The Origin of OBAFGKM: How Star Classification Evolved

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The classification of stars using the letters OBAFGKM has a rich history dating back to the 19th century. What started as an alphabetical system evolved over time through the pioneering work of astronomers at the Harvard College Observatory, leading to a sequence that inadvertently aligned with stellar surface temperatures. However, the final order was not simply arbitrary - it represented a more scientifically accurate way of categorizing the complex spectra of stars.

Why it matters

The OBAFGKM system is a fundamental tool in modern astronomy, providing a standardized way to understand the properties and evolution of stars. Tracing the origins and development of this classification scheme sheds light on the iterative nature of scientific discovery, where initial observations and categorizations are refined over time to better reflect the underlying physical reality.

The details

In the 1860s, Italian astronomer Angelo Secchi pioneered the first system for classifying stars based on their spectra. This was built upon in the late 1800s by the team at the Harvard College Observatory, led by Edward C. Pickering. They initially assigned letters based on the strength of hydrogen lines, but as they analyzed more stars, they realized the system needed refinement. The familiar OBAFGKM sequence emerged, with the final order inadvertently aligning with stellar surface temperatures, even though the Harvard team didn't know this at the time.

• In 1866, Angelo Secchi classified around 4,000 stars into four groups based on the visual appearance of their spectra.
• In the late 1800s, the Harvard College Observatory, funded by the Henry Draper Memorial Fund, launched a project to classify stellar spectra on a massive scale.