting...or is it?

An example of the spectra you actually capture looks like the ones I took of Mizar in my album

http://stargazerslounge.com/members/jsandse-albums-spectra.html

That picture actually has 6 spectra in it which are aligned one on top of the other.

But you get what I mean what can I do with this spectra?

Well you can convert it into a line profile using some software (I use vspec) and then calibrate it so that you know what the wavelengths of the line in the spectra are.

This calibration can be done using a spectra with a reference lamp with known lines in it.

Ok so you have a line profile of a spectra with a number of dips in it corresponding to the dark lines (also called absorption lines) in the original spectra. An example of this

is the one in my album of epsilon auriga

What is causing these lines and what does this tell me about the nature of the star I am looking at? I'll just answer the first part of the question in this blog. Leave the second part for another time.

What is causing these lines are atoms on the surface of the star absorbing some of the light before it escapes from the star. This part of the star is called the photosphere. It's thin the sun's photosphere is only 1000km thick.

Each type of atom (element) absorbs light of a unique characteristic wavelength. Anyway what this means is that if I know the wavelength of a dark line in my spectrum then I can in principle find out the atom responsible.

So back to stellar models. Given three basic properties of a star:

a) temperature

b) logG - surface gravity of the star - this is how heavy things are on the surface of the star (eg the surface gravity on the moon is less than that onthe earth - just look how long those astronauts can jump and hit a golf ball!)

c) metallicity - ratio of amount of metals (to astronomers this means any elements that are not hydrogen or helium - crazy isn't it?) to amount of hydrogen in the star

a stellar model can calculate for most stars

the physical properties of the star (pressure, temperature at different depths in the star) and generate a stellar spectra for it.

I won't go into how we find out what the temperature, surface gravity and metallicty of the star is here - thats another blog...

I will just say that you need these three properties to tell you what types of atoms are in the stellar photosphere (mainly comes from metallicity) and how they behave - which depends on temperature of course and the pressure they are under ( this is related to surface gravity)

Right so I have captured my spectra of Vega, I know what its temperature, surface gravity and metallicity is.

I can then use some software (I use Atlas) to generate a model for it and to produce the spectra from it.

Now comes the exciting part of comparing the spectra I took with the model

In my album please look at the picture at this link:

http://stargazerslounge.com/members/jsandse-albums-spectra-picture8198-comparison-detail-spectra-vega-i-took-against-synthetic-spectra-produced-atlas-stellar-model-vega-similarity-between-two-spec

So in the picture I have placed two line profiles next to each other. One in red which was generated from the model. And the other in black I captured myself from Vega.

It is remarkable how similar they are - virtually all the absorption lines in the model correspond with those that I had captured from the real star.

The benefit of using the model is that it gives me the atoms that cause the absorption lines. I have annotated the picture with details of some of these elements for the more prominent lines - includign ones caused by Hydrogen (H), Iron (Fe), Magnesium (Mg) and Titanium (Ti).

How cool is that! I can decode the spectrum of a star that is 150,000,000,000,000.000 miles away into the types of atoms that are absorbing the light at its surface.