Stellar Spectroscopy (Nancy Sexton)

Stellar Spectroscopy (Nancy Sexton)

Title:  Stellar Spectroscopy

Principle(s) Investigated: 
  Chemical Elements, Atomic Structure, Wave Characteristics, Electromagnetic Radiation, Light Characteristics, Kirchkoff’s Law’s, Emission spectra, Continuous Spectra, Absorption Spectra

Standards : 

Earth Science 1.e  Students know the Sun is a typical star and is powered by nuclear reactions, primarily the fusion of hydrogen to form helium.

Chemistry 4.f Students know there is no temperature lower than 0 Kelvin.

Chemistry 1. J Students know that spectral lines are the result of transitions of electrons between energy levels and that these lines correspond to photons with a frequency related to the energy spacing between levels by using Planck's relationship (E = hv).

Physics 4.e Students know radio waves, light, and X-rays are different wavelength bands in the spectrum of electromagnetic waves whose speed in a vacuum is approximately 3×108 m/s (186,000 miles/second).

Physics 4.f  Students know how to identify the characteristic properties of waves: interference (beats), diffraction, refraction, Doppler effect, and polarization.

Lab paperwork, cutouts of absorption spectra for several elements, 3 unknown star absorption spectra

Procedure:  Using the recently gained knowledge of absorption and emission spectra the students analyze three unknown stars’ absorption spectra by comparing several elements’ spectra and determining which elements are contained in the star based on identical patterns.

Student prior knowledge:
 Basic understanding of atomic structure regarding electron energy levels, basic understanding of light and electromagnetic radiation, basic understanding of wave characteristics, absorption spectra, emission spectra, continuous spectra, black body radiation.


Light and Electromagnetic Radiation:

Electromagnetic radiation is energy in the form of a wave.  Light is the visible part of the EMR spectrum.  When light travels through a prism, it is split and changes direction due to refraction.  Most objects we see have reflected light – if we turn off the lights we can’t see them.  Glowing objects are emitting light.  If you turn off the lights you still see them glow. All objects emit light, but we only see them if they’re hot enough.

The light from a star is usually concentrated in a rather narrow range of wavelengths.

The spectrum of a star’s light is approximately a thermal spectrum called a black body spectrum.

A perfect black body emitter would not reflect any radiation. Thus the name “black body”. 


Two characteristics of black body radiation are (1) it is a continuous spectrum (at least some radiation at every wavelength) and (2) the wavelength of maximum emission depends on the body’s temperature. 

 Three Types of Spectra:  Continuous, Emission, and Absorption:



The continuous spectrum:

At least some radiation at every wavelength – no absorption or emission lines



Solids – hot filament of an electric light

Liquids – molten iron

Gas – inside stars




Emission Spectra:


When electrons are pushed by a photon to a higher energy level, then fall back down, they emit a photon.  The wavelength of the photon is determined by the difference in energy of the two states.  These emitted photons form the elements emission spectra.  Each element has a unique emission spectra.


Hydrogen emission spectra


Iron emission spectra

Absorption Spectra:


When light from a glowing object (black body radiator) passes through a cooler gas which absorbs some of the wavelengths (specific photons that cause that particular elements electron to jump to an excited energy level).

The elements of the cooler gas absorb exactly the same wavelengths they would emit if they were in the form of a glowing gas.  Scientists can determine what elements are in the cooler gas by analyzing the absorption lines.


Kirchhoff’s Laws of Radiation

1 - A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.




2 - A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum.

3 - If light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

4. Inner, dense layers of a star produce a continuous (blackbody) spectrum.  Cooler surface layers absorb light at specific frequencies.   Spectra of stars are absorption spectra.


In basic terms, we can look at starlight and, using the unique emission spectra for elements, determine which elements are present in the star by the fact that they absorb the exact same pattern as they emit, and we can use these patters as a key.

Questions & Answers: 

(1)  Why are the emission spectra and absorption spectra patters identical (except in the opposite manner)?  Because they are both related to the element either absorbing energy (a photon) and electrons jumping to a higher energy level or releasing energy (the photon) after returning from an excited state.  The emission spectra shows the presence of the photon emitted, the absorption spectra shows the absence of the photo absorbed.

(2)  Are the elements we are identifying in the photosphere or the core of the star?  They are in the photosphere which is the “cooler gas” that is absorbing the photons from the inner core into the elements.

(3)  Why does the intensity of light increase dramatically with only a slight change in temperature (see graph with Black Body Radiation)?  Because it follows the inverse square law.


Applications to Everyday Life: 

1 – Neon sign is an example of electrons emitting light in an excited state

2 – Hot metalwork from a blacksmith. The yellow-orange glow is the visible part of the thermal radiation emitted due to the high temperature.

3 – Why the sky is blue.  Rayleigh scattering is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. Rayleigh scattering of sunlight in the atmosphere causes diffuse sky radiation which is the reason for the blue color of the sky and the yellow tone of the sun itself.

Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.

Norman Herr,
Oct 20, 2011, 1:58 PM
Norman Herr,
Oct 20, 2011, 1:04 PM