Project #3 Visual Binary Stars
Visual binary stars are two stars in orbit around each
other. They are far enough from each other to be seen as two separate stars,
but are close enough to be under each other's gravitational influence. In this
exercise we will do four things:
Observations
Two lists of visual binary stars.
'Wide'
pairs
|
Name |
RA (2000) |
DEC (2000) |
Mag 1 |
Mag 2 |
|
|
h m s |
' '' |
|
|
|
p And |
00 36 53 |
+33 43 10 |
4.5 |
8.5 |
|
l Ari |
01 57 56 |
+23 35 46 |
4.8 |
7.6 |
|
g And* |
02 03 53 |
+42 19 47 |
2.3 |
4.8 |
|
h Per* |
02 50 42 |
+55 53 44 |
3.8 |
8.5 |
|
1 |
04 32 00 |
+53 55 00 |
5.7 |
6.8 |
|
14 Aur |
05 15 24 |
+32 41 15 |
5.4 |
7.9 |
|
19 Lyn |
07 22 54 |
+55 17 00 |
5.6 |
6.5 |
|
i Cnc* |
08 46 41 |
+28 45 45 |
4.0 |
6.6 |
|
t Leo |
11 27 56 |
+02 51 23 |
5.2 |
7.4 |
|
a CVn |
12 56 01 |
+38 19 00 |
2.9 |
5.6 |
|
z UMa |
13 23 56 |
+54 55 25 |
2.3 |
4.0 |
|
b Cyg* |
19 30 44 |
+27 57 45 |
3.1 |
5.1 |
|
d Cep |
22 29 10 |
+58 24 55 |
4.0 |
6.0 |
|
94 Aqr |
23 19 07 |
-13 27 31 |
5.3 |
7.3 |
|
'Resolution' pairs Fall
|
'Resolution' pairs Spring
|
Listed in each table is the name of the star,
its position in the sky and the brightness of each component in magnitudes. The
wide pairs will be used for 1 through 3 above. The resolution pairs will be
used to estimate the resolution of the telescope, # 4 above.
First, spend some time focusing the
telescope as well as you can.
From the wide pairs you need to observe at
least two systems. The first should have a *. Choose the second
from those without a *. Whichever you choose, make sure you see both stars. For
each observation be careful that the brightest star is not over-exposed. Since
these are very bright stars, your exposure times will be short. However make
sure that you can see the faintest member of the system clearly. Observe each
system in B V R and I. As you take CCD pictures of the systems also write-down
carefully what they look like in the eyepiece (Include an estimate of the relative brightness and
the colors.)
The strategy for the resolution pairs is a
bit different. These two stars are much closer together than the wide pairs.
What we are trying to do is to find out how close together stars can get in the
sky before they merge together into one image. The resolution pairs are
arranged in order from the widest to the closest. Look at each system
visually in the eyepiece, writing down what you see. Or you can just 'go for
it' and try the closest one. If you see two stars in the closest one that's as
good as you will get. If not you can try one further up the list. Also take a
picture in either R or I. Even if you can't see two stars with your naked eye,
you might make out the difference in the CCD image later.
Before you start reduction make sure you have
reviewed ‘HOW TO REDUCE DATA’. Start with the wide pairs. On each image measure:
You might want to arrange a
table for this so you can do it systematically. You can use the logging ability
of the reduction software to help
you record the data. If you like spreadsheets this is a good time to use one.
The separation of each wide pair
Find the distance between the components in
pixels and convert it to arcseconds using the arcsec/pixel value you found in
Project #2.
The brightness difference
Let's do this in magnitudes
as astronomers do. Record the difference in magnitudes in each color. The differences you have measured in V should be close to the differences of the two magnitudes in the table above. You
reduction software does not have enough information to convert what it measures
into real (standard) magnitudes for the stars. To get the actual magnitudes of each star in the table we would have to calibrate the
telescope.
Look at the differences in the other colors.
How do they compare? Is there a trend from B to I, that is, do the differences
get larger or smaller going from B to I?
The difference in the magnitudes from color
to color reflects the differences in the colors of the stars. Again, to get the
actual color of the star (called the color index) we would have to calibrate
the telescope. However, the color difference of the magnitude difference is a
good approximation of difference in the star's color. Calculate both the R-I
differences and the B-V differences. Can you say more about why the stars have
different colors? How about guesses on spectral types? Which stars are the
brighter, the 'red' ones or the 'blue' ones? Explain.
The resolution of the telescope
The resolution or resolving power of a
telescope is given by
Resolution
= 2.5 105 l /diameter
where l is the wavelength
of the light (color) you observe and the diameter is the diameter of the
telescope mirror. For the UGA telescope the latter is 0.6m. For R you may use 7
x 10-7 m and for I, 9 x 10-7 m. What is the theoretical
resolution of the UGA telescope?
Now let us see how close we get to that. Look
at each of the close pairs. See if you can distinguish two stars or is it just
one blob? You might measure the length and width of the star image with the
cursor. Be careful, as any EW elongation of exposures of more than a second or
so may be due to the telescope drifting. Which images can you see two stars?
Put them in a rough order as to how far apart the stars seem -or how elongated
the image is. Compare this to the order in the table. When you have done that
come to me and find out what your best resolution is and include it in your
write up.
As usual describe your observations and your
approach to the problem. For the wide pairs you will want to include what you
wrote about your visual observations as well as your measurements. One way to
do this is 'star at a time'. Describe all your observations and results on one
object before moving to the next
For the close pairs you will need to describe
carefully what led you to decide there were two images not one. Remember, the
more negative or smaller the magnitude, the fainter the star.