The Hertzsprung-Russell Diagram (AQA A Level Physics)

The Hertzsprung–Russell (HR) Diagram

• Danish astronomer Ejnar Hertzsprung, and American astronomer Henry Noris Russell, independently plotted the luminosity of different stars against their temperature

  • Luminosity, relative to the Sun, on the y-axis, goes from dim (at the bottom) to bright (at the top)

  • Temperature, in degrees Kelvin, on the x-axis, goes from hot (on the left) to cool (on the right)

The Hertzsprung-Russell Diagram depicts the luminosity of stars against their temperature

• Hertzsprung and Russel found that the stars clustered in distinct areas

• Most stars are clustered in a band called the main sequence

  • For main sequence stars, luminosity increases with surface temperature

• A smaller number of stars clustered above the main sequence in two areas, red giants, and red supergiants

  • These stars show an increase in luminosity at cooler temperatures

  • The only explanation for this is that these stars are much larger than main sequence stars

• Below and to the left of the main sequence are the white dwarf stars

  • These stars are hot, but not very luminous

  • Therefore, they must be much smaller than main sequence stars

• The Hertzsprung-Russell Diagram only shows stars that are in stable phases

  • Transitory phases, such as supernovae, happen quickly in relation to the lifetime of a star

  • Black holes cannot be seen since they emit no light

Evolutionary Path of Sun-Like Stars

• The evolutionary path of stars similar to the Sun can be described using a H-R diagram

Evolutionary path of a solar mass star

The lifecycle of the Sun can be mapped out on a H-R Diagram

Protostar to Main Sequence (A to B): 

• The protostar collapses from a cold cloud of gas

• Initially, it is visible as a very dim cool star as it moves onto a fixed position on the main sequence

• Its position on the main sequence is determined by the star's mass

Main Sequence to Red Giant (B to C):

• On the main sequence, the star is stable while it fuses hydrogen into helium nuclei

• Once hydrogen fusion stops, the star begins to collapse under gravity

• This heats up the core until further nuclear reactions reignite the star

• The massive increase in temperature causes the star to expand into a red giant, which could be 100 times the current diameter of the Sun

• As the outer layers move further from the core, its surface temperature will be lower, at about 3000 K, and the extremely large surface area causes it to be much more luminous

Red Giant to White Dwarf (C to D):

• When the supply of helium runs out in the star, nuclear fusion stops and the star collapses into a white dwarf

• The surface temperature of a white dwarf is generally very hot ~10 000K

• Due to the small surface area of a white dwarf, its luminosity is very low

Lifetimes of Stars

• The brightest stars have very short lifetimes (a few million years)

  • These stars use up nuclear fuel at a much higher rate

• The dimmest stars have extremely long lifetimes in comparison (~10¹² years)

  • These stars use up nuclear fuel at a much slower rate

• Stars on the main sequence with high luminosities are massive and very bright

  • A star that is 10⁶ times brighter than the Sun will use up its nuclear fuel 10⁶ times faster than the Sun 

  • A star that has a mass 100 times that of the Sun will live about  or 10⁻⁴ times as long