The Hertzsprung - Russell Diagram
· The Hertzsprung-Russell diagram is an analytical tool for understanding the properties of stars.
· This diagram is a plot of stellar brightness (luminosity) versus stellar color (temperature).
· Brightness (luminosity) is plotted along the Y-axis; color (temperature) along the X-axis.
· The diagram is named for the two astronomers who created the first version around 1912, uncovering fundamental relationships between the properties of stars.
· The observational HR diagram plots the brightness and colors of stars in units of magnitudes. This version of the HR diagram is also frequently termed the color-magnitude diagram.
· The theoretical HR diagram plots the luminosity of stars expressed in solar units, against stellar surface temperature, expressed using the Kelvin temperature scale.
· The X-axis is also labeled with the stellar spectral classifications.
· Spectral class is defined by the type, number and strength of spectral lines observed in a star’s optical spectrum.
· Spectral classes are coded by letter: O, B, A, F, G, K, M.
· Spectral class and color are defined by the star’s temperature.
An HR diagram, plotted using luminosity on the Y-axis and temperature on the X-axis.
Stars, Revealed
What does the HR diagram reveal about the nature of stars ?
· Stars do not smoothly populate the HR diagram, nor are they randomly distributed.
· The HR diagram contains many “regions”, where stars are more concentrated:
4 Main sequence: A thin band extending from the top left of the diagram to the lower right. Dwarfs stars lie along the main sequence.
4 Red giant region: Located towards the upper right of the diagram. Stars located here are about 10 times larger than the Sun, and many time more luminous.
4 Supergiants: A band extending across the top of the HR diagram from blue to red. These are extremely large and luminous stars.
4 White dwarfs: A region in the lower left of the diagram. These are faint, hot, very small stars.
· 90% of stars lie along the main sequence, a consequence of the luminosity-radius-temperature relationship.
· The Sun is classed as a G2 dwarf, and is located on the main sequence.
· Stars range in temperature from the hot O stars (up to 50,000 K) to the relatively cool M stars (~3000 K).
· Stars range in luminosity from 10-4 to 104 L .
· Stars range in size from up to 500 R , labeled supergiants, to 0.01 R , the smallest M dwarfs.
· The mass of a star defines its initial location on the HR diagram.
· As stars evolve, their luminosity and temperature change — they “move” around the HR diagram.