Why do astronomers emphasize spectrscopy so much?

     -- We can't visit or touch anything outside the Solar System so we use light as a messenger telling us about conditions elsewhere.  Here are two samples from later in the course that show how useful spectroscopy is.

Solar spectrum This spectrum of the Sun shows absorption lines from cool gas above the surface of the Sun.  The presence of lines from hydrogen, magnesium, and sodium tells us right away that these elements are present in the Sun.  To determine the relative amounts of these elements requires knowledge of the Sun's temperature.
redshifts

This collection of spectra reveals that galaxies (collections of stars like the Milky Way) have spectral lines that are Doppler shifted. The shifts indicate that all of these galaxies (and virtually all that have ever been studied) are moving away from us (eg. are red-shifted).  The recession velocities are proportional to distance which implies that the Universe is expanding (more on this later in the course).

Notice that the same pair of absorption lines are seen in all the galaxies which tells us that the stars comprising these galaxies are similar (and similar to those seen nearby). 

 

 

rainbow.jpg (6687 bytes)

 

 

 

 

Rainbow, from  http://www.lioncrusher.com/ecard/

sirtflaunch.jpg (4413 bytes)

Aten, Egyptian Sun god

 

 

 

 

Aten, Egyptian god of the sun,

http://www.kenseamedia.com/egyptian_gods/

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hypertext copyright.jpg (1684 bytes) G. H. & M. J. Rieke

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