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Stellar Spectroscopy

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					                         Stellar Spectroscopy




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                                   Introduction


 Spectroscopy is a branch of science that is concerned with the investigation and
  measurement of the spectrum of light produced when matter interacts with
  electromagnetic radiation.


 In astronomy, spectroscopy is a very useful tool because telescopes can easily
  measure the spectra of far away objects, such as stars and galaxies.


 The following activity will focus on stellar spectroscopy (the spectra of stars)
  and how those spectra can be used to classify stars.




   National Schools’ Observatory
                                  Introduction


 We measure the spectra of luminous
  objects by attaching a special instrument
  to a telescope, known as a spectrograph.

 Within the spectrograph, light is passed
  through a series of prisms and mirrors in
  order to split the light into its different
  component wavelengths (its spectrum).

 The intensity of those different wavelength components is then measured to
  create a spectra. The spectra can then be investigated in detail to discover
  useful information about the target. Rainbows are the spectra of our Sun.

 The Liverpool Telescope (LT) has its own spectrograph – FRODOspec (Fibre
  fed Robotic Dual-beam Optical Spectrograph), and the following activity uses
  real data taken by the LT using FRODOspec.

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                                    FRODOspec

Light collected by the telescope enters
the FRODOspec instrument and is split
into two beams:
 Longer wavelength light is sent down
  the ‘red arm’.
 Shorter wavelength light is sent down
  the ‘blue arm’.
 The light from each arm is then split as
  it passes through a prism, and a
                                             The above image shows the inside of FRODOspec
  detector captures the resulting                        during its development.

  spectrum on a CCD device.


    National Schools’ Observatory
                                   FRODOspec

The image to the right shows the raw
spectrum of the star BD-02-5059.

 Each of the vertical lines are repeats

  (or orders) of the same spectrum.

 You can see a line feature (circled

  red) running across those orders.

 In this activity you will be provided

  with a 2-D spectrum (wavelength
  versus intensity) that has been
  extracted from the raw data.

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                                   Types of Spectra


In general, there are two types of spectra that can be easily identified, EMMISION
spectra and ABSORPTION spectra.

       An example of an emission spectra:




       An example of an absorption spectra:




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                                    Emission Spectra


Emission spectra occur when electromagnetic radiation originates from a cloud of
relatively hot (compared to its surroundings) gas.
   Due to the temperature of the gas, the atoms within it will possess lots of energy, and
    some of their electrons will occupy excited states around the nucleus.
   These electrons will de-excite (loose energy and fall to a lower energy level), with the lost
    energy being emitted as a photon at a specific wavelength.
   Such processes cause bright lines in a spectra – Emission lines.




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                                    Absorption Spectra

Absorption spectra occur when electromagnetic radiation from a background
star passes through a relatively cold gas.

   Radiation at specific wavelengths from the star interacts with (is absorbed by) atoms in the cold
    gas, causing their electrons to gain energy and enter excited states.

   These electrons quickly de-excite and emit photons at the same wavelengths. However, the
    direction of the emitted light is random and this leads to the appearance of dark lines (or
    missing light) in the resulting spectra, corresponding to the wavelengths that were absorbed by
    the gas. These lines are known as absorption lines.

   Astronomers most frequently use absorption spectra to study stars.




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                                  Absorption Spectra


With careful analysis of the absorption lines, astronomers
can learn many things about as star, including:
 Temperature
 Rotational velocity
 Translational velocity (overall motion of the star)
 Density
 Chemical composition
 Metallicity (how many heavy elements stars contain)         Image of the Pleiades
                                                                  Star Cluster

Some astronomers liken the spectrum of a star to a very basic DNA test that
allows them to determine what kind of star the light originated from.



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                                Chemical Composition

                                                   Certain elements and molecules
                                                    preferentially absorb and emit light at
                                                    different wavelengths.
                                                   It is possible to deduce from spectra
                                                    which elements are causing lines at
                                                    particular wavelengths, and hence
                                                    determine the chemical composition of
 Above: Emission spectra of different elements.     the object being observed.
                                                   Analysis of a spectrum of the Sun led to
                                                    the discovery of Helium, even before it
                                                    was discovered on Earth!


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                                    Blackbody Radiation


Before investigating real spectra, we need to talk about Blackbody radiation.

   All stars are made up of a hot, dense interior and a cooler outer layer, or atmosphere.

   The interior of the star causes a continuous spectra, known as a blackbody spectrum.

   If there was no absorption or emission from the surrounding atmosphere, a stars
    spectrum would be close to a pure blackbody spectra.

   The blackbody curve (see next slide) represents the spread of intensity of the radiation
    being emitted over all possible frequencies.

   A metal glowing red hot in a furnace is an example of blackbody radiation.

   The shape of the blackbody spectrum depends on the temperature of the body.



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                                  Blackbody Radiation

Warmer objects have blackbody spectra that peak at shorter wavelengths, whereas
cooler objects have blackbody spectra that peak at longer wavelengths.




  National Schools’ Observatory
                                    Blackbody Radiation

In reality the measured spectra is never
a perfectly smooth blackbody (BB) due
to absorption and emission features.

 The dips in the curve (where it drops

   below the BB spectra) represent
   ABSORPTION lines.

 The peaks (where it rises above the

   BB spectra) represent EMISSION
   lines.                                     An example of a spectra showing absorption and emission
                                                        features (Sloan Digital Sky Survey).




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                                   Wien’s Law

 In order to classify a star, it is important for an astronomer to know the temperature
  of that star.
 Stars range in temperature from about 2000K to 40,000K.

 Using Wien’s law, it is possible to calculate the surface temperature, or ‘effective’
  temperature, if we know the wavelength its blackbody curve peaks at.

                                            W
                                   Teff 
                                            m ax
 where W is Wien’s constant and = 2.8977685×10−3 m·K.

As an example, our Sun’s blackbody spectrum peaks at a wavelength of around 500 x 10 -9
m, giving an effective temperature of about 5800K.


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                                     The Doppler Effect

   One of these effects, the Doppler Effect, occurs due to an apparent shift in the wavelength
    and frequency of a wave due to relative motion between the source and the observer.

   For example, we experience the Doppler Effect when an ambulance drives past with its
    sirens on:

       As the ambulance (the source) approaches you (the observer), the sound wave is
        compressed (bunched up) and the resulting sound is higher pitched (shorter λ)

       Once the ambulance has passed, the wave is stretched and the pitch sounds lower
        (longer λ)


          Shorter                                                                Longer
          wavelength                                                             wavelength




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                                     The Doppler Effect

   The same effect is observed with light, when a star (the source) is moving towards or away from
    observers located on the Earth.
   We can detect this motion in the spectrum of the star.
   If the star is moving AWAY from the Earth, the light waves are stretched out and the whole
    spectra experiences a shift towards the red end of the spectrum – a red shift.
   If the star is moving TOWARDS the Earth, the light waves are bunched up and the spectra
    experiences a shift towards the blue end of the spectrum – a blue shift.




          Shorter                                                                    Longer
          wavelength                                                                 wavelength




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                                     Spectral Lines

   If there were no external factors at play, the absorption spectra of a star would show very
    thin spectral line features at the exact wavelengths corresponding to amount of energy
    transferred to the atom when one of its electrons has been excited to a higher energy level.

   In reality, there are a number of different factors that act to spread out or blur the spectral
    lines across a wider range of wavelengths, a process called spectral line broadening.




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                            Spectral Line Broadening


The images below are of the same absorption line feature, in this case due to
hydrogen, as it appears in the spectra of three different stars.
Spectral broadening can be caused by a number of different factors, including:

   Temperature (Doppler Broadening)
   Stellar Rotation
   Pressure




   National Schools’ Observatory
                     Doppler (Thermal)Broadening

The Doppler broadening of spectral lines is the major factor is stellar spectra.

   Atoms in a gas move around at different speeds and in different directions .

   This movement can cause light to be Doppler shifted as an atom in motion emits a photon,
    and because some of the atoms will be moving away from the observer and some moving
    towards, the overall effect will contain both blue-shift and red-shift components.

   In a hot gas, the atoms possess more energy and thus move around more quickly than in a
    cooler gas. This means that Doppler broadening is more significant in hot stars.




                                                               Hot Star
                                    Cool Star



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                                    Rotational Velocity


 Stars are not just static objects as they
  appear in the night sky.
 They are made up of a fluid mass of
  gas and plasma (hot gas) that rotates
  around a common axis. The Sun
  rotates once every 24.5 days.
 When we obtain the spectra of distant
  stars, however, the light is gathered
  from across the entire disc of the star
  and not from just one point.


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                                    Rotational Velocity


 This means that some of the star is

  rotating towards us and some away.

 The parts rotating TOWARDS the

  observer are blue-shifted.

 The parts rotating AWAY from the

  observer are red-shifted.

 The net effect is that the star’s

  spectral lines appear to be smeared
  out, or rotationally broadened.



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                                   Spectral Classification

 It is possible to use the absorption spectra of individual stars to classify them

  into distinct groups.

 Astronomers classify them by temperature, into groups “OBAFGKM”, with

  group O containing the hottest and M containing the coolest.



    Note that there is a direct
       correlation between
     temperature and mass.




   National Schools’ Observatory
                                   Spectral Classification

 These groups are further divided into sub-groups numbered from 0 to 9, where

  0 is the hottest; for example, B1 is hotter than B3, and F2 is hotter than F9.

 The Sun has a surface temperature of 5800K, putting it in the G2 classification.




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                                            Activity

Now you know a little more about stellar spectroscopy, you will soon get the chance to look at some
real spectra from the Liverpool Telescope! Your teacher will supply you with an Excel file containing
the 2-D spectral data for 9 stars, each under its own separate tab or page of the worksheet. You are
required to plot the spectra for each of these stars.

   Plot the number of counts against the wavelength for each star. You should produce spectra
    similar to the one seen below (don’t forget to label your axes appropriately).
   The shape of each spectra will vary due to the spectral class of the star.

    Intensity




                                            The nine spectra cover many of the different classes of stars.

                                                                                               Wavelength
                                                                                               (Angstroms)

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                                              Activity

   Once you have plotted the nine spectra, you can compare them to see how the different stellar
    classifications vary. Things to look out for are the width and depth of particular spectral lines.
   The Hydrogen line at 6563 angstroms (10-10m) is a good absorption line for spotting any differences.
   Hotter stars will have a wider absorption lines due to Doppler broadening.
   Cooler stars tend to have more erratic spectra than hotter stars.
   Some spectral lines will appear in some stars, but not in others.


Intensity




                                                                     Make notes of your observations !

                                                                                                  Wavelength
                                                                                                  (Angstroms)

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                                      Activity


Once you’ve had a good look at the spectra of the 9 known star types, it’s time to use this
knowledge to classify stars of an unknown star type.

Your teacher will now supply you with another Excel file containing the spectra for two
stars that you will have to plot and then classify.

Using the information provided in this presentation, and by making comparisons to the
spectra you have already plotted, try to classify the mystery stars.




Good luck!



p.s. If you are really stuck, your teacher will have the answer.


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