• International Year of Astronomy - The Hertzsprung-Russell Diagram
Danish astronomer Ejnar Hertzsprung (1873 – 1967) and American astronomer Henry Norris Russell (1877 – 1957), the creators of the H-R diagram. Urania Observatoriets Bibliotek/The Astronomical Society of the Pacific

International Year of Astronomy - The Hertzsprung-Russell Diagram

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Hertzsprung-Russell Diagram

The Hertzsprung-Russell (H-R) diagram is often considered
the most important graphical aid in stellar astronomy. The diagram was created
in the early 20th century by Danish astronomer Ejnar Hertzsprung and
American astronomer Henry Norris Russell, and shows the relationship between
luminosity (intrinsic brightness) versus classification, color, and surface
temperature of stars.

Usually, the H-R diagram has luminosity in the vertical axis
in solar luminosity units, where the Sun has a value of 1. More luminous stars
are at the top, and less luminous stars are at the bottom. Surface temperature,
color and/or classification run along the horizontal axis, usually going from
hotter (bluer) to cooler (redder). The Sun has a surface temperature of 5,800 K
(about 5,520 °C) and is classified as a G-type (yellow) star. 

When plotted in the H-R diagram, most stars fall along a
diagonal band called the "main sequence." This is because most stars follow a
relationship between their luminosity, their mass, and their temperature-blue
stars are very hot, luminous, and massive, while red stars are cool, dim, and
less massive. This holds true as long as the stars are converting hydrogen into
helium in their cores. Stars that are not burning hydrogen in their cores fall
in different regions of the H-R diagram-giants, supergiants, and white

A star's position in the H-R diagram may also indicate its
radius and mass, measured in terms of the Sun's radius and mass (where the Sun
has a value of 1 for each), as well as its evolutionary stage.  Main sequence stars often have sizes between
a tenth to a few times the size of the Sun. As a star runs out of hydrogen in
its core, it grows to be a red giant or a red supergiant, and its size can
increase as much as a few hundred times the size of the Sun (while its mass
stays the same or even decreases). Red giants and red supergiants are cool
stars. However, their enormous sizes make them very luminous, therefore falling
in the upper right corner of the H-R diagram. White dwarfs, plotted at the
bottom center of the diagram, are the hot-but-cooling center cores that
near-solar-mass red giants leave behind after they shed their outer layers.

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