1. No category

Physics 243 Astronomy Project Manual
Fall 2014
Astronomy Project Manual for Physics 243
Fall 2014
Contents
1
RR Lyrae stars and distances to globular clusters ........................................................................................... 1
1.1
1.2
Introduction ............................................................................................................................................. 1
Experimental procedure ........................................................................................................................ 2
1.2.1 Planning
2
1.2.2 At the telescope
5
1.3
Data reduction ......................................................................................................................................... 7
1.4
Analysis .................................................................................................................................................... 9
2
Hertzsprung-Russell diagrams of globular clusters ..................................................................................... 10
2.1
2.2
2.3
2.4
3
Active nuclei and their host galaxies: observations of Seyfert galaxies ..................................................... 15
3.1
3.2
3.3
3.4
1
1.1
Preparations for observation ............................................................................................................... 10
Before you leave for the observatory ................................................................................................. 12
At the telescope ..................................................................................................................................... 12
Back on campus..................................................................................................................................... 13
Preparations for observation ............................................................................................................... 15
Before you leave for the observatory ................................................................................................. 16
At the telescope ..................................................................................................................................... 16
Back on campus..................................................................................................................................... 17
RR Lyrae stars and distances to globular clusters
Introduction
This is an observing experiment in which we will measure light curves for the variable stars in a globular
cluster, and determine the distance to this cluster by comparison of average magnitudes of the variables
with that of RR Lyrae, which is near enough to have precisely-measurable parallax.
This observing project will involve visits to the telescope frequently over the course of 1-2 weeks,
hopefully including a few nights in a row. Everybody who opts for this project will share their data,
and receive everyone else’s in return. Each student will perform his or her own analysis and write an
independent report on the results.
Stars in the instability strip of the stellar Hertzsprung-Russell diagram have been fundamental in the
determination of the distance scale of the Universe. These stars pulsate, usually in their fundamental
acoustic mode; as we have noted in AST 142 (7 February 2013), with periods Π that vary inversely with
the mass density ρ of the star:
©2014, The University of Rochester
1
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
R
=
Π 4∫
0
dr
6π
≅
vs
γ Gρ
(uniform density star).
The lowest density stars – giants and supergiants – that lie in the instability strip thus have the longest
periods. Their low density is an expression of the size to which each star has swelled during its death
throes, so stellar luminosity is also tracked by the pulsation period. As Henrietta Leavitt was first to show
(see the AST 142 lecture notes for 2 April 2013), classical Cepheid variables – the highest-luminosity
Population I inhabitants of the instability strip – exhibit a very tight relationship between average
absolute magnitude and period. These stars, and their period-luminosity relationship, form the strongest
rung of the extragalactic distance ladder in the stretch between trig parallax and Hubble’s Law.
The instability-strip variables in globular clusters include RR Lyr stars and Population II Cepheids. RR
Lyr stars aren’t as bright as the Population II Cepheids, but they are quite a bit more numerous. 1 These
are much less luminous than Cepheids, and thus hard to detect in galaxies besides the Milky Way.
However, there is one RR Lyrae star close enough to the Solar system to measure its distance and
absolute magnitude with trigonometric parallax. This is RR Lyrae itself, for which the variability (Figure
1) was discovered, and its similarity to what were then called “cluster variables” first pointed out, by
Wilhelmina Fleming in 1901. Comparison between RR Lyrae and stars with similar light curve and
period can be used to measure the distance to globular clusters, the distribution of which is a good tracer
of the global structure of the Milky Way. As we discussed at length in AST 142, Shapley’s (1917) use of
RR Lyr stars to measure the distances to globular clusters yielded the first correct determination of the
shape of the Galaxy, despite the offset in overall scale that resulted from the (forgivable) error of taking
RR Lyrs to have the same period-luminosity relation as the classical Cepheids in the Small Magellanic
Cloud.
1.2
Experimental procedure
1.2.1
1.
Planning
Select a candidate globular cluster in which to
search for RR Lyr stars. The top 10 northernhemisphere globular clusters in terms of
numbers of variable stars is given in Table 1;
the ten brightest globular clusters in the
northern sky are listed in Table 2. The
coordinates of a larger number of selected,
bright globular clusters are listed in Table 3.
Make sure your cluster is as bright and
variable rich as possible, and above the
horizon as long as possible.
Figure 1: light curves of RR Lyrae (AAVSO). You
will reproduce this result, in this experiment, so I
won't tell you yet what the units on the
horizontal axis mean.
In M3, for example, there may be only one Pop II Cepheid (e.g. Rabidoux et al. 2010), but the cluster
contains more than 240 RR Lyrae stars (e.g. Sawyer Hogg 1973). Not all clusters are so rich in variables; M
13 has no Pop II Cepheids and only 33 RR Lyrae stars (Clement 1997).
1
©2014, The University of Rochester
2
All rights reserved
Physics 243 Astronomy Project Manual
2.
Fall 2014
Table 1: top ten variable-rich globular
clusters in the Northern sky
Table 2: ten brightest Northern globular
clusters, summed over the entire cluster
NGC 5272 (M 3)
NGC 5904 (M 5)
NGC 7078 (M 15)
NGC 7006
NGC 6934
NGC 5024 (M 53)
NGC 2419
NGC 5466
NGC 6205 (M 13)
NGC 6341 (M 92)
NGC 5904 (M 5)
NGC 6205 (M 13)
NGC 5272 (M 3)
NGC 7078 (M 15)
NGC 6341 (M 92)
NGC 5024 (M 53)
NGC 6838 (M 71)
NGC 6779 (M 56)
NGC 6934
NGC 6760
For each candidate globular cluster, identify a nearby calibration star, and the nearest really bright (V
< 2) star. The calibration star bright enough to be easily identifiable in an image, but no brighter -say, V = 6-9 -- and it's handy to use early A stars. (Unextinguished A0 stars have the same magnitude
at all visible wavelengths.) A handy tool for finding an appropriate calibration star close to your
candidate clusters is the UKIRT star-catalogue search engine,
http://www.jach.hawaii.edu/jac-bin/ukstar.pl .
The nearest very bright star will usually be obvious in TheSky, with a wide field view centered on
your candidate cluster. This star will occasionally facilitate telescope pointing checks.
3.
One unusual aspect of these observations is the need for data reduction to take place while the
observations are still in progress. Accordingly, your team should pack two laptop computers from
the undergraduate astronomy lab. One will be used to operate the CCD camera, and the other for
image alignment and photometry, so that variable stars can be identified as soon as they are detected.
4.
Scan the weather reports to find the next reasonable observing night. Look up or calculate the times
of sunset and sunrise, and the length of the night.
Table 3: globular clusters with integrated V < 8. Note how poorly provided the northern
hemisphere of the sky is with bright globular clusters. Here "radius" is that of the halfpeak-brightness isophotal contour.
Cluster
Name
RA (J2000)
Dec (J2000)
NGC 104
47 Tuc
00
24
05.67
-
72
04
52.6
Radius
(arcmin)
3.2
01
03
14.26
-
70
50
55.6
0.8
05
14
06.76
-
40
02
47.6
0.5
05
24
11.09
-
24
31
29.0
0.7
NGC 2808
09
12
03.10
-
64
51
48.6
0.8
NGC 3201
10
17
36.82
-
46
24
44.9
3.1
NGC 4372
12
25
45.40
-
72
39
32.4
3.9
12
39
27.98
-
26
44
38.6
1.5
12
59
33.92
-
70
52
35.4
2.4
NGC 362
NGC 1851
NGC 1904
NGC 4590
M 79
M 68
NGC 4833
©2014, The University of Rochester
3
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
Cluster
Name
RA (J2000)
Dec (J2000)
NGC 5024
M 53
13
12
55.25
+
18
10
05.4
Radius
(arcmin)
1.3
NGC 5139
ω Cen
M3
13
26
47.24
-
47
28
46.5
5.0
13
42
11.62
+
28
22
38.2
2.3
13
46
26.81
-
51
22
27.3
0.7
15
18
33.22
+
02
04
51.7
1.8
15
46
03.00
-
37
47
11.1
1.0
NGC 5272
NGC 5286
NGC 5904
M5
NGC 5986
NGC 6093
M 80
16
17
02.41
-
22
58
33.9
0.6
NGC 6121
M4
16
23
35.22
-
26
31
32.7
4.3
NGC 6171
M 107
16
32
31.86
-
13
03
13.6
1.7
NGC 6205
M 13
16
41
41.24
+
36
27
35.5
1.7
NGC 6218
M 12
16
47
14.18
-
01
56
54.7
1.8
NGC 6254
M 10
16
57
09.05
-
04
06
01.1
2.0
NGC 6266
M 62
17
01
12.80
-
30
06
49.4
0.9
NGC 6273
M 19
17
02
37.80
-
26
16
04.7
1.3
NGC 6341
M 92
17
17
07.39
+
43
08
09.4
1.0
NGC 6333
M9
17
19
11.26
-
18
30
57.4
1.0
NGC 6352
17
25
29.11
-
48
25
19.8
2.1
NGC 6362
17
31
54.99
-
67
02
54.0
2.1
NGC 6388
17
36
17.23
-
44
44
07.8
0.5
NGC 6402
17
37
36.10
-
03
14
45.3
1.3
NGC 6397
M 14
17
40
42.09
-
53
40
27.6
2.9
NGC 6441
17
50
13.06
-
37
03
05.2
0.6
NGC 6544
18
07
20.58
-
24
59
50.4
1.2
NGC 6541
18
08
02.36
-
43
42
53.6
1.1
NGC 6624
18
23
40.51
-
30
21
39.7
0.8
NGC 6626
M 28
18
24
32.81
-
24
52
11.2
2.0
NGC 6637
M 69
18
31
23.10
-
32
20
53.1
0.8
NGC 6656
M 22
18
36
23.94
-
23
54
17.1
3.4
NGC 6681
M 70
18
43
12.76
-
32
17
31.6
0.7
NGC 6715
M 54
18
55
03.33
-
30
28
47.5
0.8
NGC 6723
18
59
33.15
-
36
37
56.1
1.5
NGC 6752
19
10
52.11
-
59
59
04.4
1.9
NGC 6809
M 55
19
39
59.71
-
30
57
53.1
2.8
NGC 7078
M 15
21
29
58.33
+
12
10
01.2
1.0
NGC 7089
M2
21
33
27.02
-
00
49
23.7
1.1
NGC 7099
M 30
21
40
22.12
-
23
10
47.5
1.0
5.
Plan to take 3-5 minute exposures, with the V filter. Do not plan to take dark frames automatically
with every image; instead plan to take them periodically through the night.
6.
Familiarize yourself with the Mees telescope and camera startup and shutdown guide. Obtain a copy
at
©2014, The University of Rochester
4
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
http://www.pas.rochester.edu/~dmw/ast111/Projects/Chklist.pdf
and pack it along with all the other things you'll need to take.
7.
Unlike the observing projects in AST 111, this one takes all night. Therefore you will be required to
sleep at the Observatory after your observations are done and before returning to campus. Plan
accordingly by packing for an overnight stay, and discussing with your co-observers the allimportant requirement of packing sufficient Food. (Mees is unfortunately beyond the range of pizza
delivery, the nearest coffee shop is in Naples, and the nearest donut shop is in Victor.) Familiarize
yourself with the rules of the observatory sleeping quarters at the Gannett House.
8.
Plan to arrive well before sunset. You have one important task to carry out before the sky gets dark.
The telescope and camera must be ready to go, and all team members must be at their stations, as
soon as it's dark enough to acquire your first target.
1.2.2
Before you leave for the observatory
Make sure you have along your lab notebook, calculator, food, and all of the material you prepared or
downloaded in section 1.2.1. Bring also the CCD camera’s laptop computer, obtained from the
undergraduate astronomy lab instructors. If it’s after September, bring cold-weather clothing, including a
heavy coat, gloves and (especially) heavy shoes or boots, no matter how warm you think it might be.
Please note that as you will still be at the observatory at 2:30 AM, you will be required for safety’s sake
to stay until morning.
1.2.3
At the telescope
1.
Go through the telescope and CCD-camera startup instructions outlined in the Mees Telescope
Checklist. Set up appropriately-named directories on the camera computer, into which to record your
data. Decide what your file-naming convention will be. Usually the file name should have at least the
target name, the exposure time, and some sort of version number in case multiple exposures are
necessary. All files should be stored by default in FITS format. The CCD temperature should be set at
-5-0 C, and left at this setting for the entire night.
2.
With the V filter in front of the CCD, point the telescope near the zenith in a spot with no stars visible
in the camera – only blue-sky or twilight emission. Adjust the exposure time so that the signal on the
array is about half the maximum value (65535) or less. Take a series of 16 images with this exposure
time, identifying these images as V flat fields, and – as with the rest of the images you will take –
store them in FITS format.
3.
As the sky darkens, and with the CCD camera reading out short exposures continuously in Focus
Mode, point to a bright star near the zenith. Focus the telescope roughly, center this star carefully in
the camera and update the TCS coordinates. Record the width of the stellar images, in pixels, for brief
non-saturated exposures, now and periodically throughout the night. (This diameter, in arcseconds,
would be the answer if later on one of the instructors asks you what the “seeing” was during your
measurements. The plate scale of the STX-16803 camera is 0.226 arcseconds per pixel.) By moving the
telescope slightly with the guiding paddle between short exposures with the camera, make careful
note of the correspondence among the directions on the CCD display (up, down, left, right) and the
directions on the sky and buttons on the paddle (NSEW), to see that the images from the STX-16803
camera are right-side up (north up, east left) as displayed on the computer.
4.
Check to see if your target is above the telescope’s artificial horizon ( ZA < 70° ). If so, point the
telescope at your target. Bring the cluster into the center of the field, using the telescope paddle, and
©2014, The University of Rochester
5
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
adjust the focus to produce a sharp image. Record this focus readout value and the direction you
were changing the focus to achieve the sharp image (moving to larger or smaller FOC numbers).
Repeat with the eyepiece. Switch back and forth a couple of times to make sure that the telescope
operator can change the focus position between camera and eyepiece reliably and accurately. Make
sure everybody gets a chance to view the target cluster through the main telescope and eyepiece.
The field of view of the STX-16803 camera is quite large, so you should see the cluster in your first
image. But if you have trouble finding it, use TheSky to send the telescope to the previouslyidentified, relatively nearby, very bright star. A V < 2 star is bright enough to produce easily visible
scattered light that indicates its location, even when it is far from the field of view. Center this star in
the eyepiece, reinitialize the coordinates at which the telescope thinks it’s pointing using the TCS
computer, and point back to the target cluster, which should now appear smack in the middle of the
field.
5.
Return the diagonal mirror to the CCD camera’s view, and restore the Focus setting to the value last
used for taking images. Take an image of the target cluster, centered in the field, with the CCD
camera, and make sure the image is still in sharp focus. Note that the field covered by the SBIG STX16803 camera is a square 15.4 arcminutes on a side, so even very large clusters like M13 fit completely
in the picture.
6.
Take a few images of length 3-5 minutes, to determine how long an exposure still gives round stellar
images. Settle on a standard length a little less than this maximum, and take a dark frame of this
length.
7.
Now you’re ready to work. Chase everybody out of the dome and close the door to the stairway a
while before taking any images you will wish to use in your data analysis. (The sharpness of stellar
images will generally improve if you do.) Take images of this same field within your target cluster
over the course of the night. A good sequence would be 25-30 minutes of consecutive exposures on
the NE quadrant of the target, followed by calibration, this pattern repeated as long as possible.
Calibration consists of offsetting the telescope to the nearby calibration star you identified to go with
the target cluster – taking several very short exposures (a few seconds) with this star centered in the
field; taking dark frames the same length as the cluster and calibrator observations; and offsetting the
telescope back to the cluster, trying to reproduce the original position as closely as possible.
It will help a great deal in the analysis to write down the altitude ( 90° − ZA ) of your calibration star
from TheSky’s information window, each time an observation is made of this star.
8.
Interleave observations of RR Lyrae into this routine as well. (It is known to TheSky as SAO 48421.
Being in Lyra, it is close to the very bright star Vega.) Add observations of this star to your observing
routine, and write down the altitude corresponding to each observation, as you also do for your
calibrator.
9.
While this is in progress, the Reductionist, working on the other laptop and with network access to
the image files, should be producing corrected and aligned versions of the images and searching for
stars that change in brightness. The tools most useful in this task are parts of the camera-control
program, CCDSoft, and an IDL routine called ATV.
a.
A great deal of effort will be saved in the analysis if you dark-subtract and flat-field each
image as it comes in. Follow the procedure outlined below (section 1.3, item 1). Use an
addition to your file-naming convention to indicate the state of processing, such as
appending –ds-ff to the original name to designate a dark-subtracted, flat-fielded version.
Subtract the dark frame acquired closest in time to a given image.
©2014, The University of Rochester
6
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
b.
With CCDSoft one can mark each image of the target cluster with the centroid of a given star,
and then shift each member a stack of images so that the star’s centroid is on the same pixel
in each image. This is necessary for keeping track of the variable stars in the face of drifts in
the pointing of the telescope. Select that star with the Mark Centroid button, and then shift
all of the open images so that the star is in the same pixel, using the Image… Align… Align
Centroids feature. If you have previously dark-subtracted and flat-fielded the images, the
aligned versions need nothing else before final analysis. Indicate that an image is aligned by
appending a suitable flag, e.g. –al, to the file name.
c.
ATV has two useful features: Blink and Aperture Photometry, both under ATV’s
MouseMode button, and the latter under the ImExam feature of that button. Blink allows one
with the click of the mouse to toggle the ATV display between two images, which is an
excellent way to find variable stars. Aperture photometry allows one to count up the total
signal in circular area, subtracting automatically the signal from an equal-area annulus
surrounding the circle. If a star lies within the circle and the rest is nothing but sky, the result
is a number proportional to the flux from that star. Compare this result to that of the same act
performed on a star of known magnitude, and you can calculate the flux and magnitude of
that star. This is what we will use to measure the magnitude as a function of time for the
variables we identify in the target cluster.
By this means, identify as many of the target cluster’s variable stars as you can, as you go
along. Carefully note the position of each, in pixels with respect to the alignment star.
10. Whenever an object is not clearly seen in the eyepiece or camera, quickly to point to your previouslyidentified, nearby really bright star, center it in the camera and update the TCS telescope position,
and then point back to the target. You’ll have to learn how to do this in no more than a couple of
minutes.
11. When all is done or when the Sun comes up – whichever comes first – cover and stow the telescope,
list all the exposure times used during the night (and temperatures, if it had been necessary to change
CCD temperature), and make sure you have a dark frame for each exposure time. Take one, if not.
12. For safekeeping, make at least one copy, on flashdrive or DVD, of all of the night’s data.
13. Now shut down the camera, telescope and computers according to the Mees checklist, and go get
some sleep. You may not return to campus without at least the driver(s) sleeping, for at least four
hours. This safety rule will be enforced strictly.
1.3
Data reduction
The steps of reduction and calibration are fairly easy, but there are a lot of data, and it's important to go
through enough of them right away, to see if you need another night on the telescope for this project.
It will be easiest to conduct the preparation of images, and the identification of the variable stars, in the
lab using the same computers that the Reductionist used at the telescope. If all has gone well you already
identified at least 10-12 variable stars and can specify their coordinates in pixels with respect to the
alignment star. If you have only a few identified, start Blinking your images and find more. But now
comes the harder part: measurement of flux as a function of time.
1.
If you didn’t create dark-subtracted and flat-fielded images during your observations, go back to the
original V images and do that now, for the target cluster and the two stars alike. Dark subtraction can
be done with the Image… Reduce… Dark subtract… feature of CCDSoft. Flat fielding amounts to
©2014, The University of Rochester
7
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
division of a dark-subtracted image by a dark-subtracted observation of a uniform brightness object,
such as a star-free patch of daylight or twilight sky. This is why you took blue-sky images earlier
(section 1.2.2, item 2). Flat fielding removes pixel-to-pixel variation in the transmission of the optics,
which may be caused by imperfection of optical elements or (more commonly) dust on the surfaces of
optics near the CCD. The process can be implemented by another feature of CCDSoft, Image…
Reduce… Flat field… . Finally, align all of the target-cluster images, using the same alignment
star as before. Use a file-naming convention as described above to keep track of which image has
been through what stage of processing.
2.
Create a spreadsheet list in Excel of all observations for your night, including the time of observation
and object name. This can be done conveniently in Windows by opening a DOS command window,
changing to the directory containing the original (unprocessed) images, and executing a directory list
command which saves its output in a file: dir > dirlist.txt. Then open the dirlist.txt (or
whatever you called it) file in Excel, edit to leave columns with the observation time and object/file
name, and save in Excel format.
3.
Add the altitude for each observation in a new column. Generate from this a column of zenith angles,
and from this a column of values of sec ZA = 1 cos ZA . By construction your target cluster within a
few degrees of the calibrator, and it is safe to assume that its sec ZA is the same as the closest-in-time
observation of the calibration star.
4.
For each processed (ds, ff) image of the calibrator and RR Lyr, use ATV’s aperture photometry to
measure its signal. Record these numbers in a new column of the spreadsheet, beside the
corresponding time, name and zenith angle.
5.
Sort the spreadsheet to bring the observations of the calibrator together; copy them onto a new sheet
in the same workbook. In this new sheet, make an X-Y plot of the star’s signal as a function of sec ZA .
Click on the series of points in this plot, and choose Chart Tools… Layout… Trendline… to fit a line
to the points. Ask Excel to display the equation of the linear fit, and record the intercept and slope it
reports. The intercept is the atmospheric-extinction-corrected signal from the star, and the slope is
minus the product of the extinction-corrected signal and the extinction of the atmosphere toward the
f f 0 − f 0τ 0 sec ZA . (See AST 142 workshop for 5 April 2013.)
zenith: =
6.
Use this result to make a column of atmospheric-extinction-corrected signals for the two stars:
=
f corrected f original ( 1 − τ 0 sec ZA ) . Compute the average corrected signal for the calibration star,
f calib .
7.
Atmospheric-extinction-correct the images of the target cluster, by multiplying each image by
1 ( 1 − τ 0 sec ZA ) . This can be done with in CCDSoft with the feature Image… Combine… ; choose the
Multiply function from the dropdown box. Again, append an appropriate tag, e.g. –ec, to the names
of the resulting files.
8.
Sorting the original spreadsheet again, make new worksheets devoted to the target cluster and RR
Lyr, with all their filenames and observation times listed. Turn the times into columns of Date and
Coordinated Universal Time by adding four hours to EDT, remembering to adjust the date
accordingly for 8 PM through midnight EDT. Express the time in 2013 digital days.
9.
Now go through each atmospheric-extinction-corrected image of the target cluster and RR Lyr with
ATV, and use the aperture photometry feature to record the corrected signal from each variable star –
each in its own spreadsheet column – as a function of time. Identify the globular-cluster variables by
©2014, The University of Rochester
8
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
their x-y offsets in pixels from the alignment reference star.
V 8.40 + 2.5 log ( f calib f ) .
10. Convert all the signals f so obtained to V magnitudes, via=
11. (Optional) Your calibrated, aligned images would make a nice movie, in the spirit of the one on our
Project page but with much higher time resolution. Such a movie could add considerable geeky
charm to your Facebook page.
1.4
Analysis
Plot all of your light curves: your measurements of V magnitude as a function of time in digital days. In
good astronomical fashion make the magnitudes decrease up the vertical axis, as in Figure 1. Use the plots
to provide answers to the following questions in the course of writing your report.
1.
For how many variables do you have good measurements of the maximum magnitude (minimum
flux)? What is the range of maximum magnitudes? Do any of them seem anomalously small
compared to the others, and seem to change more slowly than the rest? If so, you might have a Pop II
Cepheid there; check with Dan.
2.
For which stars have you measured complete periods? Make a table of period, amplitude and
maximum magnitude for those. Is there a trend among the periods and maximum magnitudes?
3.
Did your observations of RR Lyr include a maximum magnitude and/or complete period? If so, add
this to the table. If not, you need to do some more observing.
4.
Assume that the variables you have identified in your target cluster are like RR Lyr – except for any
anomalies which might be Pop II Cepheids. From the stellar maximum magnitudes, and from the
Hipparcos-measured parallax of RR Lyr ( 4.38 × 10 −3 arcseconds; i.e. a distance of 228 parsecs),
compute the distance to your target cluster. Comment on the accuracy of this result, in light of the
range of periods and maximum magnitudes of the target-cluster variables.
©2014, The University of Rochester
9
All rights reserved
Physics 243 Astronomy Project Manual
2
Fall 2014
Hertzsprung-Russell diagrams of globular clusters
In AST 142, we became intimately familiar with the color-magnitude diagrams of open clusters:
collections of hundreds to thousands of Population I stars, all nearly the same age, loosely and
amorphously arranged and not usually gravitationally bound. A glance at a celestial map like that
available on our lab computers by running TheSky reveals many open clusters, ranging across the sky but
never found very far from the plane of the Milky Way. The stellar clusters that can be seen on the maps,
that lie many degrees away from the plane, are all globular clusters. Globular clusters are spherical,
gravitationally bound, compact collections of 10 5 − 106 Population II stars, all members of which were
formed during a span small compared to their present ages. The globular clusters we can see are rather
heavily concentrated in the direction of the constellation Sagittarius – which lies in the Galactic plane, as
well as the Zodiac – but otherwise show no tendency to follow the Milky Way. Those that lie far from the
plane provide very good targets for astronomy students who are learning how to do photometric
observations of stellar clusters: the density on the sky of cluster members is very large, and the density of
confusing, foreground and background non-members can be very small.
Globular clusters have played several crucial roles in the development of modern astronomy: their
observation has advanced our understanding of pulsating stars, of the cosmic distance scale and the
structure of our Galaxy -- most famously, their use by Harlow Shapley to identify the location of the
dynamical center of the Milky Way, which turns out to be within that heavy concentration of clusters in
Sagittarius -- and of the evolution of stars. In this observing experiment, we will take up the latter of these
topics, and assess the state of evolution of stars in globular clusters by observing the visible magnitudes
and colors of the stars, and constructing a color-magnitude, or Hertzsprung-Russell, diagram from our
results.
Please note:
This project may take more than one trip to the observatory.
This project must be completed before the end of classes. Please make plans to go to the telescope the
very next time it’s clear: it’s unwise to count on it being clear at last minute.
2.1
Preparations for observation
Well before you go to the telescope, you and your team must make a detailed plan of your observations.
1.
Review the observatory startup and shutdown procedures, which can be found at
http://www.pas.rochester.edu/~dmw/ast142/Lab_manual/Chklst.pdf . Take copies of these
documents with you to the observatory.
2.
Browse the sky for available objects. Have at hand a computer running TheSky and the list of
globular clusters in Table 3: globular clusters with integrated V < 8. Note how poorly provided
the northern hemisphere of the sky is with bright globular clusters. Here "radius" is that of the
half-peak-brightness isophotal contour.. Choose a few globular clusters from the list and find
them on TheSky using the Edit…Find… feature. You may find it handy to print for later reference
the Object information window that appears when you Find or click on an object in TheSky.
Determine what times of night during the next 2-3 weeks your choices will lie above 30°
elevation. Estimate how much (or how little!) of each target object’s field will fit on the CCD.
Remember that the field of view of the STX-16803 CCD camera is 15.4 arcminute × 15.4 arcminute
square.
©2014, The University of Rochester
10
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
3.
We will need flux-calibrated images, so we need to identify some standard stars that lie close in
the sky to the target globular clusters: stars that have accurately determined magnitudes and
colors, and cover a fairly large range of apparent magnitude. Start with the two or three stars
nearest your targets, using the procedure of Experiment #1. It would be especially helpful to
identify at least a couple of relatively nearby, early A stars (A0 or A1) – which have the useful
property that they have the same magnitude at all wavelengths –at least one of which is no
brighter than 9th or 10th magnitude. The best list of faint standards, by Landolt (2009), can be
found at http://www.noao.edu/wiyn/queue/images/tableA.html . This list can help you find
stars of given spectral type, within a small angular distance of your target and within a given
range of V magnitudes.
4.
Become familiar once again with the process of photometric calibration, so you can estimate the
magnitudes of stars while you are observing. This is summarized in Figure 2. Recall that once an
image is bias- and dark-subtracted, the sum of the signals (in ADUs) in the image of a point
object like a star – or, more simply, the peak signal in the star’s image – divided by the exposure
time ∆t , is proportional to the flux f from the star, received within the bandwidth of the filter.
Thus, for measurements of two stars at a single wavelength, the ratio of stellar fluxes at this
wavelength is
f1 ADU 1 ∆t2
=
f2
∆t1 ADU 2
Acquire images:
1. Desired image
2. Image of object(s) with known brightness,
at same wavelength, for flux calibration
3. Image of objects with known angular
separation and orientation
4. “Bias frame” at short integration time
5. “Dark frames” or “background frames”
at same integration times as 1-3
6. “Flat field” at same wavelength, with
high signal-to-noise
7. Same uniform illumination at a variety of
integration times and fractions of full
wells, for linearity test
,
(1)
On the basis of the bias-corrected images (7),
correct all others for linearity
Subtract “darks” or “backgrounds” from 1-3
Divide 1-3 by “flat field”
Determine ratio of flux to ADUs from 2;
multiply every entry for 1 by this ratio
Subtract bias frame (4) from all other
images
Determine angular scale and orientation of 1
from 3
Figure 2: Correction and calibration procedure for CCD cameras.
and for the magnitude difference,
 f 
 ADU1 ∆t2 
m2 =
m1 + 2.5 log  1  =
m1 + 2.5 log 
 .
 ∆t1 ADU 2 
 f2 
©2014, The University of Rochester
11
(2)
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
The known magnitude m1 of a standard star can be used in this way with observations of the
standard and a (starlike) target object to calculate the magnitude of the target. 2
5.
Each team must turn in a (non-binding) observing plan before leaving.
2.2
Before you leave for the observatory
Bring along your lab notebook, calculator, food, and all of the material you prepared or downloaded in
section 2.1. Bring also the CCD camera’s laptop computer, obtained from the undergraduate astronomy
lab instructors. If it’s after September, bring cold-weather clothing, including a heavy coat, gloves and
(especially) heavy shoes or boots, no matter how warm you think it might be. Please note that if you are
still at the observatory at 2:30 AM, you will be required for safety’s sake to stay until morning.
2.3
At the telescope
The following procedure is presented under the assumption that the telescope and camera have already
been started, and the telescope pointing verified.
1.
Organize the order of your observations and other activities. Note that in addition to images of
your targets you will need to acquire and save:
a.
dark frames at every exposure time you used for images.
b.
flat field frames for every wavelength you used for images. These are frames with good signal-tonoise ratio of the daytime or twilight sky, or of the white fabric just past the top of the dome slit,
illuminated by dome lights.
c.
frames of each of the standard stars you selected to correspond to the present target, at each
wavelength you used for target images. At least one of these standard stars has to be observed
with the same exposure time as your globular cluster images, at each wavelength, without being
saturated. Prepare, as before, to keep track of the zenith angles at which all the calibrator
observations take place
You can do this before or after your target observations. Teams are encouraged to share their
standard-star, flat-field and dark-current data. Save ALL of your data both in the CCD camera’s
native data format, and in 16-bit FITS format. Your analysis will be performed back on campus
after completing the observations, using the FITS versions of the images.
2.
Point the telescope at your target, using the user-interface computer in the control room running
TheSky. Those in the dome should be able to see the cluster nicely through the finder telescope.
Move the telescope so that the center of the cluster is roughly centered on the CCD. Make sure
the image is in good focus, and take note of the diameter, in pixels and in arcseconds, of the stars
in you images. (Again, this diameter, in arcseconds, would be the answer if later on one of the
instructors asks you what the “seeing” was during your measurements.) To take images on
which to make photometric measurements of the magnitudes and colors of stars, start with
Strictly speaking, the images must also be divided by a flat field after bias and dark subtraction, and the
background sky emission signal per pixel (in ADUs) in parts of the CCD near each star must be
subtracted from each pixel in the stellar image in order that we count only the ADUs produced by the
star. These smaller corrections should be made during your final data analysis.
2
©2014, The University of Rochester
12
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
exposure times around one minute, but make sure that the stellar images aren’t significantly
elongated in right ascension; if they are, adjust the telescope tracking rates so that many-minute
exposures can be taken without elongating the stellar images. Chase everybody out of the dome
and close the door to the stairway a while before taking any images you will wish to use in your
data analysis. (The sharpness of stellar images will generally improve if you do.)
3.
Take at least one good few-minute exposure on your target in each of the three filters R, V and B.
Make sure the telescope pointing offsets and drifts between images are small enough that a large
number of cluster stars are observed at both wavelengths. It will help if the exposure time is the
same at the three wavelengths.
4.
Identify the brightest few stars in each image and note their peak signals in ADUs. Do the same
for the faintest stars you can confidently identify in the image. Find the magnitude difference
between the brightest and faintest, using Equation 2.
5.
If the difference is less than seven magnitudes, calculate the exposure time that would be
necessary in order to detect stars seven magnitudes fainter at each wavelength than the brightest
stars in your image. To do this, use Equation 2, also noting that the flux signal-to-noise ratio
increases in proportion to the square root of exposure time.
6.
Take good images of your target, at all three wavelengths, at longest exposure time calculated in
step 5, thereby enabling later calculation of the magnitudes and B-V colors of cluster members
over at least a seven magnitude range.. Make sure that the telescope tracks well enough that the
stellar images are not elongated; adjust the tracking rates and try again if they are.
7.
(Optional) If there is time left over after taking data on your target and acquiring all necessary
standard star, flat-field and dark-current images, look around on the display of TheSky to see if
there are any other objects for which you’d like to try taking images. You are encouraged to
explore all different kinds of objects.
8.
Before relinquishing control of the telescope to the next team, or shutting the system down, make
sure that you’ve acquired all of the necessary data, have saved it in both formats, and have
backed it up on a DVD or flashdrive.
9.
If you are the last team to observe, make sure you shut the system down, carefully and
scrupulously according to the printed procedures. Also make sure that the dome power is shut
off and that the doors are locked before you leave. And don’t run into any deer on the drive
home.
2.4
Back on campus
10.
You will perform photometry on your cluster in automated fashion, using the IDL-based
program XStarfinder. This will most easily be done with the CCD camera’s computer. You and
your team should arrange with the instructors to be admitted to this room for the following
procedures.
a.
Run your B and V images through Xstarfinder. This will produce two lists of coordinates and
magnitudes that can be imported as text files into Excel.
b.
Put a copy of both lists in the same Excel worksheet, and match up the B and V measurements
that have the same coordinates, deleting records of stars that were not detected at both
wavelengths. (This can be tedious, obviously.)
©2014, The University of Rochester
13
All rights reserved
Physics 243 Astronomy Project Manual
c.
Fall 2014
Finally, using Excel formulas, Fill Down, Equation 2, and the signals and magnitudes of your
reference star, calculate the B and V magnitudes, and the B-V color index for all of your globular
cluster stars, recording the results in new spreadsheet columns in each case.
11.
Plot V vs. B-V for all of your stars, in an Excel chart. Make the V axis runs “backwards” (smaller
values at the top, larger values at the bottom), as is conventional in color-magnitude diagrams;
you can do this by clicking the “Values in reverse order” box on the Scale menu for the vertical
axis.
12.
Discuss the probable nature of these stars in the light of their position in this diagram: Which are
likely to be asymptotic giant branch stars? Horizontal branch stars? Red giants? Main sequence
stars? Your score on this project will increase more dramatically the further down the main
sequence your measurements go.
13.
Finally, and most importantly, compare your results to the H-R diagram for nearby stars, and
describe why, taken together, these stellar-cluster observations imply that stars evolve: that
their properties change over long periods of time, and that they do not last forever.
14.
Choose a set of dark-subtracted, flat-fielded, scaled, shifted images of your target at the three
wavelengths that all have the same exposure time. Use Image…RGB combine… to generate a
color image of your target, mapping the R, V and B bands respectively to red, green and blue. If
the shifting has been done properly (step 4) then each star will appear still to be round and will
not have a “blue end” or a “red end.” If the same display background and range is selected for
each of your monochromatic images, the resulting combined image will be in true color. Save this
image in uncompressed JPG format and include it in your report, too.
©2014, The University of Rochester
14
All rights reserved
Physics 243 Astronomy Project Manual
3
Fall 2014
Active nuclei and their host galaxies: observations of Seyfert galaxies
In the 1940s Carl Seyfert discovered the class of galaxies with active nuclei which bears his name. The
hallmark of Seyfert galaxies is a relatively bright, starlike object, easily seen in short exposures, around
which long exposures reveal the bulge and disk of a spiral galaxy. The central object’s luminosity is
generally about as large as that of the rest of the galaxy. The active nucleus also contains ionized gas for
which the emission-line spectrum indicates an ionization state substantially higher than is typical of the
H II regions associated with star formation. Profiles of forbidden lines of the ions of “metals” in Seyfert
nuclei – by which the ionization state is analyzed – are generally relatively narrow (Doppler velocities of
a few hundred km/s) and consistent with broadening primarily by the rotation of the galaxy. In Seyferts
of type 2 (like NGC 1068) this profile also characterizes the hydrogen recombination lines like Hα and
Hβ. In Seyferts of type 1 (like NGC 4151), however, the hydrogen recombination lines have in addition a
very broad component to their profile that indicates Doppler velocities of a few thousand km/s with
respect to the rest of the galaxy. Nowadays we think that the same basic supermassive-black-hole-plusaccretion-disk geometry applies to both main types of Seyferts, and the difference of type is an artifact of
the angle at which the disk is viewed: we see further in to the center of the disk for type 1 than type 2.
(This suggests that we should also see Seyfert galaxies intermediate in characteristics between these two
basic types, and that does in fact turn out to be the case.)
In this project you will repeat one of Seyfert’s classic observations, using a CCD camera instead of
photographic plates, and demonstrate that short exposures show only the active nucleus while long, or
lower-noise, exposures reveal the galaxy’s central stellar cluster as well. With observations in several
wavelengths you will also compare the colors of the active nucleus and central stellar cluster.
Please note:
This project may take more than one trip to the observatory.
This project must be completed before the end of classes. Please make plans to go to the telescope the
very next time it’s clear: it’s unwise to count on it being clear at last minute.
3.1
Preparations for observation
Well before you go to the telescope make a detailed plan of your observations.
1.
Refer to all of the “preparations” instructions in section 1 above. In particular, review the
observatory
startup
and
shutdown
procedures,
which
can
be
found
at
http://www.pas.rochester.edu/~dmw/ast142/Lab_manual/Chklst.pdf . Print copies of these
documents to take with you to the observatory.
2.
Unlike Experiments 1 and 2, there is an obvious “best” pair of galaxies to observe: NGC 1068
(a.k.a. M 77) and NGC 772, described in Figure 3.
3.
Again, we will need flux-calibrated images, so we need to identify some standard stars that lie
close in the sky to the targets. So repeat step 3 of Experiment 2 for each of the galaxies.
4.
Become familiar once again with the process of photometric calibration, so you can estimate the
magnitudes of stars while you are observing. This is described in section 1.1, and summarized in
Figure 2, above.
©2014, The University of Rochester
15
All rights reserved
Physics 243 Astronomy Project Manual
Name
NGC 772
NGC 1068
Fall 2014
Axis
δ (J2000) Hubble V (mag) Major
type
diameter ratio
(arcmin)
01 59 19.6 + 19 00 27
Sb
10.3
5
0.80
02 42 40.8 - 00 00 48
Sb
8.9
9
0.89
α (J2000)
Figure 3: NGC 772 (left) and NGC 1068 (right), and some basic information about these
two galaxies.
3.2
Before you leave for the observatory
Again: pack your lab notebook, calculator, food, and all of the material you prepared or downloaded in
section 3.1. Bring also the CCD camera’s laptop computer, obtained from the undergraduate astronomy
lab instructors. If it’s after September, bring cold-weather clothing, including a heavy coat, gloves and
(especially) heavy shoes or boots, no matter how warm you think it might be. Please note that if you are
still at the observatory at 2:30 AM, you will be required for safety’s sake to stay until morning.
3.3
At the telescope
The following procedure is presented under the assumption that the telescope and camera have already
been started, and the telescope pointing verified.
1.
Organize the order of your observations and other activities. Note that in addition to images of
your targets you will need to acquire and save:
a.
dark frames at every exposure time you used for images.
b.
flat field frames for every wavelength you used for images. These are frames with good signal-tonoise ratio of the daytime or twilight sky, or of the white fabric just past the top of the dome slit,
illuminated by dome lights.
c.
frames of each of the standard stars you selected to correspond to the present target, at each
wavelength you used for target images. At least one of these standard stars has to be observed
with the same exposure time as your galaxy images, at each wavelength, without being
saturated.
©2014, The University of Rochester
16
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
You can do this before or after your target observations. Teams are encouraged to share their
standard-star, flat-field and dark-current data. Save ALL of your data both in the CCD camera’s
native data format, and in 16-bit FITS format. Your analysis will be performed back on campus
after completing the observations, using the FITS versions of the images.
2.
Point the telescope at a reasonably faint (say, 4th-9th magnitude) star near your target Seyfert
galaxy, selected using the user-interface computer in the control room running TheSky. Move the
telescope so that this star is roughly centered on the CCD. Make sure the image is in good focus,
and take note of the diameter, in pixels and in arcseconds, of this star. (This diameter, in
arcseconds, would be the answer if later on one of the instructors asks you what the “seeing” was
during your measurements.) To take images on which to make photometric measurements of the
magnitudes and colors of stars, start with exposure times around one minute, but make sure that
the stellar images aren’t significantly elongated in right ascension; if they are, adjust the telescope
tracking rates so that many-minute exposures can be taken without elongating the stellar images.
Turn on the dome’s exhaust fan, chase everybody out of the dome, and close the door to the
stairway a while before taking any images you will wish to use in your data analysis. (The
sharpness of stellar images will generally improve if you do.)
3.
Now point the telescope at NGC 1068. Take at least one good short exposure (30 sec – 2 min) on
your target in each of the three filters R, V and B. In these images the galaxy will look starlike:
that’s the active nucleus. Make sure the telescope pointing offsets and drifts between images are
as small as possible. It will help if the exposure time is the same at the three wavelengths.
4.
Now take good images of NGC 1068, at all three wavelengths, at an exposure time long enough
to reveal that there is a spiral galaxy surrounding the starlike object seen in the short exposures.
Again it will help if the exposure times are the same at all three wavelengths. Make sure, in
particular, that the B and V images are good enough to enable accurate calculation of the total
magnitude and B-V color of the galaxy’s central stellar bulge. And make really sure that the
telescope tracks well enough that compact objects within your images (e.g. the active nucleus) are
not elongated; adjust the tracking rates and try again if they are.
5.
Repeat steps 3 and 4 for NGC 772.
6.
(Optional) If there is time left over after taking data on your targets and acquiring all necessary
standard star, flat-field and dark-current images, look around on the display of TheSky to see if
there are any other objects for which you’d like to try taking images. You are encouraged to
explore all different kinds of objects.
7.
Before relinquishing control of the telescope to the next team, or shutting the system down, make
sure that you’ve acquired all of the necessary data, have saved it in both formats, and have
backed it up on a CD or memory stick.
8.
If you are the last team to observe, make sure you shut the system down, carefully and
scrupulously according to the printed procedures. Also make sure that the dome power is shut
off and that the doors are locked before you leave.
3.4
Back on campus
Data reduction will be carried out using CCDSoft, an image-processing program that comes with TheSky.
This will most easily be done with the CCD camera’s laptop computer. You and your team should
arrange with the instructors to be admitted to this room for the following procedures. This is the most
©2014, The University of Rochester
17
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
labor-intensive part of the experiment. Insure that the tasks of measurement of magnitudes are
distributed equitably among the members of your team.
1.
Start up CCDSoft and Microsoft Excel. Under CCDSoft’s Photometry Setup, enter the information
appropriate for an f/12.5 24” telescope, a CCD camera with 20 micron square pixels, and seeing
quality appropriate for your findings in step 2 above.
2.
In CCDSoft, open all of FITS images of your standard stars at one of the wavelengths. Choosing
one of them as the reference, select Reference magnitude, click on this star’s image, and enter the
star’s magnitude at this wavelength. Then choose Determine magnitude, and click successively
on the images of all of the other standard stars. Compare these magnitudes to those listed in the
catalogues for each star. If there are many large discrepancies, repeat the process with one of the
other stars serving as a reference.
3.
Repeat for the other two wavelengths. In the end, determine as accurately as possible the
magnitude of the reference star for which the exposure time is the same as that for the target
cluster. Hopefully this won’t be too different from its catalogue magnitude.
4.
Shift all of the images of your target Seyfert galaxy so that the active nucleus is in the same
position (in pixel coordinates) on all the images. To do this, click the Mark centroid button, and
click on the nucleus in each image, recording the centroid position displayed by CCDSoft at lower
left each time you do. Then use Modify…Shift… on all of the images but one to shift the images
so that the selected star will have the same position in all images. Check to make sure it worked
by using Mark centroid on this star again in all the shifted images. Save all new images produced
thereby, and work henceforth on the shifted images.
5.
Repeat step 4 for all your images of the normal spiral galaxy.
6.
Now you are ready to measure the magnitudes and colors of the galaxy nuclei. In the short
exposures it may be possible to measure the magnitudes and colors of the active nucleus in the
same manner as the colors and magnitudes of your globular cluster stars: Open the B-band
images for your Sefert and for the magnitude reference star. Use Reference magnitude to set the
B magnitude you worked out for the reference star. Then use Determine magnitude to measure
the magnitudes of the compact nuclues. Repeat in V and R.
7.
Estimate the B-V color of the central stellar bulge in both of your galaxies. To do this, move the
cursor across your image to determine values in well-defined locations within the bulges of the
signal in ADUs, and the background level well away from any substantial galactic or stellar
emission, also in ADUs. Repeat the process for one of your standard-star images. Then use these
flux measurements in ADUs to derive the magnitudes and colors for the bulge locations you
chose, using Equations 1 and 2.
7.
Discuss the probable nature of these galaxy nuclei. What kinds of stars predominate in the galaxy
bulges? What are the similarities and differences between the bulges and nuclei in the two
galaxies you observed?
8.
Discuss the nature of the compact object in the nucleus of the Seyfert galaxy. How does it
compare in color and magnitude to the bulge? Does it have a counterpart in the normal spiral
galaxy? (Here the View – 3D surface plot feature of CCDSoft may be useful.) What kinds of stars
have colors similar to the color of the compact object? How many stars of this type would be
required to produce an object of the observed magnitude, and within what distance would they
©2014, The University of Rochester
18
All rights reserved
Physics 243 Astronomy Project Manual
Fall 2014
need to be concentrated? Given the lifetime of such stars, discuss the likelihood that the compact
object is a cluster of stars, and offer alternative models if the likelihood seems small.
8.
Choose a set of images of your target galaxies at the three wavelengths that all have the same
exposure time. Use R-G-B Combine to generate a color image of your target. If the shifting has
been done properly (step 4) then each star will appear still to be round and will not have a “blue
end” or a “red end.” Save this image and include it in your report.
©2014, The University of Rochester
19
All rights reserved