Background: Classifying Stars

The History of Stellar Classification

Before the advent of digital cameras, spectra were created using photographic plates. Though the images were black and white, calibration allowed astronomers to map wavelengths to the spectra they obtained. The results looked something like the image below.

Unlike spectra obtained using a digital device, with film there was no way of knowing how much light was being produced at each wavelength. At the Harvard College Observatory, Williamina Fleming worked with a group of women to develop a way to classify stars using only the dark lines visible in each spectrum. Even though scientists of the time did not understand what caused the absorption lines, Fleming and her colleagues were still able to create a complete classification system of 22 different classes labeled A through P.

A few years later, Annie Jump Cannon continued this work by combining the previously established classifications with new knowledge of what caused these dark absorption lines and their relationship to temperature. She reordered Fleming's original spectral classes and combined several similar groups to form the Harvard Spectral Classifications—the same classification system used by astronomers today.

Stellar Spectra, a brief review

The spectrum of a star is composed primarily of blackbody radiation—thermal radiation that produces a continuous spectrum (a continuum). The star emits light over the entire electromagnetic spectrum, from the x-ray to the radio. However, stars do not emit the same amount of energy at all wavelengths. The peak emission of their blackbody radiation comes at a wavelength determined by their surface temperature, a relationship known as Wien's Law. Most stars put out the maximum amount of radiation in and around the optical part of the electromagnetic spectrum. An ideal blackbody spectrum is shown on the left below.

In addition to the continuous spectrum, a star's spectrum will feature a number of either emission or absorption lines. Emission lines are produced by atoms when electrons drop from upper energy levels to lower ones, emitting photons at specific wavelengths. This process adds radiation to the star's spectrum, so emission lines are brighter than the region of the spectrum around them. Absorption lines are produced by atoms whose electrons absorb radiation of a specific wavelength, causing the electrons to move from a lower energy level to a higher one. Rather than adding radiation to the spectrum, this process removes some of the continuum being produced by the star and results in dark features in the spectrum. These lines are dimmer than the wavelength region around them.

An actual stellar spectrum is shown above on the right. Notice how the underlying shape, the continuum, is a blackbody radiation curve with roughly the same peak as the spectrum on the left. The big difference between these two is that an actual stellar spectrum has absorption and emission lines. See if you can spot at least one absorption line and one emission line.