Vol. Science · Astronomy · Stellar diagram

HR diagram
hot and bright

The Hertzsprung–Russell diagram plots effective temperature across and luminosity up. Hot is on the left, the astronomy convention, so a hotter main-sequence star sits up and left of a cooler one. Luminosity is L/L☉, logarithmic. The right-hand scale is bolometric magnitude, which runs backwards: a smaller number is brighter. Slide the point, or tap a cited star. Radius lines are L = 4πR²σT⁴. The letters OBAFGKM live on the stars desk. The curve behind σT⁴ is the blackbody desk. How bright a star looks from Earth is magnitude. Distance, which absolute magnitude assumes, is parallax, and the nearest of these stars are on nearest stars.

Classroom schematic. The shaded regions are an educational cartoon — main sequence, giants and supergiants, white dwarfs — not a Gaia catalog, not an ephemeris, and not a luminosity-class boundary. Markers are cited central values. Error bars are the cited uncertainties, clipped to the frame. The color chip is the blackbody desk’s sRGB approximation. Not a stellar-atmosphere model.

Interactive · Temperature versus luminosity

Hot left · bright up · tap a star or the plot · radius lines are blackbodies

Radius
Main sequence
Giants
White dwarfs

Cited presets · tap to place the marker

Temp K

L / L☉ L☉

The plot needs JavaScript. The cited anchors are still below. The nominal Sun is 5772 K and 1 L☉ (IAU 2015). Proxima Centauri is 2980 ± 80 K and 0.00151 ± 0.00008 L☉ (Ribas et al. 2017). Vega is 9560 ± 30 K and 44 ± 2 L☉ (Hill, Gulliver, and Adelman 2010). Sirius A is 9845 ± 64 K and 24.74 ± 0.70 L☉; Sirius B is 25,369 ± 46 K and 0.02448 ± 0.00033 L☉ (Bond et al. 2017). Betelgeuse is 3500 ± 200 K and log(L/L☉) = 5.10 ± 0.22 (Dolan et al. 2016). Rigel is 12,100 ± 150 K and log(L/L☉) = 5.08 (Moravveji et al. 2012). σ is the CODATA value 5.670374419… × 10⁻⁸ W m⁻² K⁻⁴.

Three families

Hotter dwarfs are brighter. Giants are large. White dwarfs are tiny.

On the main sequence, hydrogen is fusing in the core. A hotter dwarf is usually a more massive one, and it is brighter. The Sun sits in the middle of this page. Proxima is down and to the right: cool, small, faint. Vega and Sirius A are up and to the left: hotter, and more luminous.

Giants and supergiants are large. Betelgeuse is cooler than the Sun and still vastly more luminous, because the radius is hundreds of solar radii. Rigel is hot and large, so it sits with the supergiants rather than with the dwarfs at that temperature. The shaded box is a cartoon around those two cited stars, not a catalog of every giant.

A white dwarf is the exposed core of a star that has already shed its envelope. Sirius B is hotter than Sirius A and far fainter than the Sun. The temperature is high. The radius is under a hundredth of the Sun’s, so the area cannot make up for it. The stars desk is the life path that ends there. This page does not draw that path.

Literacy · the words on the axes

Hertzsprung–Russell: Temperature across, luminosity up. Hot on the left is the usual astronomical plot, so the main sequence runs from cool and faint at the lower right toward hot and bright at the upper left. This page’s primary axes are effective temperature and L/L☉. Spectral type would run the same direction, O toward M, left to right. The stars desk is where those letters are taught.
Stefan–Boltzmann: L = 4πR²σT⁴. Surface flux is σT⁴. σ = 2π⁵k⁴ / (15h³c²) = 5.670374419… × 10⁻⁸ W m⁻² K⁻⁴, the CODATA exact value. A radius line is every temperature and luminosity that share one radius. The same law is the luminosity readout on the blackbody desk.
Two solar luminosities: A radius line divides by the Stefan–Boltzmann formula at the IAU nominal solar radius and nominal effective temperature, so the Sun preset sits on the 1 R☉ line at 1 L☉. The IAU nominal luminosity, 3.828 × 10²⁶ W, is the other zero. At those nominal inputs the two agree at the digits this desk prints.
Bolometric magnitude: M_bol = 4.74 − 2.5 log₁₀(L/L☉), with 4.74 the IAU 2015 nominal solar bolometric magnitude. Brighter is smaller, and the brightest stars go negative. That is the right-hand axis. It is not visual absolute magnitude.
Visual M_V: Visual absolute magnitude is how bright the star would look, in the V band, from 10 parsecs. The magnitude desk is that scale. The Sun’s fact-sheet value is +4.83. The bolometric number on this page is +4.74. They differ because the eye does not collect the whole spectrum. Sirius A’s visual figure is the magnitude desk’s arithmetic from Ducati V = −1.46 and the ESA parallax 0.37921″, not a second catalog.
The cartoons: Main sequence, giants and supergiants, and white dwarfs are labeled regions so the eye can find the three families. The polygons were drawn so the cited presets fall in the family the caption names. They are not a Gaia selection, not a mass track, and not a boundary you could publish.

Sources · CODATA, IAU, and the papers

Cited, not invented

NIST CODATA — Stefan–Boltzmann constant — σ = 5.670374419… × 10⁻⁸ W m⁻² K⁻⁴, exact, from the defined h, c, and k. This desk computes σ = 2π⁵k⁴ / (15h³c²) using h = 6.62607015 × 10⁻³⁴ J·s, c = 299792458 m/s, and k = 1.380649 × 10⁻²³ J/K. The same computation is the blackbody desk.
IAU 2015 Resolutions B2 and B3 (Prša et al. 2016, AJ 152, 41) — nominal solar radius 6.957 × 10⁸ m, nominal solar luminosity 3.828 × 10²⁶ W, nominal solar effective temperature 5772 K, nominal solar bolometric magnitude 4.74. Conversion constants, exact, not a measurement of the Sun. The resolution text is also at the IAU archive.
NASA NSSDCA — Sun Fact Sheet — absolute magnitude +4.83. That visual figure is the magnitude desk’s, printed here beside the bolometric 4.74 so the two are not mixed.
Ribas et al. 2017, A&A 603, A58 — Proxima Centauri: T_eff = 2980 ± 80 K, R = 0.146 ± 0.007 R☉, L = 0.00151 ± 0.00008 L☉.
Hill, Gulliver, and Adelman 2010, ApJ 712, 250 — Vega: mean T_eff = 9560 ± 30 K, equatorial radius 2.75 ± 0.01 R☉, polar radius 2.40 ± 0.02 R☉, luminosity 44 ± 2 L☉. Seen nearly pole-on.
Bond et al. 2017, ApJ 840, 70 — Sirius A, adopting Davis et al. 2011 adjusted to their parallax: T_eff = 9845 ± 64 K, R = 1.7144 ± 0.0090 R☉, L = 24.74 ± 0.70 L☉. Classical type A1 V. Sirius B, from their STIS fits: T_eff = 25,369 ± 46 K, R = 0.008098 ± 0.000046 R☉, L = 0.02448 ± 0.00033 L☉, type DA2. They note the Sirius B uncertainties are internal and omit modestly larger systematics. Davis et al. 2011 is PASA 28, 58.
Moravveji et al. 2012, ApJ 747, 108 — Rigel: T_eff = 12,100 ± 150 K, R = 78.9 ± 7.4 R☉, log(L/L☉) = 5.08 with uncertainties +0.07 and −0.10.
Dolan et al. 2016, ApJ 819, 7 — Betelgeuse, adopted: T_eff = 3500 ± 200 K, R = 887 ± 203 R☉, log(L/L☉) = 5.10 ± 0.22. The radius uncertainty follows the distance 197 ± 45 pc in Harper, Brown, and Guinan 2008, AJ 135, 1430.
SIMBAD — * alf CMa — V = −1.46 from Ducati’s 2002 catalogue of Johnson photometry, the same figure the magnitude desk uses. The visual absolute magnitude here is that V minus the distance modulus at the ESA parallax, not a SIMBAD absolute magnitude.
ESA Science & Technology — Stellar distances — Sirius parallax 0.37921″, the figure on the parallax and magnitude desks. d(parsec) = 1 / p(arcsecond).
NASA Science — Star types — main sequence, red giants, and white dwarfs as the families named on this cartoon. The shapes are not taken from that page.

Classroom picture of the Hertzsprung–Russell diagram. Hot on the left. L = 4πR²σT⁴ with the CODATA constant, compared with the IAU 2015 nominal Sun. Bolometric magnitude uses M_bol,☉ = 4.74. The regions are a cartoon. Not a Gaia catalog and not an ephemeris. Earthrise preview: NASA / Bill Anders (public domain).

Keep going

Letters

HR diagram

A classroom picture. Hot on the left, bright up. The shaded regions are a cartoon. Not a Gaia catalog and not an ephemeris.

The plot

The plot
Across	Effective temperature, logarithmic, from 2,500 K to 40,000 K. Hot is left.
Up	Luminosity in solar units, logarithmic, from 10⁻⁴ to 10⁶. Bright is up.
Right	Bolometric magnitude from the IAU zero point, M_bol = 4.74 − 2.5 log₁₀(L/L☉). The number runs backwards. Hidden on a narrow screen. The readout stays.
Regions	Main sequence, giants and supergiants, white dwarfs. A sketch that puts the cited presets in the named family. Not a boundary.
Radius lines	L = 4πR²σT⁴ at 0.01, 0.1, 1, 10, 100, and 1,000 solar radii. A cited star also draws its own radius, dashed, when that radius is not already one of those lines.

Controls

Controls
Presets	Sun, Proxima, Vega, Sirius A, Sirius B, Betelgeuse, Rigel. Each button places the cited temperature and luminosity. Left and Right move through the buttons when one of them has focus.
Plot	Tap empty space to drop a custom marker. Tap a star to select it. Drag empty space to move the custom marker. A cited star does not drag off its paper.
Sliders	Temperature and luminosity, both logarithmic. The number fields accept the same ranges. Typing or sliding leaves the preset.
Layers	Radius lines and regions toggle. Both start on.

Readouts

Readouts
L / L☉	The marker. For a preset, the paper’s luminosity, including 10^5.10 and 10^5.08 where the paper quotes a logarithm.
Bolometric M	The IAU formula applied to that luminosity. Not a measured magnitude.
Visual M_V	Shown for the Sun (+4.83, fact sheet) and Sirius A (the magnitude desk’s arithmetic). Hidden when this page has no cited visual figure.
Cited radius	The paper’s radius. Blank for a custom point.
Blackbody radius	The radius whose Stefan–Boltzmann line passes through the marker, using the IAU nominal Sun as the zero.

Keyboard and touch

Keyboard and touch
Plot	The canvas takes focus. Arrow keys nudge a custom point. Shift is a coarser step.
Buttons	Enter or Space selects. The pressed button is aria-pressed. Targets are at least 44 pixels on a side.
This dialog	The question mark, or ?, opens it. Escape closes it. Tab stays inside until you close it. Click the dim page behind the card to close it.

HR diagram
hot and bright

Hotter dwarfs are brighter. Giants are large. White dwarfs are tiny.

Cited, not invented

Stars

Blackbody

Magnitude

Nearest stars

Parallax

Solar map

HR diagram

The plot

Controls

Readouts

Keyboard and touch

HR diagramhot and bright

Hotter dwarfs are brighter. Giants are large. White dwarfs are tiny.

Cited, not invented

Stars

Blackbody

Magnitude

Nearest stars

Parallax

Solar map

HR diagram
hot and bright