HR Diagrams and Cluster Evolution: An Introduction to Stellar Evolution Through Star Cluster Analysis

Star clusters are among the most useful objects in the sky — not just because they’re beautiful, but because they’re natural laboratories. Every star in a cluster formed from the same cloud of gas at roughly the same time, which means that when you look at a cluster, the differences you see between stars are real, physical differences that can be measured, modelled, and understood.

This activity takes you from that simple observation all the way through to a complete, research-grade analysis of real star clusters using professional astronomical tools and data. Along the way, you’ll learn essentially everything an introductory astronomy student needs to know about how stars work, how they age, and how they die — not through passive reading, but by actually doing the analysis yourself.

By the time you’ve worked through all the examples, you will know how to:

• process raw telescope images into calibrated, aligned, colour-composite photographs
• measure the brightnesses and colours of thousands of stars simultaneously
• use proper motion and distance data from the Gaia space mission to distinguish cluster members from unrelated foreground and background stars
• fit theoretical stellar evolution models to your data to determine a cluster’s age, metallicity, distance, and interstellar reddening
• recognise and interpret the full range of features visible in cluster HR diagrams — from the main sequence and turnoff point through red giants, horizontal branch stars, asymptotic giant branch stars, white dwarfs, and blue stragglers
• understand the physical processes responsible for each of these features in terms of the fundamental physics of stellar structure and evolution

The activity is built around six guided examples, each analysing a different cluster chosen to introduce the next layer of complexity: a young open cluster still dominated by hydrogen-fusing main sequence stars; a young star-forming region with variable extinction and ongoing star formation; a moderately old open cluster showing a well-developed red giant population and an emerging group of white dwarfs; and finally an ancient globular cluster whose HR diagram is completely dominated by evolved giant stars. Two optional examples round out the complete introduction to star cluster photometry research — covering image alignment and stacking, and the addition of infrared survey data to create multi-wavelength colour composites.

The analysis tools used throughout — Afterglow for image processing and photometry, and Clustermancer for field star removal and isochrone fitting — were developed by the Skynet Robotic Telescope Network. Clustermancer requires no login. Afterglow and the sample image data currently require a Skynet account, which for now means a quick email to me at daryl[dot]janzen[at]usask[dot]ca — account creation directly through the site is coming in a future version of Skynet.

Whether you’re a student working through this as part of a course, or someone who stumbled across it and wants to actually do astronomy rather than just read about it — welcome. This is the real thing.