Program: COSMOS UC Irvine

Year: Summer 2014

Goal: Analyze the open star cluster M11 for its physical properties such as distance, age, chemical composition, brightness, color, through analyzing CCD images


  • profile fitting photometry to measure brightness of stars

  • flat-fielding & data reduction to select candidate stars for analysis

  • HR Diagram analysis with best fitting isochrone to determine age, composition, etc of the cluster

  • error of less than 6.3% overall

Technical Details

Using the UCI 24" telescope, four I-band and three V-band images were taken, with pixel scale of 0.81 arcsec/pix and in a 7"x 7" field of view. Profile-fitting photometry was used to measure the brightness of the stars present in the open star cluster Messier 11 (M11), also known as the Wild Duck Cluster. Most stars in this cluster are unevolved main sequence stars due to its young age, with some red giants and white dwarfs.

Because each pixel has different level of quantum efficiency, flat fielding is used to calibrate the pixel efficiency and to reduce error. Once the images had been flat fielded, the average full width at half maximum of the Gaussian functions of were used to represent intensity in terms of radius. This is also known as the point-spread function (PSF).


Only the brightest, isolated stars were used, and DAOPHOT function on LINUX was used to determine the 840 stars finally used for the HR diagram. DAOPHOT provides a list of possible stars, and the stars have to be sorted and refined manually, especially for the faint stars not automatically detected by the function.

The Hertzsprung-Russell Diagram (HR Diagram) was plotted using the selected stars. The HR diagram plots the brightness of stars as a function of temperature and luminosity. The best fitting isochrone lines were found using the PADOVA stellar evolutionary models, and the best line was used to derive the age, distance, chemical composition/metallicity, and reddening of the cluster M11.

The results were verified through comparison with WEBDA results- and the maximum error was about 6%. The slight differences in value could be due to circumstances such as the cloudy weather.

© 2021 by Olivia Heui Young Park

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