Understanding Stellar Evolution: The Hertzsprung-Russell Diagram
Expert reviewed •22 November 2024• 6 minute read
Introduction
The Hertzsprung-Russell (HR) diagram is a fundamental tool in astronomy that helps scientists classify stars based on their physical properties and evolutionary stages. This powerful visualization plots stars according to their luminosity and surface temperature, revealing patterns that help us understand stellar evolution.
Structure of the HR Diagram
The HR diagram consists of two primary axes:
Vertical Axis (Y-axis):
Represents stellar luminosity
Can be expressed as absolute magnitude or luminosity relative to the Sun (L/L☉)
Scale is logarithmic
Horizontal Axis (X-axis):
Represents surface temperature (in Kelvin)
Often includes spectral class and color
Temperature scale runs from hot (left) to cool (right)
Key Stellar Properties
Luminosity
Luminosity (L) represents a star's total energy output per second, measured in watts (W). It can be expressed as:
L=4πR2σT4
where:
R is the star's radius
σ is the Stefan-Boltzmann constant
T is the surface temperature
Surface Temperature
Surface temperature determines a star's color and spectral classification. According to Wien's displacement law:
λmax=Tb
where:
λmax is the peak wavelength
b is Wien's displacement constant (2.898 × 10⁻³ m·K)
T is the surface temperature in Kelvin
Spectral Classification
The Morgan-Keenan system classifies stars into spectral types:
Class
Temperature (K)
Color
Main Characteristics
O
> 25,000
Blue
Strong ionized helium
B
10,000-25,000
Blue-white
Neutral helium
A
7,500-10,000
White
Strong hydrogen
F
6,000-7,500
Yellow-white
Weakening hydrogen
G
5,000-6,000
Yellow
Ionized calcium
K
3,500-5,000
Orange
Strong metallic lines
M
< 3,500
Red
Strong molecular bands
Major Regions on the HR Diagram
Main Sequence
Diagonal band running from top-left to bottom-right
Contains approximately 90% of observed stars
Stars spend most of their lives here
Our Sun is a G2V main sequence star
Giants and Supergiants
Located above the main sequence
Larger radius and higher luminosity
Lower surface temperature than main sequence stars
Represent late evolutionary stages
White Dwarfs
Found in bottom-left region
High temperature but low luminosity
Final evolutionary stage for low/medium-mass stars
Extremely dense with Earth-sized radius
Stellar Evolution Paths
The mass of a star determines its evolutionary path:
Low/Medium-Mass Stars (< 8M☉):
Main Sequence → Red Giant → Planetary Nebula → White Dwarf
High-Mass Stars (> 8M☉):
Main Sequence → Supergiant → Supernova → Neutron Star/Black Hole