# Understanding her work

After classifying almost a quarter of a million stars Annie Jump Cannon celebrated her accomplishments by writing a story for her Christmas cards in 1915 entitled, The Story of Star Light, which briefly explained this “new” science and her work to her friends:

At the turn of the century the new science of astrophysics was growing rapidly, due to the recent findings in photometry and spectroscopy. When Annie began at Harvard scientists knew that when light from the Sun was shone trough an prism it was dispersed into a spectrum of component colors, like a rainbow. The spectrum was broken up by a series of dark lines and bands which resulted from various atoms in the solar atmosphere. After astronomers like Prof. Pickering attached prisms to their telescopes and took photographs of other stars they discovered that the lines in the spectra of stars differed from those of the Sun. Stellar classification according to line patterns began. This was not easy to do. Annie excelled because of her sharp eye and memory. The following image is one photographic plate containing stellar spectra. For this particular field she averaged 3 stars a minute.

How would a beginning astronomy student begin understanding how to do this? Annie started at Wellesley by learning about spectroscopy. A spectroscope is a device used for splitting a beam of light or radiation into its component parts to be studied. In Astronomy 101 you learn that many hot objects emit radiation containing all wavelengths of light, known as a continuous spectrum. The spectrum of other hot gasses may not contain all the colors. Instead it includes only a few very defined pieces or slivers of the continuous spectrum, known as emission lines. This spectrum is called an emission spectrum. If the beam of radiation was passed through as cool gas it would produce absorption lines (like gaps in the continuous spectrum) at the corresponding frequencies. A German physicist, Gustav Kirchhoff, describes the relationship between these three spectra. Kirchhoff's Laws are:

• A luminous solid or liquid, or sufficiently dense gas emits light of all wavelengths and so produces a continuous spectrum of radiation.
• A low-density, hot gas emits light whose spectrum consists of a series of bright emission lines. These are characteristic of the chemical composition of the gas.
• A thin, cool gas absorbs certain wavelengths from the continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum. These lines are characteristic of the composition of the intervening gas. They occur at the same wavelengths as the emission lines produced by the gas at higher temperatures.

Scientists study these spectral lines because they can be used as a “fingerprint” to identify an element.

Electromagnetic radiation is made up of energy bundles called photons. Emission lines are caused by the emission of a photon and absorption lines are caused by the absorption of a photon by an atom. The energy of the photon is directly proportional to its frequency and depends upon its color. Atoms consist of negatively charges electrons orbiting a positively charges nucleus. The heavy nucleus is made of neutral neutrons and positively charged protons. The number of protons determine the element that the atom represents. In normal (electrically neutral) circumstances the number of protons equals the number of electrons. There are various well-defined states in which the electron can reside. Each state has a specific energy level. As electrons move between states or “energy levels” the difference in energy levels is emitted or absorbed by photons. The radiation of the groups of atoms making up molecules can be identified in a similar manner. Astronomers use these observed spectral characteristics to deduce information about the stars. For example, which lines are present can tell us the composition of the star. The intensities can tell us the composition and temperature. The peak frequency or wavelength can also tell us the temperature (in the case of continuous spectra). Line broadening or line width can tell us about the gas temperature, rotation speed, density, magnetic field or even thermal motions and gas turbulence. The Doppler shift can even tell us its radial velocity. The following picture shows a more detailed picture of stellar spectra for main sequence class stars F5-M5 (Carroll & Ostlie, An Introduction to Modern Astrophysics; Reading Massachusetts: Addison-Wesley Publishing Company, 1996.)

It is amazing that Annie Cannon was able to classify over a quarter of a million stars with such speed and accuracy. Understanding her work makes it obvious why her unique contributions to the then-new field of astrophysics were so valuable!