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Spectra by xuyuzhu


									Astronomical Spectra
         Astronomical Spectra
We've talked about collecting and focusing light using mirrors
and lenses. You can learn much more about the structure of
light-emitting objects by breaking up their light into a

We will discuss:

       Refraction and Diffraction

       Types of Spectra

       Features of Spectra

       Spectral Classification

       Uses for Spectra
                           Remember chromatic aberration?

                           A prism breaks up light into its
                           spectrum for the same reasons
                           that light focused by a lens has
                           chromatic aberration.

Refraction works on the principle that different energies of
light have different “bending” properties in glass, water, etc.

The greater the energy, the more bending that takes place.

In these days, astronomical spectra are not generally created
by means of prisms.
Light can also be broken up into a spectrum by lines that are
ruled on a surface very close together (e.g. 10,000 parallel
lines ruled in each centimeter).

These ruled surfaces, better known as diffraction gratings,
are more commonly used than prisms to create astronomical

Diffraction works on the physical
principle of interfering light waves.

The shorter the wavelength
(greater energy) the less the
light is diffracted.
A spectroscope is a spectrum-making device that usually
includes everything between a slit at one end and the
viewing eyepiece at the other.

When the resultant spectrum is recorded either with a
photographic plate or electronically (as opposed to
viewed with the eye), the device is called a spectrograph.
Different Types of Spectra I

                 A continuous spectrum
                 with a number of fine dark
                 lines (i.e. missing colors)
                 is called an absorption

                 A spectrum with more
                 radiation at some points
                 than at neighboring points
                 is an emission spectrum.
Different Types of Spectra II
                 A hot, sufficiently dense
                 gas or a hot solid object
                 produces a continuous
                 spectrum with no dark
                 spectral lines.

                 A hot, diffuse gas produces
                 bright spectral emission

                 A cooler, diffuse gas in front
                 of a hotter source of a
                 continuous spectrum
                 produces dark spectral
                 absorption lines in the
                 continuous spectrum.
    Spectra of Astronomical Objects
Stars will usually have absorption
spectra - the interior core is
sufficiently dense to create a
continuous spectrum and the stellar
atmosphere “steals” light from this
spectrum giving rise to absorption.

Planets shine by reflected sunlight
so their spectra will be absorption
in nature.

Galaxies and nebulae usually have emission
spectra because of the geometry of the stars
related to the gas.

Most astronomical objects do not have
continuous spectra because there are so
many patches of dust to get in the way.
   Features of Astronomical Spectra
The absorption or emission
lines in astronomical
spectra correspond to
certain elements.

Which element is excited
is dependant on the average
energy of the light hitting
the atoms in the star.

The average energy of the
star’s light is dependant on

The strength of the emission
does not reflect the
abundance of that element in the star, but merely which elements get
excited by the light. Most stars have similar compositions.
Absorption and Emission Explained...
                    Quantum mechanics tells
                    us that only certain size
                    electron orbits are allowed
                    around the dense nucleus
                    of an atom.

                    An electron can drop from
                    a high energy orbit to a
                    lower energy orbit,
                    emitting a single photon of

                    An electron can absorb a
                    photon and go into a
                    higher energy orbit.
       Oh Be A Fine Girl Kiss Me
        (Spectral Classification)
                              Through a very complicated
                              relationship the lines you see
                              in the spectrum are related to
                              the temperature of the object.

                                                         3/ 2
                                              2πm kT 
                                i +1 = i +1 
                               N      2Z                         − χ / kT
                                                e             e i
                                N     n Z  h2 
                                  i    e i          

       O   Strong He          Thus, based on the presence
       B   Strong H           and strength of various
       A   Strong H           elemental lines astronomers
       F   Moderate H         can lump stars into classes
Temp   G   Strong Ca          based on their temperature.
       K   Strong Metals
       M   Strong Molecules
          Other Uses of Spectra
Spectroscopy is possibly one of
the most useful tools in

Spectral lines shift as objects
move. Spectra can be used to
determine how fast something
is moving. For most distant
galaxies this can also be used
to determine how far away they

Detection of Extrasolar planets.

Detection of relics from the big bang.

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