The next several pages will guide you through the analysis of four different star clusters using tools developed by the Skynet Robotic Telescope Network for image processing and star cluster analysis. In addition to learning to work with these tools, these examples provide a foundation for recognising and understanding a broad range of physical features that are identifiable in cluster HR diagrams, which will be important if you wish to study another cluster. Along the way, these various features will be explained in terms of the relevant physical processes of stellar evolution. Therefore, by working through all the examples you will become familiar with both the practical aspects of astronomical image processing, photometry, field star removal, and isochrone modelling, as well as the various aspects of stellar evolution theory that explain the variety of features we observe in cluster HR diagrams.
In the first three examples, we will focus on a relatively young cluster (NGC 3766) — one in which active star formation has long since ended, while most stars are still in their hydrogen-fusion phase of evolution.
- In Example 1, you will use telescope data to create a colour image, measure stellar brightnesses and colours, and plot a colour-magnitude diagram, graphing the two sets of measurements which can be compared with the visual colour image.
- In Example 2, you will learn about the magnitude units astronomers use to measure brightness, and how those are used to measure colour and the reddening of starlight that travels through the interstellar medium. You will learn to recognise the main-sequence stars — those that are still fusing hydrogen in their cores — and to identify unresolved pairs of stars by their locations on colour-magnitude diagrams. And you will learn how to fit a theoretical model to your colour-magnitude data by varying the age and metallicity of the star cluster, along with its distance and interstellar reddening parameter.
- In Example 3, still working with the same cluster, you will learn how to supplement your own brightness and colour measurements with archival data for more robust measurements of the cluster’s physical parameters.
- Example 4 will introduce you to understanding the various real sources of physical scatter in cluster data. By analysing photometric data in a young star-forming region (IC 2948), you will see the effect of factors like variable reddening and age in star-forming regions.
- Example 5 will finally introduce you to the physical processes that take place in stars when they run out of hydrogen fuel in their cores, at which point they evolve to become red giants and white dwarfs. Additionally, you will learn about the effects of mass transfer on the evolution of stars in binary systems.
- Finally, in Example 6 you will analyse the globular cluster NGC 3201, which was pictured on the first page of this Learning Activity. Globular clusters are so old that their light is completely dominated by bright red giant stars, while most of the remaining main sequence stars are too dim to easily observe. This dramatically affects the shape of their HR diagrams.
In order to proceed with this list of activities and use Skynet’s image processing tool, Afterglow, currently an account on the Skynet Robotic Telescope Network is required and account creation has not been automated. If you would like to try the above activities and don’t already have an account, please email me at daryl[dot]janzen[at]usask[dot]ca and I can create one for you. The cluster analysis software itself does not require login, but access to the Sample images and free image processing software Afterglow (eg used in Examples 1, 2, and 4) does require an account.
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