Why does temperature decrease to the right on an H-R diagram?

It is historical. Hertzsprung and Russell first plotted spectral type (O, B, A, F, G, K, M) or colour index, and that sequence runs from hot to cool. When effective temperature replaced spectral type the direction was kept, so hot blue stars sit on the left and cool red ones on the right. The vertical axis is the logarithm of luminosity (or absolute magnitude) because stellar luminosities span more than ten orders of magnitude.

How are the star radii obtained?

From the Stefan-Boltzmann law L = 4πR²σT⁴, solved for R: R/R☉ = √(L/L☉) / (T/5772 K)². That keeps L, T and R exactly consistent. The same relation means stars of equal radius lie on straight lines of slope 4 in log L vs log T; the dashed lines show 0.001 to 1000 solar radii. White dwarfs sit near 0.01 R☉ while red supergiants exceed 1000 R☉.

Why is the main-sequence lifetime 10 Gyr × M/L?

On the main sequence a star shines by fusing hydrogen in its core. The available fuel scales roughly with mass and the burn rate with luminosity, so lifetime scales as M/L, normalised to the Sun's 10 billion years. Because luminosity rises steeply with mass (about M^3.5), a 15 M☉ star lives only about ten million years, while a 0.2 M☉ red dwarf lasts hundreds of billions of years, far longer than the current age of the universe.

How accurate are the evolution tracks?

They are schematic. The key phases (main sequence, subgiant, red giant branch, helium flash, horizontal branch, asymptotic giant branch, planetary nebula and white dwarf cooling, or red supergiant and supernova for massive stars) sit roughly where real models put them, but the shapes are simplified and the animation pacing is chosen for viewing, not real durations. Real stars spend about 90% of their lives on the main sequence and cross the Hertzsprung gap very quickly.

How are apparent magnitude, absolute magnitude and distance related?

Apparent magnitude m is how bright a star looks; absolute magnitude M is how bright it would look from 10 parsecs. Their difference is the distance modulus m − M = 5 log(d / 10 pc), so any two give the third. The tool also links absolute bolometric magnitude to luminosity with M_bol = 4.74 − 2.5 log(L/L☉). No bolometric correction or interstellar extinction is applied, so treat results as estimates.

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⭐Interactive H-R Diagram - Star Catalogue, Main-Sequence Calculator, Evolution Tracks

Plot the Sun, Sirius, Betelgeuse, Proxima Centauri and 80-odd other well-known stars by surface temperature and luminosity, each coloured like a blackbody at its temperature. Tap a star to see its spectral type, radius, mass, distance and magnitudes. The diagram marks the main sequence, giants, supergiants and white dwarfs with lines of constant radius, animates schematic evolution tracks for 1, 5 and 15 solar masses, and includes calculators for the mass-luminosity relation, Stefan-Boltzmann radius and distance modulus.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use H-R Diagram & Stellar Evolution

1Hover or tap a star on the diagram, or search by name or spectral type
2Read its temperature, luminosity, radius (from L and T), mass, distance and magnitudes under 'Star info'
3Open 'Evolution track', pick 1, 5 or 15 M☉ and press Play to watch the star's life on the diagram
4Use 'Calculator' to get a main-sequence star's luminosity, temperature and lifetime from its mass, or a radius from L and T
5Use 'Magnitude & distance' to solve for any one of m, M (or L) and d

FAQ

Why does temperature decrease to the right on an H-R diagram?: It is historical. Hertzsprung and Russell first plotted spectral type (O, B, A, F, G, K, M) or colour index, and that sequence runs from hot to cool. When effective temperature replaced spectral type the direction was kept, so hot blue stars sit on the left and cool red ones on the right. The vertical axis is the logarithm of luminosity (or absolute magnitude) because stellar luminosities span more than ten orders of magnitude.
How are the star radii obtained?: From the Stefan-Boltzmann law L = 4πR²σT⁴, solved for R: R/R☉ = √(L/L☉) / (T/5772 K)². That keeps L, T and R exactly consistent. The same relation means stars of equal radius lie on straight lines of slope 4 in log L vs log T; the dashed lines show 0.001 to 1000 solar radii. White dwarfs sit near 0.01 R☉ while red supergiants exceed 1000 R☉.
Why is the main-sequence lifetime 10 Gyr × M/L?: On the main sequence a star shines by fusing hydrogen in its core. The available fuel scales roughly with mass and the burn rate with luminosity, so lifetime scales as M/L, normalised to the Sun's 10 billion years. Because luminosity rises steeply with mass (about M^3.5), a 15 M☉ star lives only about ten million years, while a 0.2 M☉ red dwarf lasts hundreds of billions of years, far longer than the current age of the universe.
How accurate are the evolution tracks?: They are schematic. The key phases (main sequence, subgiant, red giant branch, helium flash, horizontal branch, asymptotic giant branch, planetary nebula and white dwarf cooling, or red supergiant and supernova for massive stars) sit roughly where real models put them, but the shapes are simplified and the animation pacing is chosen for viewing, not real durations. Real stars spend about 90% of their lives on the main sequence and cross the Hertzsprung gap very quickly.
How are apparent magnitude, absolute magnitude and distance related?: Apparent magnitude m is how bright a star looks; absolute magnitude M is how bright it would look from 10 parsecs. Their difference is the distance modulus m − M = 5 log(d / 10 pc), so any two give the third. The tool also links absolute bolometric magnitude to luminosity with M_bol = 4.74 − 2.5 log(L/L☉). No bolometric correction or interstellar extinction is applied, so treat results as estimates.

Features

80+ famous stars (approximate literature values) coloured by an integrated blackbody spectrum
Main sequence, giant, supergiant and white dwarf regions plus constant-radius lines from 0.001 to 1000 R☉
Spectral-class band on top, absolute bolometric magnitude on the right, search and tap-to-select
Animated schematic tracks for 1, 5 and 15 M☉ with each phase explained (red giant, helium flash, blue loop, supernova)
Main-sequence calculator: mass to luminosity, radius, temperature, spectral type and lifetime
Distance modulus solver for m, M (or L) and d

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