Hertzsprung-Russell Diagram

One of the most important properties of the H-R diagram is that it lets us trace the lives of the stars. A ball of gas officially becomes a star at the moment that nuclear fusion reactions begin in its core, converting hydrogen to helium. At the point the star is a brand-new main sequence object, and lies at the lower boundary of the main sequence strip. Sensibly enough, this is called the zero-age main sequence, or ZAMS.

As a star ages, it gradually gets brighter. This means the star moves upward on the H-R diagram, because it is getting more luminous. That is why the main sequence is a band and not just a line: different stars of a given spectral type are different ages and have slightly different luminosities.

When a star runs out of hydrogen, many bizarre and fascinating things begin to happen. With its hydrogen nearly exhausted, the star has to begin fusion of heavier elements like helium, carbon, and oxygen to keep its interior furnace going. This causes the surface of the star to expand greatly, and it becomes very luminous, moving to the upper parts of the H-R diagram.

Giant stars, therefore, are dying beasts. They are stars that have run out of hydrogen and are now burning heavier elements in their cores. Many giant stars are unstable and pulsate, while others shine so fiercely that matter streams away from them in a stellar wind. All these are important evolutionary states and occur in stars in specific parts of the H-R diagram.

Nowhere is stellar evolution more dramatically illustrated than in a star cluster H-R diagram. Clusters are large groups of stars that all formed at the same time. Figures 2 and 3 show the H-R diagram for two clusters, the Pleiades and M67.

These are only a few of the ways in which H-R diagrams reveal the essential properties of stars. The power and elegance of the H-R diagram in improving our understanding of stars and how they evolve has made its invention one of the great advances in twentieth-century astronomy. More importantly, it demonstrates how careful classification, often considered mundane or even boring work, can reveal the beautiful patterns hidden in nature and reward humans with a clearer understanding of the universe of which they are such a small part.