Summary
Highlights
Stars appear as twinkly dots, but closer inspection reveals differences in brightness and color. Historically, the reason behind these color variations was a mystery. The late 19th century saw astrophotography and spectroscopy become crucial scientific tools. Spectroscopy, the process of dividing light into individual colors or wavelengths, provides abundant physical data about objects. Stars emit a continuous spectrum due to their hot, dense gas cores, but their atmospheres absorb specific wavelengths, creating absorption lines that reveal elemental composition.
Early star classification was based on the strength of hydrogen lines (A, B, C, etc.). In 1901, Annie Jump Cannon introduced a new system, classifying stars by various absorption lines. Max Planck later showed how a star's color relates to its temperature, with hotter stars appearing blue and cooler ones red. Meghnad Saha explained how atoms emit light at different temperatures. Cecelia Payne-Gaposchkin then combined these findings, demonstrating that stellar spectra depend on temperature and atmospheric elements. She also proved that stars are primarily composed of hydrogen and helium, contrary to earlier beliefs.
The classification scheme, refined by Cannon and Payne-Gaposchkin, arranges stars by temperature, using letters: O (hottest), B, A, F, G, K, and M (coolest). Each letter group has 10 subgroups (e.g., G0-G9). Recent discoveries added L, T, and Y for even cooler stars. Our Sun, a G2 star, has a surface temperature of 5500° Celsius. Hot stars are blue, cool stars are red, with orange and yellow stars in between. There are no green stars due to how our eyes perceive mixed light; even stars emitting the most green light appear white.
The Sun’s spectrum peaks in green, yet we perceive it as white because our eyes mix all emitted colors. The Sun appears yellowish due to atmospheric scattering, where shorter wavelengths like blue and purple are dispersed, giving the sky its blue color. Knowing a star's spectrum and its distance allows astronomers to calculate its intrinsic luminosity – the actual amount of energy it gives off. Luminosity is related to a star's size and temperature; hotter or larger stars are more luminous.
A century ago, Ejnar Hertzsprung and Henry Norris Russell created the HR Diagram, plotting stellar luminosity against temperature, revealing a strong trend. This diagram is crucial for understanding stellar lives. Bright stars are at the top, faint ones at the bottom; hot, blue stars on the left, and cool, red stars on the right. The prominent diagonal line is the Main Sequence, where most stars, including our Sun, spend the majority of their lives fusing hydrogen into helium. More massive stars on the Main Sequence are hotter and more luminous (upper left), while less massive stars are cooler and redder (lower right).
Other groups on the HR Diagram represent later stages of stellar evolution. White dwarfs (lower left) are hot, faint remnants of stars like the Sun. Red giants (upper right) are luminous but cool, massive stars nearing the end of their lives. Above them are red and blue supergiants. Stars change position on the HR Diagram as they age, with massive and low-mass stars evolving differently. The HR Diagram provides a quick overview of a star's current state and evolutionary path.