The Hertzsprung-Russell Diagram and Stellar Evolution
Plotting luminosity against surface temperature sorts stars into a main sequence, giants and white dwarfs, and a star's track across the diagram is its life story. Position on the main sequence is set almost entirely by mass.
What a learner can do afterwards
- Places a star on the diagram from its temperature and luminosity
- Explains why main sequence lifetime falls sharply as mass rises
- Reads the age of a cluster from where its main sequence turns off
1 · Read
The Hertzsprung-Russell diagram plots each star by surface temperature against luminosity. Temperature runs backward, hottest on the left. Most stars fall on the diagonal main sequence band, with hot bright giants above and faint compact white dwarfs below.
Place a star with two readings: its spectral class gives the temperature, and its apparent brightness plus distance gives the luminosity. Drop that point onto the diagram and you know at once whether it is a main sequence star, a giant, or a white dwarf.
A star spends most of its life fusing core hydrogen on the main sequence, and its mass fixes how long that lasts. Heavy stars burn fiercely to support their weight, so a star ten times the mass of the Sun lives far shorter. Mass sets position and pace together.
A cluster gives every star the same birthday and composition, so mass is the only difference. Massive members leave the main sequence first, and the point where the sequence bends away is the turnoff. Read the turnoff mass and you read the cluster age.
Plot temperature against luminosity, let mass set the pace, and read cluster ages from the turnoff.
2 · Watch
Take it off screen
Where it sits
This opens up
Nothing builds on it yet.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.