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An Examination of the Age of and Distance to NGC 1496
Andrew Wade and Curtis Roberts
Sullivan South High School, Kingsport, TN
Teacher: Thomas Rutherford, RBSE 2005
ABSTRACT
Using the 0.9-meter SARA telescope (equipped with a CCD camera) located at the Kitt Peak
National Observatory; images were obtained of the open cluster NGC 1496 in B and V
photometric filters. From this data the age of and distance to the cluster were determined. NGC
1496 was found to lie at a distance of 1.4 x 103 parsecs (4600 light years) from Earth. In
addition, the cluster, which contains no evident spectral class O or B stars, is approximately 400
million years old.
INTRODUCTION
An open cluster is a group of gravitationally-bound stars that formed in a single gas cloud.
Members of an open cluster are approximately the same age, lie at approximately the same
distance from the earth, and are composed of similar chemicals (Frommert 2007). The study of
open clusters is crucial to astronomy because it provides insight into the nature of the formation
of stars.
All data used in this study were collected remotely on the night of February 25, 2010 using a
Charged-Coupled Device (CCD) mounted on the SARA 0.9-meter telescope at Kitt Peak
National Observatory (Tucson, AZ).
Previous research of NGC 1496 includes photoelectric UBV and photographic RGU photometry
by del Rio and Huestamendía in 1987.
OBSERVATIONS AND DATA REDUCTION
Six 60-second exposures of the cluster were taken in two filters (B and V). These images were
then calibrated and stacked using AutoStar Suite in order to increase the signal to noise ratio of
the data (LeFevre 2003).
The two resulting images (one in each filter) were then analyzed photometrically using the
software MaximDL.
The RBSE Journal 1 2010 V4 Image 1: An image of the stacked data of the cluster in the B filter is shown above. The image’s
color has been inverted in order to facilitate the nature of the photometric analysis involved with
MaximDL.
Image 2: This study used the numbering system created by del Rio (1987). Some numbered
stars were not included because of limitations involving photometry.
The RBSE Journal 2 2010 V4 To determine the magnitudes of the other cluster members, NGC1496-22 was used as a reference
in both filters. Star 22 was chosen because its magnitude was known from the WEBDA site;
however, the software should calculate the same magnitudes for the entire data set if any other
reference star had been used. After determining the other members’ magnitudes using MaximDL,
the data was entered into a spreadsheet (Microsoft Excel) in order to efficiently subtract the V
magnitudes from the B magnitudes. This gave the color index of each star. However, corrections
to the data had to be made in order to account for interstellar extinction and reddening, which
make the cluster members seem fainter and farther away than they actually are. In order to
accomplish this, 0.45 was subtracted from each color index value (WEBDA), and 1.4 was
subtracted from each V magnitude value (since the total absorption in the V band is
approximately 3.1 times greater than the reddening) (Garmany 2007).
ANALYSIS AND RESULTS
After accounting for reddening in the (B-V) calculations, the stars were classified by spectral
type. Table 1 presents the criteria used to classify the cluster members.
Table 1: The color index of a star may be used to estimate its spectral type. It may also be used
to estimate the star’s lifespan (Washington 2005, Clemens 2007).
Spectral Type Color Index (B-V) Lifespan (years)
O
-0.4
<1 x 106
B
-0.2
3 x 107
A
0.2
4 x 108
F
0.5
4 x 109
G
0.7
1 x 1010
K
1.0
6 x 1010
M
1.6
>1 x 1011
The cluster members were next plotted according to the Hertzsprung-Russell (H-R) model
(Comins 2003). Then the Schmidt-Kaler ZAMS (Zero-Age-Main-Sequence) graph (1982) was
juxtaposed onto the graph. By examining the H-R diagram in relation to the ZAMS line, the
turnoff point of NGC 1496 became apparent. The star that sits on the tip of the main sequence is
approximately the same age as the cluster itself (Rieke 2002). Using this method, the age of
NGC 1496 is no greater than 400 million years.
The RBSE Journal 3 2010 V4 Figure 1: (X=color index (b-v), Y=Brightness (v)) According to the graph, the tip of the main
sequence appears to lie around NGC 1496-1. This star’s color index is approximately 0.062,
classifying it as a spectral type A star. Based on this observation, the age of the cluster is
estimated to be no greater than 400 million years. Stars 21 and 46 appear to be outliers in the
data set. This could imply that they are not cluster members, but are in fact closer to the earth
than is the actual cluster.
Since all stars in a cluster lie at approximately the same distance from Earth (Frommert 2007),
we can determine the distance of the entire cluster from Earth based on a few stars. However, the
absolute magnitude of these stars must be known. Thus 3 standard stars, Vega (α Lyrae),
Procyon (α Canis Minoris), and Capella (α Aurigae) (“Stellar Brightness”, “The Colour of
Stars”), with known absolute magnitudes and color indices equal to that of three of the cluster
members-- stars 2, 7, and 15, respectively, were chosen and the distance modulus equation
applied to them (Seeds 2005):
d=10(m-M+5)/5
Equation 1
where m=apparent V magnitude (11.484, 14.061, 9.786), M=absolute V magnitude (0.6, 2.6,
0.4), and d=distance from Earth (in parsecs). The resulting distances from each calculation were
then averaged in order to determine the cluster’s distance from Earth, which is 1400 parsecs, or
4600 light years.
The RBSE Journal 4 2010 V4 DISCUSSION
There are some differences with the data observed in this study and del Rio’s determination of
the age of the cluster. After converting (G-B) data to (B-V) data, del Rio determined the age of
the cluster to be 630 million years (1987) while the current study arrived at an age of 400 million
years. This discrepancy may be due to differences in photometric analysis as well as in the
method of image capture (CCD vs. photoelectric photometer). The distance to the cluster
observed in del Rio’s paper also varies from the findings in this study (1230 pc versus 1400 pc)
(1987), although not by a great deal.
SUMMARY
Using modern methods of photometry to analyze images taken of NGC 1496 using the 0.9-meter
SARA telescope, the age of the cluster (400 million years) and its distance from Earth (1400 pc)
were determined. These results do vary from those of previous studies (del Rio 1987); however,
this may be due to differences in the methods used to collect data.
ACKNOWLEDGEMENTS
The researchers would like to thank the following people for their contributions to the project:
Dr. Gary Henson (ETSU), for collecting the images using the SARA telescope, the Southeastern
Association for Research in Astronomy (SARA); Curtis Roberts, for assisting with various
aspects of the project; Mr. Thomas Rutherford, for advising and guiding this endeavor; and Ms.
Melanie Calhoun, for her cooperation and support of the project.
REFERENCES
Clemens, Christina and Rachel Reece. “The Age and Distance of the Open Cluster NGC 2345.”
RBSE Journal. 2007. 23 March 2010.
“The Colour of Stars”. [Online]
http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html December
24, 2004.
Comins, Neil and William Kaufmann III. Discovering the Universe. 65th ed. New York: W.H.
Freeman and Company, 2003. 277.
Del Rio, G. and G. Huestamendía. 1988. “Photoelectric UBV and Photographic RGU Photometry of the Open Clusters NGC 1496 and NGC 1513.” Astronomy and Astrophysics
Supplement Series. Vol 73, 425.
“Distances to Open Clusters.” [Online] http://www.math.lsa.umich.edu/mmss/courses
ONLINE/Astro/Ex1.3/ 2002.
Frommert, Hartmut and Christine Kronberg. “Open Star Clusters.” [Online]
http://messier.obspm.fr/open.html August 27, 2007.
Garmany, Katy. 2007 March 22. A Photometry Question [email.] Accessed 2010 March 29.
Kaisler, Denise. “Cosmic Intrigue.” Astronomy (2000): 42-46. The RBSE Journal 5 2010 V4 “Kitt Peak National Observatory.” [Online] http://www.noao.edu/kpno/ 2010.
LeFevre, Paul.. “Aligning and Stacking Images.” [Online]
http://www.lefevre.darkhorizons.org/articles/proctutorialchap3.htm 2003.
Rieke, Marcia. “Star Clusters, Introduction to the Milky Way.” [Online]
http://ircamera.as.arizona.edu/astr_250/Lectures/lecture_18.htm March 18, 2002.
Schmidt-Kaler, Th. 1982, Landolt-Börnstein, Numerical Data and Functional Relationships in
Science and Technology, New Series, Group VI, vol. 2(b), ed. K. Schaifers, & H.H. Voigt
(Berlin: Springler Verlag), 14
“Stellar Brightness.” [Online]
http://www.astro.wisc.edu/~dolan/constellations/extra/brightest.html 2010.
University of Washington. “Cluster Color-Magnitude Diagrams and the Age of Stars.” 16 May
2005. 29 March 2010.
The RBSE Journal 6 2010 V4 APPENDIX 1. The following table presents each star used in this study. Some cluster members
were omitted.
Star Name NGC 1496‐1 B V Color Index X Position Y Position Magnitude Magnitude (B‐V) 2 3 4 5 6 7 8 9 10 11 12 15 16 17 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 42 43 The RBSE Journal 0 6.7 ‐13.6 ‐17.3 ‐12.1 1.1 10.4 1.1 ‐4.8 ‐41.5 ‐36.4 ‐14.4 ‐58.4 ‐92.7 ‐63.5 68.5 114.1 68.1 82.6 96.1 103.6 111 118.9 121.4 141.3 52.8 44.3 41.9 34 36.7 44.2 22.3 84.3 96.3 92.5 100 65 33.2 0 ‐6.5 ‐6.9 ‐11.7 ‐20 ‐27.4 ‐32.6 ‐43.7 ‐41.6 ‐34.1 ‐47.2 ‐60.7 ‐120.9 ‐89.6 ‐75.7 ‐103.1 ‐106.6 ‐63.6 ‐65.8 ‐57.2 ‐35.9 ‐49.6 ‐55.3 ‐71 ‐47.9 ‐27.7 ‐24.5 ‐15.3 ‐20.5 ‐0.5 23.4 47.6 41.3 59.1 63.1 74.8 105.6 96.5 12.535
13.452
13.913
13.5
15.141
14.241
16.397
14.291
13.562
16.086
15.366
16.398
12.31
16.185
16.601
15.797
14.486
15.79
13.719
13.957
16.83
15.668
15.017
14.616
15.33
14.903
14.154
16.132
15.239
12.346
14.652
13.196
15.617
15.239
16.218
15.873
14.337
14.423
7 12.023
12.884
13.365
12.887
14.347
13.641
15.461
13.621
12.303
15.315
14.607
15.547
11.186
15.304
15.651
14.905
13.721
14.89
13.161
13.005
14.776
14.659
14.377
13.252
14.68
14.111
13.557
15.277
14.577
11.436
14.032
12.645
14.893
14.393
15.423
14.981
13.73
13.795
0.512
0.568
0.548
0.613
0.794
0.6
0.936
0.67
1.259
0.771
0.759
0.851
1.124
0.881
0.95
0.892
0.765
0.9
0.558
0.952
2.054
1.009
0.64
1.364
0.65
0.792
0.597
0.855
0.662
0.91
0.62
0.551
0.724
0.846
0.795
0.892
0.607
0.628
B‐V (red) 0.062 0.118 0.098 0.163 0.344 0.15 0.486 0.22 0.809 0.321 0.309 0.401 0.674 0.431 0.5 0.442 0.315 0.45 0.108 0.502 1.604 0.559 0.19 0.914 0.2 0.342 0.147 0.405 0.212 0.46 0.17 0.101 0.274 0.396 0.345 0.442 0.157 0.178 Spectral V (red) Class 10.623
11.484
11.965
11.487
12.947
12.241
14.061
12.221
10.903
13.915
13.207
14.147
9.786
13.904
14.251
13.505
12.321
13.49
11.761
11.605
13.376
13.259
12.977
11.852
13.28
12.711
12.157
13.877
13.177
10.036
12.632
11.245
13.493
12.993
14.023
13.581
12.33
12.395
A A A A A A F A G A A F G F F F G F A F M F A M A A A F A F A A A F A F A A 2010 V4 44 45 46 47 48 49 50 51 52 53 54 55 The RBSE Journal 20 31.3 ‐12 ‐6.5 ‐34.4 ‐57.1 ‐68.7 ‐78.4 ‐135.8 ‐143.3 ‐143.2 ‐91.1 113.9 153.3 154.6 79.1 41.5 43.5 57.2 34.3 86.8 48.2 ‐39.8 ‐10.7 14.926
14.939
15.473
15.009
14.356
15.616
16.372
16.583
13.712
14.905
14.874
15.355
8 14.073
14.025
14.69
14.085
13.766
14.883
15.495
14.768
13.123
14.192
14.232
14.674
0.853
0.914
0.783
0.924
0.59
0.733
0.877
1.815
0.589
0.713
0.642
0.681
0.403 0.464 0.333 0.474 0.14 0.283 0.427 1.365 0.139 0.263 0.192 0.231 12.673
12.625
13.29
12.685
12.366
13.483
14.095
13.368
11.723
12.792
12.832
13.274
F F A F A A F M A A A A 2010 V4