Spectroscopy: Information Concealed in Starlight

Introduction

Ancient astrologers attempted to foretell the future by observing the night sky and interpreting hidden messages concealed in the changing position of the planets, which they believed to be wandering stars.

In modern times we are a bit more pragmatic. Horoscopes are just for fun and do not predict the future for the individuals who read them. Points of light in the night sky do not contain concealed information that will predict the future fate of any individual; not for kings and queens, and not for ordinary human beings.

In an ironic twist, however, information is concealed in starlight; information that reveals a wealth of information about individual stars and contributing to our understanding of the universe as a whole. To unlock these secrets would have to wait until the development of spectroscopy.

The story begins in 1610, when Isaac Newton showed that colours were concealed in "white" sunlight. By using a lens to recombine the spectral "rainbow" of colours that resulted from passing sunlight through a prism, Newton demonstrated that coloured light was a constituent component of sunlight.

Building on the work of Newton and those who would follow, spectroscopy evolved to become an invaluable tool used to penetrate the concealed secrets of the universe. This essay will explore the history of spectroscopy; including what has been learned from the application of this tool, and hint at what might yet be revealed.

19th Century Spectroscopy

In 1802, William Hyde Wallaston used a narrow slit to admit sunlight before passing it through a prism. The resulting spectra was similar to the rainbow seen by Newton, except that it contained numerous dark lines. Fifty-seven years later, Gustav Kirchhoff and Robert Bunson reproduced similar emission lines in the laboratory by passing the light emitted from a burning pure element through a prism. Modern physics has demonstrated that these lines are produced when electrons absorb or emit a particular wavelength of light as determined by Kirchhoff's Laws. Electrons in a given element absorb (or emit) a fixed set of frequencies, such that a known pattern of absorption lines forms a unique spectral signature for that element.

It is possible to determine the elements that are present in a star's atmosphere by comparing its spectral signature to that of known elements in the laboratory. Such an analysis shows that stars contain hydrogen, helium, carbon, nitrogen, and other elements astronomers call metals. As an interesting footnote, spectroscopy detected the element helium in the solar spectrum before it was eventually isolated on Earth, and provides conclusive evidence that the sun is an ordinary star.

In 1842, Christian Doppler showed that the perceived wavelength of an object emitting light or sound waves is effected by the motion of the source relative to the observer. The so-called Doppler effect is familiar to all who have heard the changing pitch of an ambulance siren as it moves towards and then past the listener.

As the source moves toward the observer, waves are effectively compressed. In the case of light, this means that frequencies are shifted towards the blue end of the spectrum. As the source and the observer move apart, waves are effectively stretched. In the case of light, this means that frequencies are shifted toward the red end of the spectrum.

Doppler shift is the extent to which absorption lines are shifted towards the red or blue end of the spectrum. From a Doppler analysis of a star's spectrum, it is possible to determine how fast an object is moving towards or away from the observer.

What other attributes of the star can be determined by analyzing absoprtion lines in stellar spectra?

• Thermal broadening
  Temperature is a measure of the kinetic energy resulting from the motion of particles in a gas (Bless, p 252). The component of each particle's velocity along the observer's line of sight contributes to the Doppler shift. In a hot gas, more particles will be moving at a wider range of velocities along the observers line of sight, resulting in a broadening of the associated spectral lines.
  
• Pressure broadening
  At the moment at which atoms collide, they can absorb additional photons. Higher density and pressure increases the likelihood of collisions and hence the absorption of photons (Bless, p 253). This leads to a broadening of absorption lines in the spectra.
  
• Stellar Rotation
  As stars rotate on their axis, particles that are moving toward an observer will be blue shifted. Particles that are moving away from the observer will be red shifted. Stars in which the axis of rotation is along the observer's line of sight will exhibit no Doppler shift. By performing observations of many stars, astronomers can determine the average stellar rotation velocities for stars of similar temperature and spectral type (Bless, p 256).
  
• Binary Stars
  When stars in a binary system rotate about their common centre of mass, stars alternate between being red and then blue shifted as they move away and then towards the observer. Assuming that spectral lines from both stars can be resolved, pairs of absorption lines will appear to merge and then split. From this, it is possible to determine the ratio of the stellar masses and the period of the orbit (Kaufmann and Freedman, pp. 481-483).

In 1893, Wilhelm Wien determined that stellar surface temperature is inversely proportional to the wavelength of maximum emission. When the temperature of the star is high, the wavelength of maximum emission will be towards the lower end of the spectrum. When the temperature of the star is relatively low, the wavelength of maximum emission will be towards the longer end of the spectrum. Ignoring absorption effects, plotting a star's intensity as a function of wavelength results in a hill-shaped "blackbody" curve. By determining the wavelength of maximum emission, it is possible to accurately determine the temperature of the star (Kaufmann and Freedman, pp 109-110).

In the late 19th century, Joseph Stephan and Ludwig Boltzmann demonstrated that flux is proportional to fourth power of temperature, where flux is the energy flow, usually represented in joules per square meter per second (Kaufmann and Freedman, p 110-111). If the luminosity and temperature of a star can be calculated, then the radius of the star can also be determined (Kaufmann and Freedman, pp 472-474).

20th Century Spectroscopy

Early in the 20th century, Ejnar Hertzsprung and Henry Norris Russell independently showed that when the luminosity of a stars were plotted as a function of spectral type or temperature, stars were found in three major groupings. Such a plot is known as a Hertzsprung-Russell (H-R) Diagram. We now understand that the grouping of stars on the H-R diagram are due to stellar age and mass. Older stars appear on the plot as red giants (low temperature, high luminosity) or white dwarfs (low luminosity, high temperature). Young to middle-aged stars fall in a diagonal band along the central portion of the plot , a region known as the main sequence. More massive main sequence stars are more luminous than less luminous main sequence stars. The H-R diagram has assisted in understanding the lifecycle of stars and developing models of stellar evolution.

In addition to using spectroscopy to study the stars, it can also be used to study distant galaxies.

In 1929, Edwin Hubble plotted the Doppler red-shift of galaxies as a function of distance and found them to be linearly related. The accepted interpretation of this result is that the universe is expanding and that galaxies are generally getting further away from other galaxies.

In the 1930's, Jan Oort showed that much of the Galactic mass was not visible using Doppler-based techniques to study the velocity of nearby stars. With respect to the study of other galaxies, spectral analysis also enables the rate of rotation to be calculated as blue-shifted portions of the galaxy rotate toward the observer, and red-shifted portions rotate away. For the majority of galaxies, plotting galactic rotation curves as a function of the distance from the galactic centre suggests that 90% of galactic mass is typically due to matter other than that contributed by visible stars, the so-called dark matter. When studying the velocities of galactic clusters, a similar conclusion was reached by Fristz Zwicky and Sinclair Smith, who were both contemporaries of Oort.

Interestingly, the amount of matter in the universe may ultimately play a role in its fate. Will the universe expand forever, or is there sufficient mass to slow the expansion and begin contracting towards a "big crunch"?

In addition to providing clues regarding the ultimate fate of the universe, spectral analysis can also help to confirm theories regarding its origin. If the universe was created in a big bang, then as the universe cools it should radiate cosmic background energy in a spectral curve consistent with that of a blackbody. In 1989, the Cosmic Background Explorer (COBE) satellite showed that the cosmic background radiation did indeed exhibit the expected blackbody shape, with the wavelength of peak radiation occurring at the temperature predicted by theory.

Spectroscopy at the Dawn of the 21st Century

Spectroscopy has already detected evidence of massive extrasolar planets outside our solar system. If NASA scientists have their way, early in the new century the technology should exist to detect Earth-like extrasolar planets with the spectral signature indicative of biological activity.

• Detection of Jupiter-sized planets
  Researchers are currently able to infer the presences of Jupiter-sized extrasolar planets since they induce a Doppler wobble in the spectrum of the parent star as they orbit their common centre of mass. Over 30 extrasolar planets have been discovered through Doppler-based spectral analysis.
  
• Potential detection of Earth-like extrasolar planets
  Planetary spectroscopy enables scientist to detect the gases found in a planet's atmosphere. Venus, Earth, and Mars, show carbon dioxide in their spectrum. Not surprisingly, the spectrum of Venus and Mars lack the signature of ozone evident in the Earth's spectrum, a strong indication of biological activity in the latter. The spectrum of Mars and Venus lacks the signature of any significant amount water, which is necessary for life to exist as we know it. Using a fleet of space-probes and a technique to significantly reduce the glare of the parent star, NASA scientists have proposed the Terrestrial Planet Finder (TFP) mission to to detect Earth-like extrasolar planets. TFP could begin looking for the spectral signature of Earth-like extrasolar planets as soon as 2011.
  
• Optical SETI
  Researchers have suggested that spectral wobble data collected in planet hunting studies should be reanalyzed to search for evidence of optical beacons operated by extraterrestrial civilizations. These beacons would be designed to attract the attention of other intelligent beings. While a spectral analysis is not a requirement for conducting a SETI study at optical wavelengths, an obviously artificial spike at the ultra narrow band laser frequency chosen by an extraterrestrial civilization would outshine the blackbody spectral curve of the parent star. Research projects for Optical SETI research are currently underway at UC Berkeley, Harvard, the University of Western Sydney, and by amateurs in Columbus, Ohio.

Conclusion

As spectroscopy and related observational tools have evolved, so too has our science and our view of our place in the universe. Through spectroscopy, we have come to understand the life cycle of stars, produced compelling evidence that the universe was created in a big bang, determined that the universe is expanding, and hinted at its ultimate fate. In the 21st century, we may even come to know if we are alone, or live in a universe built for life. The secrets of the universe have always been there, concealed in starlight.

REFERENCES

Bless, RC (1996) Discovering the Cosmos, University Science Books, ISBN 0-935702-67-9.

Dewhirst, D, and Hoskin, M (1999) The message of starlight: the rise of astrophysics, in The Cambridge Concise History of Astronomy, Hoskin, M, ed., Cambridge University Press, ISBN 0-521-57600-8.

Kaler, J (1997) Stars and their spectra: an introduction to the spectral sequence, Cambridge University Press, First paperback ed (with corrections), ISBN 0-521-58570-8.

Kaufmann III, WJ, Freedman, RA (1999) Universe, 5th ed., WH Freeman and Company, ISBN 0-7167-3495-8.