On the 20th April, I managed to make a spectroscopic observation of a BE type star 4 Hercules.  BE stars are a category of stars which show very distinctive emission/absorption profiles cause by the movement of material surrounding the star.  For example often a star is rotating so quickly that a disk of material known as an accretion disk surrounds the star. Depending on the orientation with respect to our line of sight this accrection disk can be seen in the spectral profile.  In particular around the hydroden-alpha line at 6563 angstroms.  The following observation of the BE star 4 hercules illustrates a star with an edge on accretion disk.

(click on the image for a full res image.)

Click Here To Read The Full Article

We pay particular attention to the shape of the hydrogen alpha profile (the m shaped area around 6563 on the x axis).

Note the twin peaks (emissions).  These represent the movement of the material in the accretion disk surrounding the star.)  Let's look a little more closely.  The light coming from all parts of the star forms a point which we examine with the spectrograph.  However some of this light is emitted from material in the accretion disk which is moving away from us and some is emitted from material moving toward us

A diagram explains better;

As the disk of material rotates about the star some of it is moving towards us (left side of the above diagram) and some is moving away (right side).  The doppler effect causes light from material moving towards to be shifted to the blue end of the spectrum (blueshifted) and material moving away from us to be redshifted.  In the spectral profile above we see this in evidence the left 'hump' is the result of material moving towards us i.e. blueshifted.  The right 'hump' is the material moving away from us. i.e. redshifted.  Because we can see that the difference between the two is 6.585 angstroms we can calculate the rotational speed of this circumstellar material using the simple formula

v = c x Δλ / λ

Where Δλ is the rest wavelength i.e 6563angstroms and λ is the shifted displacement.  c is the speed of light i.e 3 x 10⁵ km/s

Since this displacement comprises of both red and blue shifted components we must use half the value for our calculation the result is the rotational velocity of the outer edge of the accretion disk.  This works out based on this observation that the outer edge of the disk is rotating at 150km/s

The rotational velocity of the inner edge will be significantly faster and is given by measuring the width of the 'wings' of the profile.  i.e 19.2A  This gives a value of 429km/s for the speed of the material at the inner edge of the disk.  The average rotational velocity is thus derived as the average of the two, 294km/s which is in keeping with the published data for this star of 300km/s.