The stars evolve. The H-R diagram charts the stages of their existence. Gravitational collapse begins the creation of a star, from a vast cloud of gas and dust. As it coalesces, its growing density adds energy to newly packed gases until its matter begins to transmute to new atomic forms. Hydrogen fuses, through deuterium to helium. Through millennia, each star begins to radiate light and to shine as if it were afire.
That is not the end of a star’s transformation. It is the beginning. The Hertzsprung – Russell diagram charts the transformations of the stars, from fiery birth to dense heavy death. An H-R diagram, or HRD, is a catalog of stars, arranged by color and magnitude, or by effective surface temperature and brightness, and a predictor of the future of any known star. For astronomers, the color of a star is directly related to its temperature, and brightness is correlated to magnitude.
An HRD is constructed with luminosity increasing from bottom to top, and temperature decreasing from left to right. Luminosity is measured by comparison with our sun, which has a value of one. Temperature is measured on the Kelvin scale. Color and spectral class vary along with temperature and absolute magnitude varies with luminosity.
Stars are graphed by placing each on the point where its luminosity, its brightness, crosses its effective surface temperature. Most stars fall along a curve that goes from upper left to lower right, from hot and bright to cool and dark (for a star). This curve is called the main sequence. Above it and to the right branch off the enormous stars, the giants, bright giants, and hypergiants. These are huge stars, but cooler than many on the main sequence. Below and to the left of the main sequence are found the white dwarfs, faint dense older stars.
The most massive stars live a few billion years. The less massive may live many billions, longer than the universe has existed so far. The smallest proto-stars shine dimly, though they change hydrogen to deuterium, heavy hydrogen. They shrink in on themselves slowly over the years, and remain brown dwarfs (astronomer Jill Tarter named them). They are not bright stars. Brown dwarfs and proto-stars would be located at the bottom right of the HRD. They are dark, cool, and small.
More massive accretions of material are able to fuse hydrogen to helium, and the pressure of the energy released by this reaction halts their shrinkage. They take a place on the HR diagram, on its main sequence. They have become stars.
The H-R diagram graphs the span of their existence. Stars enter the HRD at the lower right as proto-stars. When they coalesce, they join the main sequence, as a rule, where they may continue for billions of years. When the hydrogen of their core has all been converted to helium, they swell and become giants, leaving the main sequence and moving above it. After billions of years as a giant, a star may lose its outer shell into a planetary nebula, an illusion of rings. It becomes a white dwarf. Then it will be found towards the bottom right of the diagram.
If it is massive enough, a giant may continue to swell, fusing its material to layers of heavier and heavier elements. A truly massive star may become a supernova in the right circumstances, and may fall in upon itself as a neutron star, or even become a black hole.
Hypothetically, an old star may become an object called a black dwarf, but the universe is not yet old enough for any of these to exist. White dwarfs are probably destined to become black dwarfs. A white dwarf is a remnant, still radiating the stored heat of its former reactions. In a black dwarf, the heat will be gone.
Our own sun seems to be following a path that will lead it to become a white dwarf someday. Right now it is near the middle of the main sequence, with perhaps only five and a half billion years to go before it becomes a red giant. Then, in theory, it may cast away its outer shell in a planetary nebula and fade away as a white dwarf.
Ejnar Hertzsprung and Henry Norris Russell created the HRD in about 1910. It is used to organize stars by their size and type, but it also has more sophisticated applications. Looking at a Hertzsprung-Russell diagram, an observer is able to trace the evolution of the stars.
