
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:
- To align the stars, point the red arrow at one of the stars.
Then blink to the second nebula image and use the slider to rotate
the image until the red arrow once agains points to the star.
- To measure the rotation angle of the nebula, use nebular points
that are far from the center of the nebula. Once you have
determined the displacement and distance from center, then the angle
of rotation (in degrees) will be 180*(displacement/distance)/pi.
- Results are generally better if you measure several points,
compute the rotation angle for each point, and then determine the
mean rotation angle for all points. Van Maanen measured 87
different points, but you may not want to go that far!
Nebula Frame
- Controls
- Blink: switches back and forth between the two images of M 101.
- Displacement: shows the length of the blue displacement
vector (in arbitrary units).
- Distance from Center: shows the length of the red position
vector (in arbitrary units).
- Rotate Second Image to Align 'Stars': slider to rotate the
second image in order to align the foreground 'stars' (red
dots). This slider is only activated when the second image
is displayed.
- Visual Elements
- Background image: one of two images (labeled 1 and 2) of
the spiral nebula Messier 101. The images were both derived
from a Hubble Space Telescope image of M 101 [Credit: NASA, ESA, CFHT, NOAO; Acknowledgement - K.Kuntz (GSFC), F.Bresolin (U.Hawaii), J.Trauger (JPL), J.Mould (NOAO), Y.-H.Chu (U. Illinois)].
- Red dots: represent foreground stars that are not part of
the nebula. These stars must be aligned before the rotation
of the nebula can be measured.
- Red arrow: vector from the center of the nebula to the
nebular point being measured. Click and drag to place the
head of this arrow on the nebular point.
- Blue arrow: vector showing the displacement of a nebular
point between the two images. Click and drag the tail of
the vector to place it on the nebular point in the first
image. Then click and drag the head of the arrow to place
it on the same nebular point in the second image.
Todd K. Timberlake (ttimberlake@berry.edu)