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.