7. Star Spectrum

    Science is a way of understanding nature, and the spectrum of a star tells us a great deal about such things as temperature, motion, and composition.

    Each element has its own spectrum, unique as a human fingerprint, and it can be recognized by its spectrum across trillions of miles.

    Whatever kind of spectrum astronomers look at, the most common spectral lines are the Balmer lines of hydrogen, the only lines we can study from the earth's surface.

    Balmer lines can be used like a thermometer to find the temperatures of stellar surfaces.  We know we can estimate stellar temperatures from color - red stars are cool and blue stars are hot.  But Balmer lines give us much greater accuracy.

    NOTE: any discussion on the temperature of a star, means surface temperature - typically 40,000 - 2,000 K.  The centers of stars are much hotter - many millions of degrees - but the spectra tell us about the surface layers from which the light originates.

   The strength of the Balmer lines depends on the temperature of the star's surface layers.  Both hot and cool stars have weak Balmer lines, but medium temperature stars have strong Balmer lines.  The same process affects the spectral lines of other elements, such as ionized calcium.  That is, their lines are weak at high and low temperatures and strong at some intermediate temperature.  But the temperature at which their lines reach maximum strength is different for each element.   If a number of elements are applied to a graph, a powerful tool for finding the temperature of stars is created.

   
(a) The strength of the Balmer lines in a stellar spectrum
depends on the temperature of the star.  A star with
medium-strength Balmer lines could have one of
two possible temperatures.

  
(b) The curve for lines of once ionized calcium
reaches maximum strength at a different temperature.

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