SpiralNebulaRotation


General Description

This simulation illustrates Adrian van Maanen's method for measuring the rotations (or internal motions) of spiral nebulae. Van Maanen used a device called a "blink stereocomparator" that allowed him to rapidly switch between viewing one image of a spiral nebula and another image of the same nebula taken at a later date. After aligning the foreground stars (which were not part of the nebula) in the two images, van Maanen made measurements of the displacements of various points in the nebula from one image to the other. His measurements indicated a noticeable rotation in several spiral nebulae, and these results were used to argue against the theory that spiral nebulae are separate galaxies (because if they were, then they must be so large and so distant that any rotations would be completely undetectable in the span of time between the two images used by van Maanen). Later, Edwin Hubble showed that van Maanen's analysis was flawed and that the rotations he had measured were not real. By that time most astronomers had already come to accept that spiral nebulae are separate galaxies.

The simulation provides two images of the spiral nebula Messier 101. The user can "blink" back and forth between these two images. Foreground "stars" (red dots) have been superimposed on the two images. The user must first align the foreground stars by rotating the second image of the nebula. Then the displacements and distances from center of various points in the nebula can be measured using the draggable arrows in the simulation. If the nebula undergoes a pure rotation then the displacements should be perpendicular to the direction from center. The length of the displacement divided by the distance from center gives the angle (in radians) by which the nebula has rotated between the two images (assuming the displacements are small compared to the distance from center). This procedure can be repeated for several nebular points in order to test the consistency of the measurements and determine a statistical average for the rotation angle.

It should be emphasized that the rotation effect illustrated in this simulation is NOT REAL. Although spiral galaxies do rotate, the angle by which they rotate during a human lifetime is so small as to be undetectable. The purpose of this simulation is simply to illustrate van Maanen's method, because van Maanen's (spurious) results are historically important in that they led several astronomers to reject the idea that spiral nebulae are independent galaxies (at least for a while).

Here are a few tips for using this simulation:


Nebula Frame


Todd K. Timberlake (ttimberlake@berry.edu)