Download Photometry

Make-up Lab: Photometry of the Pleiades
Name: _________________________________
Section: _________________________________
Lab Partner(s): ______________________________________
“Photometry” is basically just a fancy word for measuring the number of photons coming from
an object. Often, we will want to know the number of photons at a specific wavelength. When
this is the case, we can use special filters so that the only photons we see are of a particular
wavelength. This wavelength is identified with a color.
The number of photons observed can be converted into an apparent magnitude (m), which is
simply a measure of how bright the stars look from Earth. The smaller the number, the brighter
the star. Each color has a different apparent magnitude. (This simply means that some stars
appear brighter in, say, the blue filter than the yellow filter.) In this lab, we will be concerned
with the apparent blue magnitude (B) and the apparent visual magnitude (V). The quantity B-V
is called the color index.
Remember the Hertzsprung-Russel (HR) diagram? You’re probably familiar with something that
looks like this:
Well, it turns out that we can plot the HR diagram with the color index on the x-axis and the
apparent magnitude on the y-axis. Note that when you do this, you want to plot the brightest stars
up top. This means that you’ll want to plot the smallest apparent magnitudes higher on the y axis
than the larger magnitudes. As you move up the y-axis, therefore, the numbers will get smaller.
We’ll come back to the HR diagram later.
Absolute magnitude (M) is a measure of how bright a star actually is. Again, the lower the
magnitude, the brighter the star. (The drawn-out definition of absolute magnitude is the apparent
magnitude, m, an object would have if it were 10 pc away from us.)
If you know both the apparent magnitude and the absolute magnitude, you can calculate the
distance in parsecs using the following formula:
d  10 ( m  M 5) / 5
In this lab, we’ll assume all of the stars are the same distance away. This is a reasonable
assumption, since all of the stars are members of the same cluster.
Instructions
If not already done, start
the program by clicking
the Clea_pho icon.
Select login from the
menu bar, click okay,
and then click yes. To
begin the lab, choose
Start the menu bar.
Open the dome by
clicking the dome
button, and turn the
tracking on by clicking
on tracking. Your
screen should be similar
to the one on the left.
Instead of “Monitor”
your screen may have a
“Change View” button.
Instead of the
photometer on the right side of the screen, you may have a “Take Reading” button.
You are in the finder mode of the telescope. To take measurements with the photometer, you
must be in the photometer/instrument mode. Switch into this by clicking on the monitor / change
view button. Since the aperture (the little red circle) covers a much larger area than the star under
study, and the sky is not entirely dark, the sky within the aperture contributes a certain number of
photons. These unwanted photons are counted by taking a sky reading. You must do this for both
the B and V filters BEFORE you take any data on the stars. The computer will subtract the sky
reading from the star reading for you.
Make sure that the aperture is free of any star. Click on the take reading button if necessary.
Select the U filter by clicking on the “Filter” button. Set “Seconds” to ten seconds and
“Integration” to 5. Click the “Take Reading” button if you haven’t already; otherwise, click the
“Start Count” button. Repeat for the V filter.
Filter
Mean Sky (Counts / sec)
B
V
Now you are ready to find the apparent B and V magnitudes. Move to one of the stars indicated
on the data sheet on the following page. You may use the monitor / change view button to
navigate the area in finder mode. Switch back to photometer / instrument mode before taking
your readings. Make sure that the entire star is within the red circle; otherwise, some of the light
from the star spills out of the aperture and is missed during measurement.
Select the B filter. Choose an integration time (the amount of time the object will be viewed for)
by clicking the “Seconds” button, and the number of integrations (the number of times the object
will be observed) by clicking the “Integrations” button. Choose the integration time and number
of integrations wisely so that the SN Ratio is 100 or more, if possible. (The SN Ratio will be
calculated after you take your observation.) To take the observation(s), click the take reading /
start count button. Record the B magnitude in the data sheet to the nearest .001 magnitude.
If your SN Ratio is too low, increase the integration time and/or the number of integrations. The
dimmer the star, the longer you will have to integrate and the more integrations you will have to
take.
Set the filter to V. Repeat the above to determine the V magnitude and record it to the nearest
.001 magnitude. Determine the B and V magnitudes of each star on the data sheet in the same
way. Then calculate the color index (B-V) by subtracting the two values.
The following stars are rather faint: 5, 6, 10
The following stars are VERY faint: 13, 17
I suggest you find the B and V magnitudes for all of the brighter stars first. Then, while the data
for the above 5 faint stars are being taken, you can be calculating the color index for the bright
stars.
Once you have B and V, plot the stars on the HR diagram provided below the data sheet.
Data Sheet: Apparent Magnitudes and Color Index B-V
Object No.
RA
hr min sec
Dec
deg min sec
1
3
41
5
24
5
11
2
3
42
15
24
19
57
3
3
42
33
24
18
55
4
3
42
41
24
28
22
5
3
43
8
24
42
47
6
3
43
42
23
20
34
7
3
43
56
23
25
46
8
3
44
3
24
25
54
9
3
44
11
24
7
23
10
3
44
19
24
14
16
11
3
44
27
23
57
57
12
3
44
39
23
27
17
13
3
44
39
24
34
47
14
3
44
45
23
24
52
15
3
45
27
17
17
57
16
3
45
33
12
12
59
17
3
46
26
49
49
58
B Value
V Value
B-V
HR Diagram: apparent magnitude
0
5
V
10
15
20
-0.3
0.2
0.7
1.2
25
B-V
Identify the main sequence. Sketch a line through it, and label it clearly.
Identify at least two possible red giant stars by circling them.
Consider the star near RA 3h 44m and Dec 24h 35m. It seems curiously out of place with respect
to the main sequence. What type of object might this be?
Distance to the Cluster
Place the clear plastic over your graph, and using the ruler trace both x and y axes. Label and
scale the x axis the same as the graph paper, but number the scale of the y axis of the plastic
overlay to range from -8 (at the top) to +17 (at the bottom). Label this new y axis V ABSOLUTE
MAGNITUDE. Leave the plastic laying on the graph paper so you can use the grid lines.
Now plot the following calibration stars on the plastic overlay. They are main sequence stars for
which absolute visual magnitudes have been determined.
(V ) A bsolute
Mag nitude
B-V
Spectral Ty pe
-5.8
-0.35
O5
-4.1
-0.31
BO
-1.1
-0.16
B5
-0.7
00.0
AO
2.0
0.13
A5
2.6
0.27
F0
3.4
0.42
F5
4.4
0.58
GO
5.1
0.70
G5
5.9
0.89
KO
7.3
1.18
K5
9.0
1.45
MO
11.8
1.63
M5
16.0
1.80
M8
Slide the plastic overlay up and down until the main sequence on the overlay best aligns with the
main sequence on your paper graph. Keep the y axes precisely parallel and over top one another.
Seek a best fit for the central portion of the combined patterns. (The cool red stars in the lower
right of your paper graph are quite scattered and may not fit very well.)
Each star of the main sequence can now be described either in terms of m (by reading the y-axis
on the graph paper) or M (by reading the y-axis on the plastic overlay). It just depends on which
scale you read. Pick a star towards the center of the main sequence, and record both it’s apparent
magnitude m and absolute magnitude M.
m = ___________________________
M = ___________________________
Use the equation provided in the introduction to calculate the distance to the cluster in parsecs.
Then convert your answer to light years. (Note that 1 pc = 3.26163626 light years.) Show all
work in the space provided.
Distance to cluster: ____________________________ parsecs
Distance to cluster: _____________________________ light years
In 1958, H.L. Johnson and R.I. Mitchell calculated the distance to this cluster to be about 410
light-years. As a percent-age, how does your calculated value compare? SHOW YOUR WORK.