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NUMBER
IV.F. TITLE:
Photoelectric Photometry of the Pleiades (M-45)
Adapted
from CLEA, a project sponsored in part by NSF |
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PURPOSE:
·
To use a simulated photometer to measure the B & V magnitudes of stars.
·
To make and compare H-R diagrams to find the relationship
between absolute and apparent magnitudes.
·
To determine the distance to a star cluster.
INTRODUCTION/DESCRIPTION:
The computer
program you will use is a realistic simulation of a UBV photometer attached to
a moderate sized research telescope. The
telescope is controlled by a computer that allows you to move from star to star
and make measurements. Different filters
can be selected for each observation, and the length of time the photometer
samples the starlight (integration time) is adjustable. The computer also does much of the busy work
needed to convert photon counts into apparent magnitude and provides an
estimate of the quality of the collected data.
You will
use the instrument to collect data on 24 stars in the region of the Pleiades
star cluster (See Table A). The
apparent magnitudes will be measured for each star, in each of two colors (B
& V). We will assume all of the
stars are approximately the same distance away. This is a necessary assumption, and reasonable
because all the stars are members of the same cluster. If we did not make this general assumption,
the apparent magnitudes of the stars would also depend on their individual
distances, an effect we cannot easily take into account in this lab.
From this information
you will plot a Hertzsprung-Russell (H-R) diagram which will display the
apparent magnitude of the cluster of stars as a function of their color index. The color index, B-V, is the apparent blue
magnitude (B) minus the apparent visual magnitude (V). Make this plot on GRAPH m provided. Recall that the dimmer a star is, the
numerically greater the apparent magnitude.
Graph M
is created on a transparency and generated from the calibration star data in
Table B.
USING THE
COMPUTER SIMULATION AND PROCEDURES:
The instructor will guide you
through the following steps in data collection:
·
Starting the Program and Settings
·
Setting Coordinates from Table A
·
Taking Sky Counts
·
Signal to Noise Ratios
(s/n = 100 or more)
·
Taking B and V Counts
- record to 0.001 magnitudes
·
Calculating B-V Color Index record to 0.01 magnitudes
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Constructing
Graphs m and M
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Determining Distance to Cluster ----- d = 10x10(m-M)/5 or d =
10(m-M+5)/5
TABLE A PHOTOELECTRIC PHOTOMETRY DATA SHEET
M-45 PLEIADES STAR CLUSTER
All
Sky
5 Integrations
at 10 seconds each
Sky
Filter B Mean
Sky Counts/Second = _______
Filter V
Mean Sky Counts/Second = _______
All
Star
3 Integrations
at 10 seconds each
Note
(x) s/n ratios which are less than 100
|
STAR |
RA hr
min sec |
Dec deg min sec |
s/n < 100 |
B |
V |
B-V |
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1 |
3 41 05 |
24 05 11 |
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2 |
3 42 15 |
24 19 57 |
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3 |
3 42 33 |
24 18 55 |
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4 |
3 42 41 |
24 28 22 |
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5 |
3 43 08 |
24 42 47 |
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6 |
3 43 08 |
25 00 46 |
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7 |
3 43 39 |
23 28 58 |
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8 |
3 43 42 |
23 20 34 |
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9 |
3 43 56 |
23 25 46 |
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10 |
3 44 03 |
24 25 54 |
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11 |
3 44 11 |
24 07 23 |
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12 |
3 44 19 |
24 14 16 |
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13 |
3 44 27 |
23 57
57 |
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14 |
3 44 39 |
23 27 17 |
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15 |
3 44 39 |
24 34 47 |
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16 |
3 44 45 |
23 24 52 |
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17 |
3 45
09 |
24 50 59 |
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18 |
3 45 27 |
23 17 57 |
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19 |
3 45 28 |
23 53 41 |
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20 |
3 45 33 |
24 12 59 |
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21 |
3 46 26 |
23 41 11 |
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22 |
3 46 26 |
23 49 58 |
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23 |
3 46 57 |
24 04 51 |
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24 |
3 47 29 |
24 20 34 |
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-0.5 0 0.5 1.0 1.5
2.0
B V (Editors note: do
not print -- must do graph by hand)
DATA ANALYSIS/QUESTIONS:
1. Identify the
Main Sequence on Graph m by sketching a line through it and label it
clearly.
2. Identify by
Star Number (Table) two possible red giant stars.
3. Note Star
15. Its data point seems curiously out
of place with respect to the main sequence.
a. What type of
star might this be?
b. Upon what did
you base your decision?
4. Determine the
distance to M-45 in Parsecs and Light Years.
Slide the plastic overlay (Graph M)
up and down until the main sequence
on the overlay best aligns with the main
sequence on your graph (Graph m).
Keep the
y axes (V Magnitudes) in precise
coincidence with one another.
Seek
a best fit for the central portion of the combined patterns. The cool red stars in the lower right of
Graph m (paper) may be quite scattered and not fit very well.
Notice that once the two main sequences
are aligned, a fixed relationship is established between the apparent and
absolute magnitude scales, no matter where you read the y axis or which star
you pick. So pick any convenient
magnitude on the absolute magnitude scale (Graph M) and read its
corresponding apparent magnitude on the paper graph (Graph m).
V
Absolute Magnitude = _______(big M read from plastic overlay)
Corresponding
V
Apparent Magnitude =
_______(small m read from graph paper)
Now use the equation on page 1 to
calculate the distance to the Pleiades (M-45).
d = _________ parsecs (observed)
d = _________ Light Years (observed)
d = _________
Light Years (published)
5. Calculate and briefly explain % Discrepancy: