Space: H-R Diagrams

Space: H-R Diagrams

H-R Diagram: Overview

  • The Hertzsprung-Russell (H-R) diagram is a key tool in the study of stellar evolution.
  • It plots the luminosity of stars against their surface temperature (or colour).
  • This chart reveals relationships between the properties of stars and provides a way of classifying them based on these properties.

Axes and Measurements

  • The vertical axis of an H-R diagram represents a star’s absolute magnitude or luminosity i.e., the intrinsic brightness of a star.
  • The horizontal axis signifies the star’s temperature. Note that temperature decreases from left to right.
  • These axes can also present data in other forms; for instance: luminosity can be replaced with absolute magnitude, and spectral class or colour index can take the place of temperature.

Understanding the Diagram

  • The majority of the stars, about 90%, lie along a stretch known as the Main Sequence - the area where stars spend most of their lives.
  • At the upper right of the H-R chart are the Red Giants, stars that have spent their hydrogen fuel and expanded in size.
  • Down towards the lower left are small, hot, faintly luminous stars known as White Dwarfs.
  • Stars that fall outside the main sequence, red giants, and white dwarfs are relatively rare.

Importance of the H-R Diagram

  • The H-R Diagram gives an insight into the lifecycle of stars as they progress from the main sequence to red giants or white dwarfs.
  • By studying the H-R Diagram astrophysicists can estimate a star’s lifespan, mass, size, and eventual fate.
  • The diagram is crucial to determining distances to far-off galaxies, tracking star clusters’ evolution, and understanding the scale and size of the universe.

Drawbacks and Limitations

  • One key limitation is that it can only plot observable stars. Many stars are too faint or obscured to be detected.
  • Another issue is that the H-R Diagram gives instant snap-shot information, but stars evolve over millions to billions of years, so it doesn’t directly show time changes.
  • The diagram does not give direct information about a star’s mass or size, though these can be inferred from other data.

The Colour-Magnitude Diagram

  • The H-R Diagram’s variant is the Colour-Magnitude Diagram (CMD), used for star clusters.
  • Instead of temperature, it uses colour index providing a clearer picture of a star cluster’s age and distance.
  • The CMD plays a vital role in establishing the distance scale of the universe.