Lecture 13: Stellar Spectra
Astronomy 101/103
Terry Herter, Cornell University
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Lecture Topics

  • Stars
    • What is a star?
    • Emission from Stars
    • Stellar Spectra

Stars?

What is a Star?

  • Stars "shine" at night.
  • A star is a self-luminous sphere of gas.
  • It is held together by gravity.
    • But what keeps it from collapsing?
      (More on this later)

 

The Spectra
of Stars
A telescope with a spectrograph measures the spectrum of a star and gives the brightness at different wavelengths.
    • Like we discussed with blackbodies.
  • Almost all stars show a "continuum" spectrum with "absorption" lines.
  • Some stars show "emission" lines.
    • All stars do not have the same spectrum!

Spectra of Blackbodies


Spectra --
  • Almost all stars show a "continuum" spectum with "absorption" lines.
  • Some stars show "emission" lines.
  • All stars do not have the same spectrum!

    Schematic Spectra of Star


Continuum Spectrum
  • Despite having absorption lines, the spectrum of a star is close to that of a blackbody.
  • What we see is produced by the hot surface called the photosphere.
  • For the Sun:
    • Photosphere is ~ 100 km deep
    • T ~ 6000 K

Stars as Blackbodies

  • If stars are similar to blackbodies, then the spectrum will be close to Planck's law.
  • => Spectrum will have a peak
  • With Wien's law (lambdapeak = 2900 microns/T) we can estimate the temperature.


Stellar Spectra

  • The spectral (absorption) lines we see in stars are very important.
  • The "missing" photons give us info on:
    • Chemistry
    • Temperature
    • Density
  • Kirchhoff's laws tell us about the region which gives rise to the spectrum.


Formation
of Stellar
Spectrum


Hydrogen
Balmer
Spectrum

The hydrogen Balmer spectrum is visible for most stars.


Classification of Stars

In the late 19th century astronomers catagorized stars according to the strength of the hydrogen absorption lines in the spectrum.
  • They labels these A, B, ... from strongest to weakest.
  • Unfortunately, this was the wrong way to do it!

Annie Jump Cannon arranged the spectra of stars in a sequence which corresponds to their temperatures (She classified over 500,000 stars in her career!)

  • The spectral sequence is:
    • O, B, A, F, G, K, M
    • Hotter to cooler (A temperature sequence)
For more information and pictures on Annie Jump Cannon visit: http://cannon.sfsu.edu/~gmarcy/cswa/history/pick.html

 


Classification of Stars...


  • Each of these classes (O, B, etc.) can be subdivided into tenths, i.e.
    • G0, G1, ... G9, K0, K1, ... K9
      (G0 is hotter than G9)
  • The Sun is a G2 star.

More pictures and information on spectral sequences can be found at:
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/sptypes.html

 


Changes
to the
Spectral
Sequence
  • For the first time in over 100 years the spectral sequence is in need of another letter.
  • Very low temperature stars discovered with infrared surveys of the sky.
  • Now an L and T stars have been added!

O, B, A, F, G, K, M, L, T

Stellar
Photometry
and
Colors

  • It is not necessary to measure the entire spectrum to determine the spectral peak of a star.
  • We can use color filters to determine the "colors" of a star.
    • By this we mean how much flux is seen in each color filter.
    • A green filter transmits only green photons.


      The UBV system:

  • A set of color filters which give coarse spectral information.


Temperature
and Colors
  • U at 3500 A => ultraviolet
  • B at 4300 A => blue.
  • V at 5500 A => visible
  • A hot star will have more flux in the U filter than the V filter compared to a cool star.

Colors of a Hot Star vs. a Cool Star

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