Cosmic Playground

HR Diagram Inference Lab

Measurement-built map: observer CMD versus theorist HR coordinates.

Observer CMD

Use arrow keys to move selection on the plot. Press Home/End to jump to boundary stars.

Guided Inference Controls

Start Here

Use the workflow below to move from pattern recognition to physical inference.

Find the main sequence
Click a star
Compare temperature and luminosity
Turn on radius lines
Reveal mass colors

Explore the Diagram

Core controls

Start in Observer CMD mode, then switch to Theorist HR mode when you are ready to infer radius.

Guide mode

Annotates the plot and keeps the learning workflow visually in front.

Show radius lines

Constant-radius guides appear in Theorist HR mode so you can infer stellar size from luminosity and temperature.

Reveal mass colors

Keep this off until after you have predicted which end of the main sequence should contain high-mass stars.

Selected Star

Inference

Click a star or use the keyboard on the plot to inspect a specific point.

Stage

Effective temperature (K)

Luminosity (L/L_sun)

Radius (R/R_sun)

Mass

Plain-language interpretation

Click a star to begin the inference workflow.

Ask whether this star is unusually bright or faint for its temperature.

Experiment Controls Experiment Controls Advanced simulation + evolution

Preset scenarios

Synthetic population

Seed Same seed plus same settings gives the same synthetic population. Population size N Larger samples reveal the sequence structure more clearly, but add crowding. Distance (pc) Distance changes how faint the stars appear before photometric scatter is added. Photometric error sigma (mag) Higher uncertainty broadens the CMD and makes the sequence look noisier. Metallicity Z Metallicity Z (chemical composition) shifts the stellar structure built into the model. Cluster mode

In cluster mode, stars share one age so the turnoff and evolved stars line up as a population.

Cluster age (Gyr)

Current value: 0.70 Gyr

Older clusters lose their hottest main-sequence stars first.

Binary fraction

Current value: 28%

Higher unresolved binary fractions push some stars above the main sequence.

Evolve a star

Mass (M_sun) Compare how different masses change both the path and the lifetime.

Time along track

Current value: 0.000 Gyr

Move forward in time to see where the chosen mass leaves the main sequence.

Main-sequence lifetime estimate: 10.000 Gyr

Current stage: Main sequence

Mass determines both the path and the timescale.

Inference Log

Classroom notes

Add one-sentence claim Use one claim per inference step so you can compare your reasoning before and after reveals.

Sentence starters

Step Guide 3 prompts

Identify the main sequence, giant branch, and white dwarf region from the synthetic population map.
Pick one hot-faint star and infer its radius using Stefan-Boltzmann scaling.
Reveal mass colors and infer the mass-gradient direction along the main sequence.

Model Notes Tout + CMD calibration

Main sequence properties use Tout et al. (1996) ZAMS fits for L(M,Z) and R(M,Z), with Teff from Stefan-Boltzmann closure.
Observer CMD mapping uses empirical bolometric correction and Teff-color calibration, plus deterministic Gaussian photometric scatter.
Radius guides follow L/L_sun = (R/R_sun)^2 (T/T_sun)^4.