Stellar Spectra

Caroline Fletcher
 The process of dispersion of an object's light into its
component colors.
 Dispersion is the spatial separation of a white light into components
of different wavelengths (different colors)
 Originally the study of the interaction between radiation and
matter as a function of wavelength (λ).
 Physicists classify light waves by their energies
(wavelengths). Labeled in increasing energy, we might draw
the entire electromagnetic spectrum as shown in the figure
 1814 Joseph Frauenhofer, an optician in Munich, discovered
many dark lines crossing the spectrum of the Sun.
 In 1814, Fraunhofer invented the spectroscope.
 This instrument is used to measure the properties of light.
 These are usually used to conduct a spectral analysis to identify
 A Spectroscope works by breaking light
into the different wavelengths.
 Spectroscopes are made of prisms,
as light passes through the glass, the
different wavelengths slow down by
different amounts and are bent
into their colors.
 1910 Annie Jump Cannon and others working at
Harvard developed an empirical scheme for classifying
the spectra.
 1930s Cecilia Payne-Gaposchkin applied quantum
mechanical calculations to stellar spectrum and
demonstrated the importance of temperature to the
appearance of a spectrum and showed that stars are
almost entirely H.
 She became the first woman to become a full professor
at Harvard.
 Spectral lines are formed in the photosphere of
a star so we must understand the structure of a
Three types of Spectra
Continuous Spectrum
 A spectrum in which all wavelengths are present between
certain limits.
 It is produced by electrons undergoing free boundtransitions in a hot gas.
Free bound- transition is the emission of radiation when a free
electron (not attached to an atom) is captured by an ion.
 The recombination may be to an excited energy level, with the
emission of a photon, after which the electron cascades down through
the excited states to the ground state, producing emission lines
characteristic of that ion or atom.
 How do they form?
 Caused by drops of water falling through the
This creates a natural continuous spectrum.
 Why are they bent, and always in the same order?
 Roy G. Biv (Red, Orange, Yellow, Green, Blue, Indigo, and
 The light exits the raindrop at a 40-42 degree angle away from
the angle it entered the raindrop.
 The violets and blues bend at a 40 degree angle, and the
oranges and reds bend at a 42 degree angle.
This is due to the light being bent as it enters and then again when it
exits the water drop.
Emission Spectra
 Each element's emission spectrum is unique.
 Spectroscopy can be used to identify the elements in matter of
unknown composition. Similarly, the emission spectra of
molecules can be used in chemical analysis of substances.
 The emission spectrum characteristics of some elements are
plainly visible to the naked eye when these elements are
heated. For example, when platinum wire is dipped into a
strontium nitrate solution and then inserted into a flame, the
strontium atoms emit a red color. Similarly, when copper is
inserted into a flame, the flame becomes green.
Absorption Spectrum
 Over 100,000 absorption lines are visible in the Sun's
 Stars have absorption line spectra.
 We can think of stars as a hot continuum source with a
"cool" atmosphere of absorbing gas.
 The chemical make-up of the stellar atmosphere
determines the wavelengths that get absorbed.
Dark hydrogen absorption lines appear against a continuous visual
spectrum, the light in the spectrum absorbed by intervening hydrogen
atoms. From "Astronomy! A Brief Edition," J. B. Kaler, Addison-Wesley,
Stellar spectral sequence
 The appearances of the spectra of stars is dependent
on the temperature of the star.
Is that star moving?
 Shifts in the lines will show astronomers the
movement of distant stars.
 A red shift means that it is moving away, while a
blue/violet shift means that it is moving toward us.
Work Cited