Rotation Demonstration



The way we get information from stars is through spectroscopy (the study of spectra), so understanding how rotation affects a star and its spectrum is critical for understanding rapidly rotating stars. This web application displays the effect of rotation on the shape, temperature and spectrum of a star.

Quick Overview of What Is Happening

A rapidly rotating star becomes oblate due to centrifugal forces, with an equatorial radius as much as 1.5 times the polar radius. A consequence of the star having an expanded equatorial region is that the matter at the equator becomes less dense and cooler, while the matter at the pole becomes hotter. This change in the surface temperature of a rotating star is known as GravityDarkening.

The strength of the spectral lines we see from a star depends on the temperature of the emitting gas. Consequently, the strength of the spectral lines coming from the hot pole of a rotating star will be different from the strength of the spectral lines coming from the cooler equator. As a result, the spectrum of a rotating star will depend on the angle at which we observe the star (viewing angle). When we look pole-on, the hot pole will be the largest contributor to the spectrum. When we look equator-on, the cool equator will be the largest contributor to the spectrum.

When a star is rotating, the spectrum of the star is also affected by Doppler Broadening. When we are looking equator-on, half of the rotating star is moving towards us and the other half is moving away from us. The spectra from the parts of the star that are moving towards us get blue-shifted (i.e., spectral features will be shifted to shorter wavelengths) while parts of the star that are moving away from us are red-shifted. This causes the spectral lines from the star to get smeared out into shallow bowls instead of sharp lines. On the other hand, when we are looking pole-on, the surface of the star is moving perpendicular to our line of sight (i.e. no part of the star is moving towards or away from us) and the Doppler effect is not observed.

What You Will Be Seeing

When you run the application you have two sliders that allow you to control the viewing angle (inclination) (0 is pole-on, 90 is equator-on), and the rotation rate expressed as a percent of the critical angular velocity (the velocity at which the surface layers become detached from the star). Moving the sliders will cause the model star to change shape and temperature. The regions that turn red are cooler than the initial temperature, and the regions that turn blue are hotter than the initial temperature. Additionally, there is a Doppler Broadening check box which allows you to neglect the Doppler effect-- giving you a chance to see how the spectral lines change with viewing angle and rotation rate, but without the Doppler effect smearing everything out.

Browser notes

The applet has been tested using Internet Explorer, Firefox, and Safari.You will need Java 1.5 (or higher) run time library. If the applet is too large, adjust the zoom controls (lower right on IE) until the applet fits onto the screen.


The spectral data is taken from the TLUSTY database (

The application was written by Charles Warren with the help of Dr. D. John Hillier. It can be used by anyone who is interested as long as credit is given to the authors and the TLUSTY database. This material is based upon work supported by the National Science Foundation under Grant No. 0507328. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.