A single star is a wonder. A million stars is a story.
A star can burn for billions, even trillions of years. With human history spanning mere centuries, how can we possibly understand the lifespan of a star? If we only had the Sun to study, understanding it’s history would be difficult, but we can observe millions of stars, some ancient and some still forming. By looking at these stars as a whole we can piece together the history and evolution of a star. It is similar to taking pictures of a single day on Earth, and using it to piece together the story of how humans are born, live and die.
One of the ways this is done is through a Hertzsprung-Russell (HR) diagram. The brightness of a star is plotted against its color. When we make such a plot, most stars lie along a diagonal line where the bluer the star the brighter it is. Given a large enough sample of stars, we can presume that the ages of stars are randomly distributed. Since most stars lie along this line (known as the main sequence) they must spend most of their lives there. So it’s clear that stars have a long stable period where they burn steadily. Other stars are red, but still quite bright. One would expect red stars to be dimmer than blue stars since they have a lower temperature. In order to be so bright, they must be quite large. These red giants are stars that have swollen up as their cores heat up in a last-ditch effort to continue fusing hydrogen. Some stars are large enough to start fusing helium in this stage. Since helium burns hotter, these stars brighten into blue giants. In the end, however, most stars collapse into white dwarfs when core fusion ends. They become hot but small stars, blue-white in color but quite dim.
While an HR diagram gives us a snapshot of stellar lifetimes, they don’t tell the whole story. Another way to categorize stars is through their spectra. Different elements in a star’s atmosphere absorb particular wavelengths of light. By looking at the pattern of wavelengths absorbed we can determine which elements the star contains. On a basic level can categorize stars by their metallicity. While stars are mainly hydrogen and helium, they contain traces of other elements (which astronomers call metals). The metallicity of a star is by its ratio of iron to helium, known as [Fe/He]. This is expressed on logarithmic scale relative to the ratio of our Sun. So the [Fe/He] of our Sun is zero. Stars with lower metallicity will have negative [Fe/He] values, and ones with higher metallicity have positive values. Since “metals” are formed by fusion in the cores of stars, those stars with higher metallicity must have formed from the remnants of earlier stars. Our Sun is likely a third generation star. One of the things metallicity tells us is that stars toward the center of our galaxy formed earlier than stars in the outer regions. Through millions of stars we not only understand the history of stars but the history of galaxies.
As we continue to gather more data on stars, they continue to tell us a rich collective story.