
Leavitt's Cepheid Variables
General Description
This simulation allows the user to explore the relationship between
period and apparent magnitude for Cepheid variables in the Magellanic
Clouds, first discovered by Henrietta Levitt in 1912. The main window
displays a photographic negative of a star field. When the simulation
is played, the main window displays an animated sequence of photo
negatives taken at 1 day intervals. Each star field contains one
Cepheid variable star, which can be identified by its changing size in
the photographic negative.
A tool for measuring the apparent magnitude of the Cepheid variable
(or any other star) is included. In addition, the user can display a
plot of apparent magnitude versus time (known as a light curve), modulo a specified test
period. A slider is provided to adjust the value of the test period.
When the test period corresponds to the actual period of the Cepheid
variable the plot will display a single full period of variation in
apparent magnitude (although the data is noisy, so there is some
scatter in the light curve).
Resetting the simulation generates a new star field with a new Cepheid
variable. By measuring the period and median apparent magnitude of
many Cepheid variables, and constructing a plot of magnitude versus period, the user can discover a relation between period
and apparent magnitude like the one found by Leavitt for the variable
stars in the Small Magellanic Cloud.
Photographic Negative Frame
This frame displays a series of photographic negative of a star
field. The user can measure apparent magnitudes of these stars,
identify the variable star, and get a rough estimate of the variable's
period in this frame.
- Options Menu
- Display Magnitude Measuring Tool: displays a red circle
that can be dragged onto a star, and a slider that adjusts
the size of the circle until it matches that of the star.
The user can then read off the corresponding apparent
magnitude for that star.
- Show Light Curve Plot: displays a plot of apparent
magnitude for the Cepheid variable versus time, modulo a
specified test period.
- Visual Elements
- Black filled circles: stars (shown in photographic
negative).
- Red open circle (optional): magnitude measurement circle.
- Controls
- Buttons
- Play/Pause: start and stop the simulation.
- Reset: resets the simulation, generating a new star field
with a new Cepheid variable.
- Step Forward: advances the simulation by one day.
- Step Backward: move the simulation back by one day.
- Sliders
- Day: the time since the first photograph in the series
was taken (in days).
- m: apparent magnitude corresponding to the size of the
red measuring circle.
Light Curve Frame
This frame displays a plot of the Cepheid variable's apparent
magnitude as a function of time, modulo a specified test period. The
user can use the Min and Max settings to specify a range of possible
periods, and then use the slider to search for the correct period.
When the test period is equal to the actual period of the variable the
plot will display a single full period of the light curve (with some
noise).
- Visual Elements
- Blue dots: apparent magnitude of the Cepheid variable at a
given time, modul to the test period.
- Controls
- Period: test period (in days). Times in the light curve
are all displayed modulo this test period.
- Min: minimum value for the test period slider (in days).
- Max: maximum value for the test period slider (in days).
Todd K. Timberlake (ttimberlake@berry.edu)