Hertzsprung – Russell Diagram

H-R Diagram Einar Hertzsprung and Henry Russell independently studied the relation between true and brightness (absolute magnitude ) and temperature of the stars in the 20th century. H-R diagram was developed from this research. It exhibits intrinsic stellar properties. H-R diagram can tell a great deal about the sizes, colors and temperatures of stars.

Astronomers survey a portion of the sky and plot each star according to its luminosity and temperature to produce an H-R diagram.

Main-sequence stars The 90 percent of all stars fall along a band that runs from the upper-left corner to the lower-right corner of the H-R diagram. These so called “ordinary stars” are called main-sequence stars. The hottest main-sequence stars are intrinsically the brightest, and vice versa.

Main-sequence stars (mass) The luminosity of the main-sequence stars is also related to their mass. The hottest (blue ) stars are about 50 times more massive than the sun. Whereas, the coolest red stars are only as 1/10 as massive. Therefore, on the H-R diagram, the main-sequence stars appear in a decreasing order, from hotter, more massive blue stars to cooler, less massive red stars.

Main-sequence stars (Sun) The Sun is a yellow, main sequence star with an absolute magnitude of about 5. Because the magnitude of a vast majority of main-sequence stars lie between -5 and 15, and because the Sun falls midway in this range, the Sun is often considered an average star. More main sequence stars are cooler and less massive than our Sun.

Giants (Red giants) Some stars do not fit in the main-sequence stars. The giant is located above and to the right of the main sequence in the H-R diagram . On the basis of their color they are called red giants.

Giants (Size) The size of the giants can be estimated by comparing them with stars of known size that have the same surface temperature. Objects having equal surface temperatures radiate the same amount of energy per unit area. Therefore any difference in the brightness of two stars having the same surface temperatures is attributable to their relative sizes.

Giants (Size) Example: Let us compare the Sun, which has a luminosity of 1 with another yellow star with a luminosity of 100. Both stars have the same surface temperature. Because both stars have the same surface temperature, they both radiate the same amount of energy per unit area. Therefore, for the more luminous star to be 100 times brighter than the Sun, it must have 100 times more surface area. It should be clear why stars whose plots fall in the upper-right portion of the H-R diagram are called giants.

Supergiants Extremely large bright star. Betelgeuse a bright red supergiant in the constellation Orion. It has a radius of about 800 times that of the Sun. The star placed at the center of the solar system would extend beyond the orbit of Mars. Other red giants that are easy to locate in our sky are Arcturus in the constellation Bootes and Antares in Scorpius.

Dwarfs (White dwarfs) In the lower-central portion of the H-R diagram, the opposite situation occurs. These stars are much fainter than main-sequence stars of the same temperature. They must be much smaller . Some probably approximate Earth in size. The group has come to be called white dwarfs. Although not all are white.

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