Investigate the Hertzsprung-Russell diagram

Investigate the Hertzsprung-Russell diagram


– Surface temperature

Stars on the left hand side of the H-R diagram have a higher surface temperature than stars towards the right.

– luminosity

Stars towards the top are very luminous. The luminosity decreases as we go down the diagram.

– colour

The stars on the right-hand end of an HR diagram (spectral class O, B and A) are blue in colour due to their temperatures. Stars classed F are white, and G are white to yellow. Those classified K and M are red.

– characteristics and evolutionary stage

Most stars lie on the diagonal trend line called main sequence.

One can determine age by turn off point i.e. where it leaves main sequence

  • Older – lower turn off
  • Younger – higher

When the hydrogen in the core is exhausted, the star moves off the main sequence as it cools and expands into a red giant.

When a star about the size of the Sun throws off most of the remaining hydrogen and helium to form a planetary nebula, it can no longer sustain fusion and cools as a white dwarf, now below the main sequence as it is dim but still quite hot.


The above diagram above shows sample stellar evolutionary tracks for single stars, zero initial rotational velocity, and solar metallicity


This diagram shows the primary evolutionary stages of a solar-mass star. In this diagram the Sun currently lies along the diagonal, main-sequence line.

The star begins as a Bok globule (a) that contracts to form a condensing protostar. When it begins burning Hydrogen about 30 million years later, it stabilizes into a zero-age main sequence star. Thereafter it remains a main sequence star for about the next 10 billion years. The proportion of Helium at the core of this star steadily increases over this period, causing a gradual rise in luminosity.

As the Hydrogen at the core becomes exhausted, the star blossoms into a massive red giant (b). The star increases in luminosity, burning Hydrogen in a shell around a degenerate Helium core. Finally the Helium flash triggers Helium fusion to begin at the core.

One to two billion years after leaving the main sequence, the giant star now begins to experience a period of instability. The envelope is gently ejected (c), forming a planetary nebula and exposing the hot, compact core. Once Helium fusion has ceased, the star becomes a white dwarf (d) and slowly radiates away its remaining energy.

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)