The objects of the exercise are to:
1. appreciate that the
sky is not unchanging;
2. examine directly part of the way in which most stars die;
3. explore how scientific data are acquired and used.
To
these ends, you will look at the variations of a "long-period
variable" (LPV) star, otherwise known, after the first discovered,
as a "Mira" variable. These are giant stars that undergo huge
variations in brightness over a period of a year or so (different
ones from about 100 to 1000 days). We will discuss them in class.
Mira variables are dying stars with carbon-oxygen cores that have
lost their equilibria and are pulsating and changing their
brightnesses. They are in this state for 100,000 years or so. At
the same time they are losing matter through massive winds, and
will eventually lose their entire outer envelopes, nearly exposing
their old nuclear-burning cores. At the end, they will produce
"planetary nebulae" that are seen as shells of gas surrounding hot
stars, and then will die as "white dwarfs," these to be studied in
class as well.
The observations will be made using a camera
system called "Stardial" that is permanently mounted on the roof of
the Astronomy Building at the University of Illinois @ Champaign/Urbana
and that takes a digital picture of the sky
every 15 minutes throughout the night. The field of view is about
9 degrees wide (about twice that of the bowl of the Big Dipper),
and the camera is pointed toward the celestial meridian about 5
degrees south of the celestial equator (at a declination of 5
degrees south). Each picture is indexed and posted on the Web
under "Stardial," and presented
so that north is to the right and east is up. The system has been in operation since July of
1996 and all the data have been archived for your use. New data
are acquired each clear night, so that if your star is available
for observation, you can see what it is doing as of the moment.
The data are not made up or artificial, but real.
Your job
is to construct a "light curve" for your star in which you make a
graph of magnitude plotted against time. Magnitude measurements
are to be made using the oldest device, the human eye.
Accompanying these notes are prepared images called "finding
charts" of two fields of view. One contains a variable star known
as T Virginis (in the constellation
Virgo). The other contains two variable stars, S Virginis and V Virginis. You may pick any of the
three stars to work on. Each image shows the variable star
(labelled "S", "T", or "V") and several "comparison stars" whose
magnitudes have been pre-measured. Each magnitude is given to a
tenth of a unit, but the decimal points have been left out to avoid
confusion with stars; for example, 97 means 9.7, etc. (Technical
details: To minimize
color differences, all the comparison
stars are late G or K stars; the fundamental
reference star against which the other comparison stars were measured
is labelled with its spectral class. Since Stardial is a red-sensitive
camera, the variable, which is cool and red, will appear brighter than it
would to the eye, and your magnitudes will be lower than they would be
were you using your eye at the telescope.)
Your
data are on a link called Stardial.
Read the
introduction to stardial ("What is Stardial?") to learn how stardial works. You do not
have to absorb it all, just read to get a feel for the system. The
images are organized by date and also by position around the sky
from west to east according to the field of view's "right
ascension," which you can read about in your
textbook. Right ascension is simply an angle around the sky to the
east of the Vernal Equinox measured in time units rather than
degrees, where "one hour" = 15 degrees, and so on. T Virginis is
found in the field with the right ascension of 12 hours and 15
minutes, or 1215. S and V Virginis are found at 13 hours 30
minutes, or 1330.
To find images for measurement do the
following:
1. Go to Stardial's main menu.
2. Click on "Data."
3. Click on "Archive via the Web."
4. Click on "jpg" (a format for pictures).
5. Click on "RA" (for right ascension mode).
6. Scroll down until you find your chosen field (the one that
contains your variable star), either "1215" or "1330."
7. Select a year (e.g., 1998). On the left will be a list of dates with
month and day (mmdd, 0117 = January 17, 0225 = February 25, etc.).
On the right will be a date of posting and the file size in
kilobytes. The camera takes pictures whether clear or cloudy.
Cloudy nights are automatically set to "6K." These are useless.
Clear nights are normally in the range "20K" to "30K" or so.
These are the ones to use. You must
still examine them (compare several), as some will still be taken
under partly cloudy or murky conditions. You are looking for the
better nights.
Interpolating between comparison stars on the
finding chart, make an eye estimate of the magnitude of your star
(to a tenth of a unit) for each observing night. You should have
a minimum of 10 observations spaced throughout the semester. Begin
with data from the archive, that is, earlier in the semester, and
then keep track of your star during the remainder of the semester.
This is actually the way variable stars are examined, except that
professionally, electronic measuring devices are used; generations
of amateurs, however, have made useful observations using the
"naked eye' technique at the telescope.
This is not a
project with canned data. We do not know what the star will do,
whether it will brighten, fade, or how fast it will change. You
will determine that by observation as the data arrive. As you
observe, record your data in a table and then plot your data on a
graph with the date on the bottom axis (the x-axis) and the
magnitude INCREASING DOWNWARD (so fainter is down) on the side (the
y-axis). At the end, connect your observations with a smooth
curve.
No measuring device is perfect, and all measurements
necessarily contain errors. When you plot your graph you will see
that the points do not exactly follow a smooth path. The
deviations of your points from the smooth curve indicate your
typical "error of measurement," which should always be cited.
Estimate your typical errors from the graph before turning it in.
There are formal ways of assigning errors; you need only make your
best estimate.
Reports should include:
1. a TABLE of measurements that includes the date
and the magnitude;
2. a GRAPH with magnitudes increasing
downward;
3. your ERROR ESTIMATE (the size of a typical
error);
4. and a brief STATEMENT about the behavior of your
star.
This should be fun; you are looking into the
unknown.
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