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12.9 Spectra of Stars

We learn about the Sun's atmosphere and surface layers by studying its spectrum, which is the distribution of light into different colors (wavelengths). The same is true of other stars. Spectroscopy — the study of spectra — is a vital tool for understanding the physical properties of astronomical objects, and many astronomers devote their entire careers to it. Spectra can be used to reveal the physical nature of stars. It is the ultimate form of remote sensing. We do not have to visit the stars or bring back stellar material into the lab; we can diagnose what they are made of and how they shine purely by gathering light and dispersing it into a spectrum.


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Solar spectrum showing the dark absorption lines. Click here for original source URL



Astronomers make a spectrum when they spread starlight into a range of wavelengths (or frequencies) with a spectrograph. A spectrum is usually presented in graphical form. By convention, short wavelengths are at the left and long wavelengths are at the right. Absorption lines appear as sharp valleys or notches in the spectrum and emission lines appear as sharp peaks. The graph shows intensity as a function of wavelength. The intensities of the image represent intensities of light, or radiant energy. Usually the blue end is to the left. The general level of brightness between absorption or emission lines is called the continuum. For stars, the continuum has the broad distribution of wavelengths that is typical of thermal emission from gas at a temperature of a few thousand degrees.

In the earliest days of spectroscopy, spectra could only be viewed by eye. By the late 1870s, astronomers began to use photographic plates to record spectra observed through a telescope. For a hundred years, astronomical data could only be recorded by photography. The pioneering work on understanding stars was done in the early 20th Century using photographic plates. Photographic spectra have now been superseded by spectra taken with electronic detectors, or CCDs. The wavelength range of the spectrum that can be recorded depends on the wavelength range of sensitivity of the material of the CCD — typically silicon. It usually covers the range the eye can see and often extends to even bluer and redder wavelengths.

 

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Harvard College Observatory "computers." These women classified thousands of stellar spectra. Click here for original source URL.

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Edward C. Pickering. Click here for original source URL.

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Henry Draper. Click here for original source URL.

 

In 1872, Henry Draper was the first to photograph stellar spectra. This represented a tremendous advance. Instead of sketching or verbally describing spectra, astronomers could directly record, compare, and measure spectral features. Draper began an ambitious project to photograph and catalog all the bright stars in the sky, but he died long before it was complete. His widow then donated enough money to Harvard to continue the work. Harvard Observatory astronomer Edward C. Pickering headed the project to create the Henry Draper Catalog. He hired a large group of women, who were called “computers,” to do the painstaking work of measuring the spectra of thousands of stars. 
 

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Representative stellar spectra from a range of spectral classes. Click here for original source URL.

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Annie Cannon. Click here for original source URL.

 

Annie Cannon and a group of young women assistants invented a system of spectral classes based on the appearance of spectral lines. Spectra were classified alphabetically according to the strength of the hydrogen absorption lines. Stars with the deepest hydrogen lines were A stars, stars with the next deepest hydrogen lines were B stars, and so on up the alphabet (the original sequence stopped at P). The most numerous spectral features were lines of hydrogen and lines of helium. Absorption lines of other elements are also seen, and astronomers traditionally lump together all elements heavier than helium under the term "metals" — this is clearly a misnomer since only a few of the heavier elements are actually metals! The spectral classification was eventually changed to be a sequence in temperature, which did not follow the alphabet since the depth of hydrogen lines does not change in a simple way with temperature. 

At the time that Annie Cannon did her survey, modern atomic theory had not been developed and astronomers had a poor understanding of the causes of spectral lines. Nobody knew that emission and absorption lines depended on the orbital structure of electrons in atoms. The spectra of stars (including the Sun) show many spectral features — this tells us that the photospheres are composed of many elements. Each element makes a unique pattern of lines, which helps us determine which elements are present.

Consider the simplest and most abundant type of atom in the universe: hydrogen. Hydrogen atoms’ orbits can be numbered. Since a neutral hydrogen atom has just one electron, an atom in the ground state would have one electron in the orbit n = 1. If the atom had been bumped by other atoms and gained energy or if it had absorbed radiation, the electron might be in the orbit n = 2, 3, and so on. Further absorption of energy might cause it to jump from n = 3 to n = 4, creating an absorption line, or it might spontaneously revert from n = 3 to n = 2, creating an emission line. Each possible transition (1 to 2, 2 to 3, 4 to 2, and so on) creates a different line. Transitions between the n = 2 level and higher levels create the lines prominent in the visible part of the electromagnetic spectrum. The famous Hydrogen-alpha (Hα) line lends its brilliant red color to many astronomical gases, including the solar chromosphere and many nebulae.

 

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Hydrogen energy diagram. Click here for original source URL.