“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
stars 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
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