星團由於其生成的機制，其成員星皆有著相似的特性，例如其化學組成、年齡、金屬豐度、自行量等。星團可作為研究銀河系之結構與演化的工具。而疏散星團則是由於其分布於接近銀盤，可代表銀盤面的特性與演化過程。

以UBV測光系統估計疏散星團之年齡、金屬豐度、紅化消光等需要成員星的資訊。我們測量各恆星自行量，並進行視差影響之修正。再從各恆星的自行量在向量點圖中的分布，擬和二元常態分布的方式得出各星之成員星概率。我們以這些可能的成員星以參考文獻之方法進行紅化消光的修正，最後再以金氏模型之曲線擬合的方式獲得疏散星團的質量與潮汐半徑。本文以鹿林山天文台取得NGC 7142疏散星團之觀測，選擇774顆恆星為可能成員星並進行紅化消光修正。結果顯示當金屬豐度Z = 0.010時平均紅化值為E(B−V) = 0.42、E(U−B) = 0.45，平均消光值Av = 1.30。所得星團距離模式(m−M) = 11.60 ± 0.1，對數年齡約落在9.50 ~ 9.80之間。金氏模型求得潮汐半徑22秒差距、質量約4100太陽質量。

The forming mechanisms of star clusters make the member stars of open cluster have similar properties such as chemical composition, age, metallicity and motion in the sky. Star clusters are useful tools in studying structure and evolution of Milky Way Galaxy because of these properties. Open cluster is a type of star cluster that mainly located on the Galactic disk which can represent the Galactic disk evolution progress and characteristic.

Using UBV image photometry to estimate the age, metallicity and reddening/extinction correction require membership information. Our method bases on astrometry measurement of proper motion with parallax correction. From fitting the bivariate distribution of proper motion in vector-point diagram, we acquire the membership probability of each star. We estimate the reddening and extinction with the probabilities information and UBV photometry data by literature method. Finally, we also estimated the mass and tidal radius the open cluster by the King model. To our target we observed with Lulin one-meter telescope, NGC 7142, we selected 774 star as probable member stars and result shows the average reddening E(B−V) = 0.42, E(U−B) = 0.45, mean extinction Av = 1.30 while Z = 0.010, distance modulus (m−M) = 11.60 ± 0.1 and logarithm age = 9.50 ~ 9.80. Tidal radius of the cluster is ~ 22 pc. The mass of NGC 7142 from the King model is ~ 4100 Mʘ.

Abell, G. O., 1959, ASPL, 8, 121
Balaguer-Núňez, L., Tian, K. P. & Zhao J. L., 1998, A&ASS, 133, 387
Blanco, V, M., 1956, ApJ, 123, 64
Cardelli, J. A., Clayton, G. C. & Mathis, J. S., 1989, 345, 245
Crinklaw, G & Talbert, F. D., 1991, PASP103, 536
Feast, M. & Whitelock, P., 1997, MNRAS, 291,683
Hiltner, W. A. & Johnson, H. L., 1956, ApJ, 124, 367
Hoag, A. A. et al., 1961, PUSNO, 13, 343
Hsieh, H.-Y., 2013, NTNU master thesis
Huang, S.-T., 2007, NTNU master thesis
Jennens, P. A. & Helfer, H. L., 1975, MNRAS, 172, 681
Johnson, H. L. & Morgan, W. W., 1953, ApJ, 117, 313
Johnson, H. L. & Morgan, W. W., 1955, ApJ, 122, 142
Johnson, H. L. et al., 1961, LowOB, 5, 133
King, I. R., 1962, AJ, 67, 471
King, I. R., 1966, AJ, 71, 64
Lasker, B. M. et al, 1990, AJ, 99, 2019
Lindholm, E. H., 1957, ApJ, 126, 588
Morgan, W. W., Harris, D. L. & Johnson, H. L., 1953, ApJ, 118, 92
Oort, F. H., 1927, BAN, 3, 275
Palmer, J. & Davenhall, 2011, StarC, 6
Punanova, A. F. et al. 2012, AASP, 2, 11
Sanders, W. L., 1971, A&A, 14, 226
Savage, B. D. & Mathis, J. S., 1979, ARA&A, 17, 73
Sharov, A. S., 1965, SvA, 8, 780
Sharov, A. S., 1968, SvA, 12, 550
Slovak, M. H., 1977, AJ, 82, 818
Smart, W. A., 1949, Text-book on Spherical Astronomy, Cambrige universty press, UK
Straižys, V., Sūdžius, J & Kurilinenė, G. 1976, A&A, 50, 431
Straižys, V. et al, 2014, MNRAS, 437, 1628
van den Bergh, S. 1962, JRASC, 56, 41
van den Bergh, S. & Heeringa, R., 1970, A&A, 9, 209
Vasilevskis, S., Klemola, A. & Perston, G., 1958, AJ, 63, 387
Vasilevskis, S., Sanders, W. L. & van Altena, W. F., 1965, AJ, 70, 806
Wang, J.-H., 2003, NTNU master thesis
Wu, C.-K., 2004, NTNU master thesis
Zhao, J. L. & He, Y. P., 1990, A&A, 237, 54
WEBDA, 2006, http://www.univie.ac.at/webda/
CMD 2.8, 2016, http://stev.oapd.inaf.it/cgi-bin/cmd