Exercise 4 - Measurement of the Age and Distance of a Globular Star Cluster

We measure blue (m_B) and green (visual, m_V) magnitudes of selected stars in the outer regions of a globular cluster shown on VLT images, convert the (m_B-m_V) values into stellar surface temperatures (T) and plot the m_V values as a function of the T values on a Hertzsprung-Russell diagram. The cluster's Main Sequence, seen in the plotted diagram, is compared with a distance-calibrated standard Main Sequence from the nearby Hyades cluster. The distance to the cluster can be determined by Main Sequence fitting and using the distance modulus. The cluster's age, which incidentally places a lower limit on the age of the Universe, can be estimated from the position of the turn-off point on the Main Sequence.

Two FITS files and one Excel file with additional data for this exercise can be downloaded here.

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Exercise 4 PDF files

• Exercise 4 in low resolution (3.2 MB)
• Exercise 4 in high resolution (10.9 MB)

Unfortunately some errors exist in the printed version of Exercise 4. Please use the correction sheet (12 Kb) together with the printed version. In the new PDF files the errors were corrected.