Stellar Spectra

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Stellar Spectra

• Dark lines in Spectra
• As light moves from a stars core through the gas
  in its surface layers, atoms there absorb the
  radiation at some wavelengths, creating dark
  absorption lines in the stars spectrum
• Each type of atom hydrogen, helium, calcium,
  and so on absorbs a unique set of wavelengths.
• Because each atom absorbs a unique combination of
  wavelengths of light, each has a unique set of
  absorption lines. From such absorption lines, we
  can determine what a star is made of.
• To measure a stars composition, we make a
  spectrum of its light and compare the absorption
  lines we see with tables that list the lines made
  by each atom.
• To find the quantity of each atom in the star
  each elements abundance we use the darkness of
  the absorption lines.

2
Classification of Stellar Spectra

• 1866 Pietro Angelo Secchi
• Devised a system of classifying stars according
  to color white, yellow, red, deep red
• 1872 Henry Draper
• Recorded stellar spectra on photographs
• E.C. Pickering
• Used letters to subdivide Secchis classes.
• A-D for white, E-L for yellow, M-N for red
• 1901 Annie Jump Cannon
• Re-ordered the sequence according to temperature
• O, B, A, F, G, K, M, hottest-to-coolest

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Definition of Spectral Classes

• Stellar spectral classes are based on the
  appearance of the spectrum.
• O-Type stars have weak absorption lines of
  hydrogen, but strong absorption lines of helium.
• A-Type stars have very strong absorption lines of
  hydrogen.
• F-type stars are distinguished by the multitude
  of lines from metals such as calcium and iron.
• G-type stars have weak hydrogen lines and strong
  calcium lines.
• K-Type stars are cooler and the hydrogen lines
  are almost invisible. They do show lines of
  numerous different carbon compounds.
• M-Type stars are the coolest and show lines of
  carbon compounds

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Definition of Spectral Classes

• Temperatures of Spectral Classes
• O-type hotter than 25,000 K
• B-type 11,000 K 25,000 K
• A-type 7500 K 11,000 K
• F-type 6000 K 7500 K
• G-type 5000 K 6000 K
• K-type 3500 K 5000 K
• M-type 2200 K -3500 K
• Two recent additions
• L-type 1300 K 2200 K
• T-type 900 K 1300 K

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Binary Stars

• Stars that orbit around each other
• Mutual gravitational attraction
• Astronomers can use binary stars to calculate
  stellar masses.
• Gravitational force between two bodies depends on
  their masses
• Gravitational force, in turn, determines the
  stars orbital motion
• If that motion can be measured, we can work
  backwards to determine the masses

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Eclipsing Binaries

• Binary stars that rotate on edge with respect to
  the Earth.
• One member of the binary passes between the other
  member and Earth.
• The duration of the eclipses depends on the
  stars radii.
• Astronomers can calculate the diameter of each
  member of the binary.

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H-R Diagram

• 1912 Working independently on separate sides of
  the Atlantic Ocean, Ejnar Hertzsprung and Henry
  Norris Russell found that if stars are plotted on
  a diagram according to their luminosity and
  temperature, then most of them lie along a smooth
  curve.
• Stars fall in the diagram along a diagonal line,
  with hot luminous stars at the upper left and
  cool dim stars at the lower right
• Hertzsprung-Russell Diagram (H-R)

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H-R Diagram
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H-R Diagram

• By tradition, bright stars are at the top of the
  diagram and dim stars at the bottom
• Hot stars are to the left and cool stars are to
  the right.
• The approximately straight line along which the
  majority of the stars lie is called the main
  sequence.

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H-R Diagram

• A stars luminosity depends on its surface area
  and its temperature
• If two stars have the same temp, any difference
  in luminosity must reflect a difference in area.
• A stars area depends on its radius large area
  large radius
• Stars that lie in the upper right of the diagram
  have the same temperature as those on the lower
  main sequence, but because they are more
  luminous, they must be larger
• These stars emit thousands of times more energy
  than main sequence stars and therefore must be
  immensely larger.
• The region in the H-R diagram populated by giants
  is sometimes called the giant branch
• Because many of these stars are cool and
  therefore red, they are called red giants.

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H-R Diagram

• Stars lying below the main sequence must have
  tiny radii if they are both hot and dim
• Radii of these stars are typically 100 times
  smaller than the Suns about the size of the
  Earth
• Because they are so hot they glow with a white
  heat and are called white dwarfs.

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H-R Diagram

• Giants and dwarfs differ dramatically in density
• Density is mass divided by volume
• For a given mass, a larger body will have a lower
  density
• If a giant star has a similar mass to a white
  dwarf, its density will be much lower.
• Luminosity classes
• A relationship between spectral line width and
  luminosity
• The width of the absorption lines in a stars
  spectrum depends on its density.
• The density of a star depends on its luminosity.
• Roman numerals are used for the luminosity
  classes
• Ia Supergiants (Brightest)
• Ib Dimmer Supergiants
• II Bright Giants
• III Ordinary Giants
• IV Subgiants
• V Main sequence (Dimmest)