Surface Flux & Colors of Stars

Section 7 of 8

Color and the HR Diagram

Part 7: Color and the HR Diagram (Preview)

Hot stars are blue (short peak, high $T$ ); cool stars are red (long peak, low $T$ ); the relationship is quantitative, $\lambda_{\text{peak}} = b/T$ . Astronomers use color indices — brightness in two filter bands — to estimate temperature without finding the exact peak.

Hertzsprung-Russell diagram with logarithmic luminosity on the vertical axis (relative to the Sun) and surface temperature on the horizontal axis decreasing from about 30000 K at left to 3000 K at right. Colored stellar points and shaded regions mark the main sequence, giants, supergiants, and white dwarfs, with the Sun labeled near luminosity 1 and temperature about 5800 K. — Figure 8The H-R diagram separates stars into main sequence, giants/supergiants, and white dwarfs; temperature decreases left-to-right while luminosity increases upward.ESO

The Hertzsprung-Russell diagram plots luminosity (vertical) against temperature/color (horizontal). The key insight: the horizontal axis is a temperature axis — hot blue stars on the left, cool red stars on the right (historical convention). Since $L \propto R^2 T^4$ , lines of constant radius are diagonal curves across it; every star lies on some constant- $R$ curve.

The More You Know: Enrichment: why OBAFGKM is a temperature sequence

The spectral sequence O B A F G K M, defined by prominent spectral lines, is fundamentally sorted by temperature:

Type	Temperature	Color	Example
O	30,000–50,000 K	Blue-UV	θ Orionis
B	10,000–30,000 K	Blue-white	Rigel, Spica
A	7,000–10,000 K	White	Vega, Sirius
F	6,000–7,500 K	Yellow-white	Procyon A
G	5,200–6,000 K	Yellow	Sun
K	3,700–5,200 K	Orange	Aldebaran
M	<3,700 K	Red	Betelgeuse

Later spectral type = cooler temperature. Which lines appear is set by ionization equilibrium (covered in later spectroscopy lectures), but the ordering is temperature.

When you plot many stars, they cluster: the main sequence (hydrogen-burning, where the Sun lives), the giant branch (cool but luminous — large radii compensate), and the white dwarf sequence (hot but dim — tiny radii, ~ $0.01\,R_\odot$ ). Lines of constant radius separate these populations.