About this experiment
Plot stars by how hot their surface is and how much light they give out and they do not scatter randomly. Most fall on a diagonal band, the main sequence; a few cool but very bright giants sit above it and hot, faint white dwarfs below. Explore 32 real stars, from Proxima Centauri to Rigel, build a star of any mass and see where it lands, then age a star cluster and watch the main sequence peel away from the top.
Learning goals
- Read luminosity, temperature and spectral class from a Hertzsprung-Russell diagram
- Use L = 4πR²σT⁴ to explain why giants are cool but bright and white dwarfs hot but faint
- Describe how mass sets a main-sequence star's position, luminosity and lifetime
- Explain how the turn-off point of a cluster gives its age
- Name the main regions of the diagram and the stages of stellar evolution they represent
How to use it
- 1Click the Sun, then Sirius B and Betelgeuse. Which is hottest, which is brightest, which is largest? Use the lines of equal radius.
- 2Find two stars with nearly the same temperature but very different luminosities. What must be different about them?
- 3Make a star of 0.1 solar masses and one of 10 solar masses. Compare their luminosities, colours and lifetimes.
- 4Find the mass whose main-sequence life equals the age of the universe, about 13.8 billion years.
- 5Age the cluster from 3 million to 10 billion years. Describe how the turn-off point moves and what appears below the main sequence.
- 6Use the cluster buttons. Estimate the turn-off mass of the Pleiades, the Hyades and M67 and put the clusters in order of age.
The model
Assumptions
- Star data are rounded literature values. Many bright stars vary or have uncertain distances, so published luminosities differ by 10 to 30 %.
- Main-sequence stars follow piecewise power laws for luminosity and radius against mass. They are good to a few tens of percent, not stellar models.
- The cluster has 700 stars drawn from the Salpeter mass function between 0.4 and 20 solar masses with a fixed seed.
- Leaving the main sequence, stars cross to the giant region and lighter ones end as white dwarfs; stars above 8 solar masses end as supernovae. These later positions are schematic, not evolutionary tracks.
- Dot sizes grow with the logarithm of the radius, so a supergiant a thousand times wider than the Sun is drawn only a few times larger.
Try this in class
- Q1Star X has the same temperature as the Sun and 100 times its luminosity. How does its radius compare with the Sun?
- Q2Sirius B is hotter than the Sun but 18 times fainter. What does this tell you about its size?
- Q3Why do massive stars live shorter lives even though they have more fuel?
- Q4A cluster has no stars above 2 solar masses on the main sequence. Estimate its age.
- Q5Why are there no white dwarfs in a cluster that is only 10 million years old?
Frequently asked questions
Why is temperature plotted backwards?
The diagram began as a plot of spectral type, ordered O B A F G K M from hot to cool. Astronomers kept that order when they switched to temperature, so hot stars stay on the left.
Do stars move along the main sequence as they age?
Hardly. A star stays near one point on the main sequence for most of its life, brightening slowly as helium builds up in its core. Its position is set mainly by its mass. Large moves only happen when the core hydrogen runs out.
Who made the first diagram?
Ejnar Hertzsprung (1911) and Henry Norris Russell (1913) plotted brightness against colour or spectral type independently. Russell noticed that giant and dwarf stars of the same colour differ hugely in brightness, which pointed to very different sizes.