Astr 351 course Stellar radiation characteristics Part I: Spectroscopy Chapter 1 Spectroscopy: Unlocking the secret in starlight

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Astr 351 course

Stellar radiation characteristics
Part I: Spectroscopy

Chapter 1

Spectroscopy: Unlocking the secret in starlight
1.1 Spectroscopy is a key tool in astronomy

By obtaining and analyzing the spectrum from a distant object, astronomers can identify what type of object it is and determine a wealth of characteristics for the object. These include its effective temperature, how fast it is rotating and whether it is moving towards or away from us, how large and dense it is and what it is made of. Within the last decade planets beyond our Solar System have been discovered via their effect on the parent star's spectrum.

1.2 Historical introduction to spectroscopy

Before looking in detail at how spectra are formed and what they can tell us about stars and other celestial objects it is worth briefly discussing the rise of spectroscopy in astronomy.

Isaac Newton showed that a glass prism could be used to split sunlight into a spectrum in 1666. Further studies by William Wollaston in 1802 revealed some black lines on the component colors of the solar spectrum. More detailed observations by Joseph von Fraunhofer resulted in 574 of these lines being mapped by 1815. These lines were named "Fraunhofer lines" in his honor. The image below shows a solar spectrum with Fraunhofer lines.

Sun's Spectrum showing Fraunhofer lines

Two key questions arise from studying these lines - what do they represent and how are they formed? The solutions to these questions were to take some time. Leon Focault matched the lines produced by a sodium lamp with some of the dark lines in a solar spectrum in 1849. In 1857 Gustav Kirchoff and Robert Bunsen identified sodium in a solar spectrum. They found that a luminous solid or highly compressed hot gas could produce a continuous spectrum whilst a diffuse hot gas produced a spectrum with narrow bright lines on a black background.

As spectroscopes were coupled to telescopes additional chemicals were identified in the spectra of stars and nebulae. Sir Norman Lockyer and Jules Janssen independently discovered the element helium in solar spectra before it was isolated in a laboratory on Earth in 1895. The use of spectroscopy, coupled with the spread of photography for astronomical purposes gave rise to the science of astrophysics from astronomy in the second half of the nineteenth century.

Three general types of spectra were now known, a continuous spectrum, showing all the component colors of the rainbow, and two types of line spectra, the first, dark-line spectra like the solar spectrum and those from stars and the second, bright-line spectra as emitted from gas discharge tubes and some nebulae. The Swiss school teacher, Johann Balmer in 1885 developed an empirical formula that determined the wavelengths of the four visible lines in hydrogen's spectrum. Five years later the Swedish physicist, Johannes Rydberg expanded Balmer's formula to apply to some other elements. The Danish physicist, Niels Bohr, finally provided an explanation as to how spectral lines formed in the 1920s. His work relied on quantum physics and the concept of energy shells or orbits for electrons.


  1. How the dark lines in the solar spectrum are formed?

  2. What do the dark lines in the solar spectrum represent?

  3. Why the spectroscopy is a key tool in astronomy?

Chapter 2

Electromagnetic radiation
Almost all astronomical information from beyond the Solar System comes to us from some form of electromagnetic radiation (EMR). (Can you think of any sources of information from beyond the Solar system that do not involve EMR in some form?) We can now detect and study EMR over a range of wavelength or, equivalently, photon energy, covering a range of at least 1016 (ten thousand trillion sounds more impressive) - from short wavelength, high photon energy gamma rays to long wavelength, low energy radio photons. Out of all this vast range of wavelengths, our eyes are sensitive to a tiny slice of wavelengths- roughly from 4500 to 6500 Å. The range of wavelengths our eyes are sensitive to is called the visible wavelength range. We will define a wavelength region reaching somewhat shorter (to about 3200 Å) to somewhat longer (about 10000 Å) than the visible as the optical part of the spectrum. (Note: Physicists measure optical wavelengths in nanometers (nm). Astronomers tend to use Angstroms1. Thus, a physicist would say the optical region is from 320 to 1000 nm).

All EMR comes in discrete lumps called photons. A photon has a definite energy and frequency or wavelength. The relation between photon energy (Eph) and photon frequency (ν) is given by:


Or, since


where h2 is Planck's constant and is the wavelength, and c3 is the speed of light. The energy of visible photons is around a few eV4 (electron volts). (An "electron volt" is a non- metric unit of energy that is a good size for measuring energies associated with changes of electron levels in atoms, and also for measuring energy of visible light photons.)

The optical region of the spectrum, although only a tiny sliver of the complete EMR spectrum, is extremely important to astronomy for several reasons. Since our eyes are sensitive to this region, we have direct sensory experience with this region. Today, virtually no research level astronomical observations are made with the human eye as the primary detecting device. However, the fact that we see in visible light has driven a vast technological effort over the past century or two to develop devices “photographic emulsions, photomultipliers, video cameras, various solid state imagers”­ that detect and record visible light. The second overriding reason to study optical light is that the Earths atmosphere is at least partially transparent to this region of the spectrum- otherwise you couldn't see the stars at night (or the Sun during the day)! Much of the EMR spectrum is blocked by the atmosphere, and can only be studied using telescopes placed above the atmosphere. Only in the optical and radio regions of the spectrum are there large atmospheric windows - portions of the EMR spectrum for which the atmosphere is at least partially transparent- which allow us to study the universe. Study of wavelengths that don't penetrate the atmosphere using telescopes and detectors out in space- which we will call space astronomy - is an extremely important part of modern astronomy which has fantastically enriched our view of the universe over the past few decades. However, space astronomy is very expensive and difficult to carry out.
In purely astronomical terms, the optical portion of the spectrum is important because most stars and galaxies emit a significant fraction of their energy in this part of the spectrum. (This is not true for objects significantly colder than stars - e.g. planets, interstellar dust and molecular clouds, which emit in the infrared or at longer wavelengths - or significantly hotter- e. g. ionized gas clouds or neutron stars, which emit in the ultraviolet and x-ray regions of the spectrum. Now, the next time you see the brilliant planet Venus and think we are being invaded by space aliens, you may ask yourself why I included planets along with dust clouds in the above sentence. The reason is that the bright visible light you are seeing from Venus is reflected sunlight and not light emitted by Venus itself). Another reason the optical region is important is that many molecules and atoms have diagnostic electronic transitions in the optical wavelength region.

  1. What do radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays have in common? How do they differ?

  2. What is the relationship between wavelength, wave frequency, and wave velocity?

  3. What is the type of relation between the photon energy and the wavelength of light?

  4. What is the type of relation between the photon energy and the frequency of light?

  5. Why the optical region of the spectrum is extremely important?

  6. Test the physical units in equations (2.1) and (2.2)

  7. In what regions of the electromagnetic spectrum is the atmosphere transparent enough to allow observations from the ground?

  8. What we mean by space astronomy?

  9. What we mean by “atmospheric window”?

  10. Arrange the EMR from the shortest wavelength to the longest wavelength

  11. Why are gamma rays generally harmful to life forms, but radio waves generally harmless?

  12. What is the energy of a photon in the middle of the visible spectrum (λ = 550 nm)?

  13. What would be the frequency of an electromagnetic wave having a wavelength equal to Earth's diameter? In what part of the electromagnetic spectrum would such a wave lie?

  14. What is the energy (in joules and electron volts) of a 450-nm blue photon? A 200-nm ultraviolet photon?

  15. A sound wave moving through water has a frequency of 256 Hz and a wavelength of 5.77 m. What is the speed of sound in water?

  16. What is the wavelength of a 100-MHz ("FM 100") radio signal?

Chapter 3

How are spectra produced?

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