Astronomy with SalsaJ Print

Life cycle of stars - Plotting your own HR diagram!

Measure the true colours of stars and guess the age of the cluster!

NGC 957, @ Robert Gendler 2004An open cluster (sometimes called a Galactic Cluster) is a group of 10s or 100s of stars that were born from the same initial cloud of gas (mainly Hydrogen) and dust. When they are young (a few million or tens of millions of years old), these clusters contain some very large, bright stars (called O or B-type stars). The very youngest clusters (usually less than 10 million years old) often still contain the remains of the gas cloud from which the stars were born – this is seen as nebulosity.

Cluster stars are very useful as they were all formed from the same giant cloud (so they have the same chemistry), and they are all at about the same distance from us (although they are typically hundreds or thousands of light years away!). By observing a group of stars in a cluster, we can assume they are all made of the same stuff, and they are all the same distance away from us – so any differences between them are really caused by their different mass.

In this project you will be shown how to use Faulkes Telescope data to measure the intensity of light from the stars in a cluster (a process known as photometry), through different filters, and plot the colours of the stars on a 'colour-magnitude diagram'. Once a measure of how “red” or “blue” the stars are is made, more informtion about them can be obtained– massive stars are usually very blue (hot), intermediate mass stars (like the Sun) are yellow, and the very lowest mass stars are red (cool).




UNKG0001.GIF Explore the star population inside an open cluster!



Starting the Exercise

Below are the documents and data files which you will need to carry out this exercise - start by downloading and reading through the 'Background Information' and 'Photometry of star clusters with SalsaJ' documents. Then run through the photometry instructions using the NGC 957 sample data given below.


Background Information (.pdf) - This document gives some background information on Hertzsprung-Russell diagrams, Colour-Magnitude diagrams and open and globular clusters

Photometry of star clusters with SalsaJ (.pdf) - This short document explains how to carry out photometry of star clusters using the free software package, SalsaJ. In this document, the aperture size used to carry out the photometry has been suggested for you, cutting out the need to find this yourselves. 

Finding the aperture radius for photometry with SalsaJ (.pdf) - This short document explains how you can find the best aperture radius to use when carrying out photometry with SalsaJ. 


CMD Plotter (.xls) - This Excel spreadsheet is used to calculate the magnitudes of the stars in the star cluster from their intensities, and plots a colour-magnitude diagram with the results.

To download the FITS data below, RIGHT CLICK on the link and select 'Save As...'. DO NOT just click on the link as you will get a screen with strange symbols on!

NGC 957 B data file

NGC 957 V data file (.fits)

 Extra Documents

Photometry with SalsaJ (full) (.pdf) - This longer document combines the instructions for finding the aperture radius and carrying out photometry with SalsaJ. It also explains how to plot a colour-magnitude diagram either by hand or with the Excel spreadsheet, 'CMD plotter' given above. . 

Further Information

For more information on advanced projects involving photometry of star clusters, please go to the Faulkes Telescope Project resources website here

If you want to go further and observe your own open cluster (or retrieve data from archives), you can find the positions of 120 of them in this paper .