1910

Main Sequence

Ejnar Hertzsprung (1873–1967), Henry Norris Russell (1877–1957)

In the early part of the twentieth century, astronomers worldwide were characterizing and classifying enormous numbers of stars in terms of their colors and spectroscopic lines, expanding on the methods pioneered by Edward Pickering’s group at Harvard. Among the most important advances was the observation, noticed independently by the Danish astronomer Ejnar Hertzsprung and the American astronomer Henry Norris Russell, that when the spectral classes or temperatures of the stars were plotted against their actual brightness (that is, after their apparent brightness in the sky was corrected for their distance from us), most of the stars cluster in a broad sequence from upper left to lower right. Hertzsprung coined the term “main sequence” to describe this prominent trend among the stars. Such plots began being used around 1910 and are called Hertzsprung-Russell (H-R) diagrams.

Over the next few decades astronomers began to understand that the main sequence was more than just a random clustering—it represents an evolutionary pathway for tracking the age and eventual fate of the stars. Most stars are born when their central pressures and temperatures are high enough for the nuclear fusion of hydrogen atoms into helium. During this hydrogen-fusing phase of its lifetime, a normal star will plot on the main sequence at a position that depends on its mass, with luminous stars a few to ten times the mass of the Sun (blue giants) on the upper left end of the plot and dim stars from about one-tenth to one-half the Sun’s mass (red dwarfs) on the lower right. As stars age and run out of hydrogen fuel, they diverge off the main sequence and eventually “die” in characteristic (and often spectacular) ways that again depend on their mass.

As the details of stellar interiors later became understood by astrophysicists such as Arthur Eddington and Hans Bethe, it became possible to predict how stars of specific masses would live and die. Our Sun turns out to be an average mass, middle-aged, main sequence star that appears destined, in about 5 billion more years, to bloat up into a red giant, expel its outer layers into a planetary nebula, and then fade away as a white dwarf.

Plots of the intrinsic luminosity of stars (on the y axis, normalized so the Sun’s luminosity = 1) versus their color—or equivalently, their temperature (on the x axis)—reveal a prominent diagonal band of stars known as the main sequence, bracketed by brighter blue and red giants and dimmer white dwarfs.