H-R Diagram Explorer

Three modes: place any star on the Hertzsprung-Russell diagram by entering its temperature, radius, and mass. Compare up to 8 stars side by side or browse the complete spectral classification atlas from O-type blue giants to M-type red dwarfs. 23 real reference stars are plotted for context.

Mode

Presets

5,778 K
2 000 K50 000 K
1.00 R☉
0.001 R☉1 000 R☉
1.00 M☉
0.08 M☉60 M☉

Hertzsprung-Russell Diagram

OBAFGKMMMain SequenceGiantsSupergiantsWhite Dwarfs2,0003,0005,0007,00010k20k30k50kTemperature (K) → decreasing10⁻⁴10⁻³10⁻²10⁻¹10⁰10¹10²10³10⁴10⁵10⁶Luminosity (L☉)SunSirius AVegaAlpha Centauri AProcyon AEpsilon Eridani61 Cygni AProxima CentauriBarnard's StarTau CetiRigelSpicaAchernarAltairArcturusAldebaranPolluxBetelgeuseAntaresDenebSirius B40 Eri BProcyon BYour Star
O/B-type (blue-white, hot)
G-type (yellow, sun-like)
K-type (orange)
M-type (red, cool)

Temperature increases to the left (astronomers' convention). Each major gridline represents a factor of 10 in luminosity. 23 reference stars are shown as small dots.

Computed Properties

Type GMain Sequence
5,778 K
G - Yellow · 1.00 R☉ · 1.00 M☉
Luminosity
1.00 L☉
Abs. Magnitude
4.83
HR Region
Main Sequence
MS Lifetime
10.0 Gyr

Formulas Used

Luminosity
Abs. Mag
Lifetime

Spectral colour

O
B
A
F
G
K
M
Luminosity
1.00 L☉
Lifetime
10.0 Gyr
Abs. Magnitude
4.83
Spectral Type
G (5,778 K)

Reference Guide

The HR Diagram

The Hertzsprung-Russell (HR) diagram is the single most important tool in stellar astronomy. It plots stellar luminosity against surface temperature, revealing that stars are not randomly distributed but cluster into distinct groups.

Developed independently by Ejnar Hertzsprung (1911) and Henry Norris Russell (1913), the diagram showed for the first time that stars follow predictable patterns governed by their mass and evolutionary stage.

L∝R2 T4L \propto R^2 \, T^4

The main sequence runs diagonally from hot, luminous stars (upper left) to cool, dim stars (lower right). About 90% of all stars lie on the main sequence at any given time.

Spectral Classification

The Morgan-Keenan (MK) system classifies stars into spectral types O, B, A, F, G, K, M based on their surface temperature and absorption line patterns. Each class is subdivided 0-9 (e.g. G2 for the Sun).

O (blue-white)30 000 - 50 000+ K B (blue-white)10 000 - 30 000 K A (white)7 500 - 10 000 K F (yellow-white)6 000 - 7 500 K G (yellow, Sun)5 200 - 6 000 K K (orange)3 700 - 5 200 K M (red)2 000 - 3 700 K

Mnemonic: Oh Be A Fine Girl/Guy, Kiss Me.

Stefan-Boltzmann Law

The Stefan-Boltzmann law relates a star's luminosity to its radius and surface temperature. It is the key equation that connects a star's position on the HR diagram to its physical size.

L=4πR2σT4L = 4\pi R^2 \sigma T^4

In solar units this simplifies to:

LL⊙=(RR⊙) ⁣2(TT⊙) ⁣4\frac{L}{L_\odot} = \left(\frac{R}{R_\odot}\right)^{\!2} \left(\frac{T}{T_\odot}\right)^{\!4}

σ = 5.670 × 10⁻⁸ W/(m²·K⁴), T☉ = 5778 K

Main Sequence Properties

Stars on the main sequence fuse hydrogen in their cores. Their luminosity scales steeply with mass, while their lifetime scales inversely, because massive stars burn fuel far faster.

tMS≈1010×(M⊙M)2.5 yrt_{\text{MS}} \approx 10^{10} \times \left(\frac{M_\odot}{M}\right)^{2.5} \text{ yr}
MassLuminosityLifetime
0.1 M☉ ~0.001 L☉ ~3 trillion yr
1 M☉ (Sun) 1 L☉ ~10 Gyr
10 M☉ ~3 162 L☉ ~32 Myr
50 M☉ ~560 000 L☉ ~0.6 Myr

Absolute magnitude measures intrinsic brightness:

M=4.83−2.5 log⁡10(L/L⊙)M = 4.83 - 2.5\,\log_{10}(L / L_\odot)