Name: ________________________________
Measuring Colors of Stars - ImageJ
HANDS-ON UNIVERSE™
MEASURING THE COLOR OF STARS WITH IMAGES UNIT
In this activity you will determine the color of stars that have been observed with the Leuschner telescope.
There are four different stars with two images of each, one using a blue filter (B) and one using a yellow or
visible filter (V). These stars will be referred to as your target stars. For each target star there is also a standard
star that was observed at nearly the same time so you can assume the observing conditions are the same. Given
the known apparent brightness of the standard stars, you can calibrate the target stars to determine their apparent
brightness.
1. What is the difference between apparent brightness and absolute brightness? ______________________
Filters are used on telescopes to determine the brightness of an object in a specific color. One use of this
information is to estimate the color of stars. Astronomers generally use a set of standard filters, meaning that the
color of light each filter lets through is very well known. This is so one observer can compare data with another
observer.
To determine the color of a star, a combination of filters that will show a sharp distinction between stars must be
used. The diagram shows plots of brightness versus color for three stars. Vertical boxes are drawn to show the
approximate position of red, yellow and blue filters. Astronomers generally use the yellow and blue filters to
measure the color of stars because the difference of light through these filters changes significantly for different
stars.
Star 1
Star 2
Star 3
Name: ________________________________
Measuring Colors of Stars - ImageJ
The naming convention for filters is relatively simple: the blue filter is called B, the red filter is called R, but
the yellow filter is called V. This is because yellow is in the center of the visible color spectrum. To determine
the color of a star using filters, the B filter is used to measure the amount of blue light coming from the star and
the V filter is used to measure the amount of yellow light. These two values are used to get the B–V index of
the star:
B–V index = (magnitude through B) – (magnitude through V)
The B–V index uses magnitudes, which are units astronomers use to quantify brightness. For converting
magnitude to brightness use the Brightness Conversion Table at the back of this packet.
Your job is to find the temp & color of the target stars. To do this, you will need the B-V index, for which you
will need B & V magnitudes. To get B & V magnitudes, you will need apparent brightness values (W/m2). To
get apparent brightness you will need to compare your target star to a standard star (one you already know
information about). To compare an unknown and a known, you need a telescope picture of each under the same
conditions.
The images you will use are listed below along with the apparent magnitudes of the standard stars in both
the visible and blue light.
Chart 1
target star in B
btarg1
btarg2
btarg3
btarg4
target star in V
vtarg1
vtarg2
vtarg3
vtarg4
standard star in B
bstan1
bstan2
bstan3
bstan4
standard star in V
vstan1
vstan2
vstan3
vstan4
Chart 2
standard star
bstan1
vstan1
bstan2
vstan2
bstan3
vstan3
bstan4
vstan4
apparent magnitude
8.0
7.0
9.2
7.2
7.8
7.5
7.0
7.0
Name: ________________________________
Measuring Colors of Stars - ImageJ
Perform the following procedure to get the apparent brightness of each target star through each filter. Record
all of your data in the table provided.
1. Open the standard and target star images for star 1 for both the blue and visible filters (bstan1, btarg1,
vstan1, vtarg1). In most cases the target or standard star is the only star in the image; if not, it is clearly the
brightest star in the image. You can adjust the image’s Brightness & Contrast or use Process, Math, Log.
2. Under Analyze, use Measure to get the counts for each of the stars. You can make a circle using the circle
tool on the toolbar and use Measure (command M). You want to include as much light as you can. The
Results window will give you the Mean Brightness of the area you’ve circled.
3. Use Chart 2 to get the magnitude for the standard stars and then use Chart 3, the Brightness Conversion
Table, to get the apparent brightness (W/m2) of each standard star.
4. Figure out the apparent brightness of each target star using the following fact:
ratio of Counts of target and standard stars = ratio of apparent brightness of the target and standard stars
Ct / Cs = Bt / Bs
solve for Bt (apparent brightness of target star)
These ratios are equal because the two stars were observed at nearly the same time by the same equipment,
so presumably under the same observing conditions.
Note: There is one exception you must account for. The exposure time for bstan2 is three times longer than the
exposure time for btarg2. This means the telescope collected three times more light from the standard star than
it would if the exposure time had been the same as the target star's. You must correct for this in your ratio. You
can find the exposure time for any image yourself by going to Image and selecting Show Info. Scroll down
and find “exp. time.” The time is given in seconds.
How can you correct for this difference in time? ________________________________________________
5. Using the apparent brightness you found and the Brightness Conversion Table, find the apparent magnitude
of each target star. Find the closest magnitude that corresponds to your value for apparent brightness.
6. Calculate the B–V index for each target star. Your value could be either negative or positive.
7. Use Chart 4 to approximate the color and temperature of each target star. If your B–V index falls between
two values in the table, estimate the answer.
Name: ________________________________
Star1
1. Counts of
Btarg
Counts of
Bstan
Counts of
Vtarg
Counts of
Vstan
2. Apparent
Brightness
of Bstan
Apparent
Brightness
of Vstan
3. Apparent
Brightness
of Btarg
Apparent
Brightness
of Vtarg
4. Apparent
Mag of Btarg
Apparent
Mag of Vtarg
5. B–V Index
of Target
6. Color and
Temperature
of Target
Measuring Colors of Stars - ImageJ
Star2
Star3
Star4
Name: ________________________________
Measuring Colors of Stars - ImageJ
Chart 3
Brightness Conversion Table
Magnitudes to Brightness (W/m2)
Mag.
I
R
10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
1.65
1.51
1.37
1.25
1.14
1.04
9.52
8.69
7.92
7.23
x
10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
6.60
6.02
5.49
5.01
4.57
4.17
3.81
3.47
3.17
2.89
x
10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
2.64
2.41
2.20
2.00
1.83
1.67
x
10-12
x 10-12
x 10-12
x 10-12
1.52
1.39
1.27
1.16
x
5.0
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
2.40
2.19
2.00
1.82
1.66
1.52
1.38
1.26
1.15
1.05
x
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
9.57
8.76
7.99
7.29
6.65
6.07
5.54
5.05
4.61
4.21
x
7.0
7.1
7.2
7.3
7.4
7.5
3.84
3.50
3.20
2.91
2.66
2.43
x
7.6
7.7
7.8
7.9
2.21
2.02
1.84
1.68
x
V
10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-12
x 10-12
x 10-12
x 10-12
3.61
3.29
3.01
2.74
2.50
2.28
2.08
1.90
1.73
1.58
x 10-11
x 10-11
10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
1.44
1.32
1.20
1.10
1.00
9.13
8.33
7.60
6.93
6.33
x 10-11
x 10-11
10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
5.77
5.27
4.80
4.38
4.00
3.65
x 10-12
x 10-12
10-12
x 10-12
x 10-12
x 10-12
3.33
3.04
2.77
2.53
x 10-12
x 10-12
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
B
9.24
8.43
7.69
7.02
6.40
5.84
5.33
4.86
4.44
4.05
x 10-11
x 10-11
3.69
3.37
3.07
2.80
2.56
2.33
2.13
1.94
1.77
1.61
x 10-11
x 10-11
1.47
1.34
1.23
1.12
1.02
9.29
x 10-11
x 10-11
8.48
7.38
7.06
6.44
x 10-12
x 10-12
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-12
x 10-12
x 10-12
U
Bolometric
1.83
1.67
1.52
1.39
1.27
1.16
1.06
9.63
8.79
8.02
x 10-11
x 10-11
2.40
2.19
2.00
1.82
1.66
1.52
1.38
1.26
1.15
1.05
x 10-10
x 10-10
7.32
6.68
6.09
5.56
5.07
4.63
4.22
3.85
3.51
3.21
x 10-12
x 10-12
9.57
8.76
7.99
7.29
6.65
6.07
5.54
5.05
4.61
4.21
x 10-11
x 10-11
2.93
2.67
2.44
2.23
2.03
1.85
x 10-12
x 10-12
3.84
3.50
3.20
2.91
2.66
2.43
x 10-11
x 10-11
1.69
1.54
1.41
1.28
x 10-12
x 10-12
2.21
2.02
1.84
1.68
x 10-11
x 10-11
Brightness Conversion Table
Magnitudes to Brightness (W/m2)
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-10
x 10-10
x 10-10
x 10-10
x 10-10
x 10-10
x 10-10
x 10-10
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
Name: ________________________________
Mag.
8.0
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
9.0
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
10.0
I
1.51
1.38
1.26
1.15
1.05
9.55
8.71
7.95
7.25
6.62
6.04
5.51
5.03
4.59
4.18
3.82
3.48
3.18
2.90
2.65
2.41
R
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
1.04
9.49
9.66
7.90
7.21
6.58
6.00
5.47
5.00
4.46
4.16
3.79
3.46
3.16
2.88
2.63
2.40
2.19
2.00
1.82
1.66
Measuring Colors of Stars - ImageJ
V
x 10-12
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
2.28
2.08
1.90
1.73
1.58
1.44
1.32
1.20
1.10
9.99
9.12
8.32
7.59
6.93
6.32
5.77
5.26
4.80
4.38
4.00
3.65
B
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
5.83
5.32
4.85
4.43
4.04
3.69
3.36
3.07
2.80
2.55
2.33
2.12
1.94
1.77
1.61
1.47
1.34
1.22
1.12
1.02
9.30
U
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-13
1.16
1.06
9.66
8.81
8.04
7.34
6.69
6.11
5.57
5.08
4.64
4.23
3.86
3.52
3.21
2.93
2.68
2.44
2.23
2.03
1.85
x 10-12
x 10-12
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
x 10-13
Bolometric
1.51
1.38
1.26
1.15
1.05
9.55
8.71
7.95
7.25
6.62
6.04
5.51
5.03
4.59
4.18
3.82
3.48
3.18
2.90
2.65
2.41
x 10-11
x 10-11
x 10-11
x 10-11
x 10-11
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
x 10-12
Chart 4 B–V index, Temperature and Color Data for Familiar Examples for Main Sequence Stars
B–V Index
Surface
Temperature (K)
Color
Familiar
Examples
Blue
Spica
- 0.31
21,000
- 0.17
13,500
White- Blue
0.00
10,000
White
Sirius, Vega
0.16
8,100
Yellow- White
Altair
0.45
6,500
Yellow
Procyon
0.57
6,000
Orange
Sun
1.24
3,300
Red-Orange
Kapteyn's Star
1.61
2,600
Red
Barnard's Star
Regulus