Starts With A Bang

How do we classify the stars in the Universe? (Synopsis)

The stars found in NGC 3532 show a rich variety of colors and brightnesses. Image credit: ESO/G. Beccari.

“Teaching man his relatively small sphere in the creation, it also encourages him by its lessons of the unity of Nature and shows him that his power of comprehension allies him with the great intelligence over-reaching all.” -Annie Jump Cannon

A look up at the stars in the night sky shows a clear distinction: some are fainter while others are brighter, some are redder while others are bluer, some are closer while others are much farther away. But what accounts for the differences — some real and some only apparent — between these stars? For most of human history, not only didn’t we know, but any distinction or classification scheme seemed arbitrary.

The original three Secchi classes, and the accompanying spectra that go along with them. Image credit: from a colored lithograph in a book published around 1870, retrieved from AIP.

In the 1800s, a new tool, stellar spectroscopy, enabled us to break up the light from stars into its individual wavelengths. By observing a number of “dark” features in these spectra, corresponding to atoms, ions and their absorption lines, we could finally start to make sense of it, and a more objective system.

Annie Jump Cannon sitting at her desk at Harvard College Observatory, sometime in the early 20th century. Image credit: Smithsonian Institution from the United States.

The person who pieced it all together was Annie Jump Cannon, and her 1901 system is still in use today!