The picture above is a star field containing our project star, FM
CAS. The white arrow points to the star. FM CAS is a Cepheid variable
star, an interesting class of stars that allows us to determine distance
using just magnitude and brightness.
Our project sought to measure the distance to FM CAS.
By observing the star frequently we can determine the period of variability,
which can reveal its absolute magnitude and distance.
first Cepheid Variable found was Delta Cephei (named according to
the constellation it is a part of), discovered by John Gooricke
in the 1780s. This discovery earned him the Copley Medal of the
Royal Society in 1784. Unfortunately, Goodricke caught pneumonia
while observing DCephei and died at the age of 21. The potential
of this discovery was not fully realized, however, until Henrietta
Leavitt appeared on the scene in the late 1800s. Leavitt volunteered
at the Harvard College Observatory in 1895, concentrating her efforts
on an intense study of variable stars in the Small Megellanic Cloud.
In 1912, Leavitt realized that the relationship between the apparent
magnitudes and periods of variability was clearly linear.
of Women in Astronomy)
Facts about Cepheids
- These types
of stars are most commonly found in open star clusters.
- The mass of
a Cepheid variable is anywhere from 5 to 20 solar masses (that
is, 5 to 20-times the mass of our sun).
- Cepheid variables
can have periods anywhere from 1 to 50 days. FM CAS has a period
of approximately 5.8 days (Fourier
- They can be
measured at distances of up to 20 million light years, and can
have temperatures ranging from 6000-8000 Kelvin.
- The brightest
Cepheids have an absolute magnitude of approximately -6 magnitudes.
- The light
curve of Cepheid variables resembles a shark fin. It has rises
sharply, and then the rate it falls is a lot slower.
- Cepheid variables
are giants and super giants that are unstable. The stars are continuously
expanding and contracting, when they are too large, gravity pulls
them inward, and when they are too small, the gas has more pressure
on it, so it pushes outward.
- Due to the
change in size, the temperature varies. The brightness is a function
of surface temperature to the fourth power, so the brightness
changes much more compared to the size of the star.
- The distance
to far away objects in the sky often have to be found using stepping
stones, by comparing objects of a known distance to the change
in distance to a farther object. Cepheid variables are a starting
point for finding the distance to other objects.
- After the
period and apparent magnitudes are measured by observation, the
period-luminosity, found by Leavitt, gives us the absolute magnitude
of the star. Using the inverse-square law the distance to the
star can be found.
- This gives
us a primary stepping stone to find the distances to far away
galaxies. This is a very accurate way to find distances because
it is just one step away, not multiple, like other ways of calculating
distances. This accuracy means, Cepheids are very important for
|| Clear until
10:50 (immediately after FM CAS was found), at which time
it became overcast.
|| Clear. Found
FM CAS; by the time the camera was appropriately focused,
it became overcast.
hazy. Found FM CAS; took pictures, but FM CAS was not in field
of view. No time to correct, as the telescope had to be shared
|| Clear, waxing
gib. moon. Took 10 (good)lights and 3 darks with 8 sec. exposures
in clear filter.
|| Clear, no
moon. Took 10 good lights and 3 darks with 8 sec. exposures
in clear filter.
hazy, no moon. Had trouble getting the camera to focus well.
Took 10 lights and 3 darks with 8 sec. exposures in clear
|| Quite hazy,
crescent moon. Again, troubles with focusing the camera and
frost on the lens. Took 10 lights and 4 darks with 8 sec.
exposures in clear filter.
|| Thin layer
of clouds. Located FM CAS, but before pictures could be taken,
it became overcast...once again.
We took all of the pictures
from each night and condensed them down into one. We did this using CCD
Soft to reduce the good images. There were bad pictures due to frost,
the camera out of focus, the camera not fully aligned on FM CAS and other
complications. Each of the pictures were reduced by subtracting the darks
to remove any noise from the camera. The background and range were adjusted
to show clarity in the picture. The photographs were then aligned by CCD
Soft and added together. This process was done for each of the days that
data was collected.
We used CCD Soft fits
files to compare the magnitudes of a known star that was always in our
pictures, with FM CAS. The known star we used was SAO 21238. This star
has a constant magnitude, so plotting the difference between it, and F
M CAS, on different days, will show the light curve of FM CAS.
We used excel
to plot these differences in magnitudes versus time, in days wrapped
around into the known period of 5.8 days (see chart below). The
average brightness is used. The period of FM CAS is 5.8 days, it
has an absolute magnitude of about -4 and an apparent magnitude
We found that
the distance to FM CAS is 4100pc. This does not agree with Hipparcos
catalog. They both say that the distance to FM CAS is 10000pc using
a parallax of 0.1 milliarcseconds (SIMBAD).
Due to poor weather conditions and strenuous time constraints, solid
data was difficult to obtain.
Our observed data does not correspond to other
verified reports. This is largely due to a limited number of observations,
caused by frequent cloud cover.
of FM CAS that shows variability correctly, with the characteristic
"shark fin" shape.
North is to the left and east is down. The angular dimensions are
12.75 by 8.5 arcminutes (765 by 510 arcseconds).
RA: 00h14m53s, +56deg13'10". FM-CAS can be found in the Cassiopeia
Team: Elise Crull , Matthew Koop, Maggie Leonard