One of the most powerful tools in astrophysics is also one you can create from your own backyard images — the Hertzsprung-Russell (H-R) diagram. This seemingly simple plot reveals the life story of stars and can tell us the age of entire star clusters.
M5 globular cluster in Serpens — 30x180sec exposures with an 8″ Newtonian
What is a Hertzsprung-Russell Diagram?
The H-R diagram plots stars by their luminosity (or magnitude) versus their temperature (or color). Developed independently by Ejnar Hertzsprung and Henry Norris Russell in the early 1900s, it’s one of the most important tools in stellar astronomy.
When you plot stars this way, patterns emerge:
- Main Sequence: A diagonal band where stars spend most of their lives fusing hydrogen. Our Sun sits here.
- Red Giants: Upper right — cool but very luminous stars nearing the end of their lives
- White Dwarfs: Lower left — hot but dim stellar remnants
- Turn-off Point: Where stars leave the main sequence — this tells us the cluster’s age!
Why Globular Clusters are Perfect for H-R Diagrams
Globular clusters like M5 contain thousands of stars that all formed at the same time from the same material, at roughly the same distance from Earth. This makes them ideal for H-R diagrams because:
- All stars are at the same distance, so apparent magnitude differences reflect true luminosity differences
- All stars are the same age, so the turn-off point reveals the cluster’s age
- The cluster contains stars in different evolutionary stages
H-R diagram of M5 generated in Siril — notice the clear main sequence and turn-off point
Creating an H-R Diagram in Siril
First install the script by going to Preferences and Scripts. Locate the Hertzprungrussell.py script, select it and click Apply.
1. Capture Your Data
You need properly calibrated images of a star cluster. For M5, I captured 30 x 180sec exposures with my Orion 8″ Newtonian and Canon T3i.
2. Stack in Siril
Process your lights, darks, and flats in Siril as you normally would. The result is a calibrated stacked image.
3. Plate Solve
Use Siril’s Image Plate Solver tool to identify stars in your image. This gives you access to catalog data.
4. Run Spectrophotometric Color Calibration
The H-R script requires that you run Spectrophotometric Color Calibration as a pre-step.
5. Generate the Plot
Run the Hertzprungrussell.py script and on the left are some options you can adjust. The default work fine to start out with. Enjoy!
What Can You Learn?
From M5’s H-R diagram, we can see:
- Clear main sequence: The diagonal band of hydrogen-fusing stars
- Turn-off point: Stars leaving the main sequence indicate M5 is about 10.5 billion years old
- Red giant branch: The upward curve of evolved stars
- Horizontal branch: Stars in a later helium-burning phase
Try It Yourself
Globular clusters are excellent targets for amateur H-R diagrams:
- M13 (Hercules Cluster): Bright and well-positioned in summer
- M3: Beautiful spring target
- M5: The one featured here — excellent in spring/summer
- M92: Compact but rewarding
The best part? You’re not just taking pretty pictures — you’re doing real science. Amateur H-R diagrams have contributed to our understanding of stellar evolution and cluster dynamics.
See more astrophotography in the gallery →
Equipment Used for M5
- Telescope: Orion 8″ 1000mm f/4.9 Newtonian reflector
- Mount: Orion Atlas EQ-G
- Camera: Canon T3i (full spectrum modification)
- Guide Scope: Meade 800mm f/10 reduced to f/5
- Guide Camera: Meade DSI Pro monochrome
- Software: N.I.N.A 3.1 for capture, Siril 1.4.0 for processing
