ie-Physics

Experiment II-9

It had been long noted that adding various chemicals to fire modifies the flame color.  But not until long after Newton's synthesis of earthly and heavenly phenomena did astronomers connect starlight with chemical composition.

Christiaan Huygens (1629-1695) in Holland speculated that stars were but distant suns.  He drilled a series of different diameter holes in a brass disk, viewed the sun through the holes, and attempted to match the brightness to his memory of the brightness of stars then calculated an estimate of the distance to nearest stars.  Huygens did many things: Among them, he made improved telescopes, invented the pendulum clock to provide the accurate timings needed for astronomy, measure the size of another planet, speculate Venus is covered in clouds, drew pictures of the surface of Mars and determine its rotation period (roughly 24 hours), discovered the largest moon of Saturn and recognize the true nature of Saturn's rings (which Galileo had thought looked like ears).

• A luminous solid or liquid, or sufficiently dense gas emits light of all wavelengths and so produces a continuous spectrum of radiation.
• A low-density, hot gas emits light whose spectrum consists of a series of bright emission lines.  These are characteristic of the chemical composition of the gas.
• A thin, cool gas absorbs certain wavelengths from the continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum.  These lines are characteristic of the composition of the intervening gas.  They occur at the same wavelengths as the emission lines produced by the gas at higher temperatures.

In 1864, Sir William Huggins matched some of the dark Fraunhofer lines in spectra from several stars with terrestrial substances, demonstrating that stars are made of the same earthly elements rather than exotic substances.

French astronomer Pierre-Jules-César Janssen (1824-1907) went to India to observe the 1868 total solar eclipse and to make the first spectroscopic study of the Sun's chromosphere.  He noted a yellow spectral line which did not quite match Sodium or any other element.  The English astronomer Sir Norman Lockyer (1836-1920) proposed that the spectral line was due to an element on the Sun, but unknown on Earth, which he proposed to call Helium after Helios, the Greek name for the Sun.  Between 1888 and 1903 Helium was identified on Earth produced by Uranium ores, contained in volcanic gases, and as much as several percent of natural gas.  And thus analysis of the spectra of starlight collected by telescope became an effective method of remotely analyzing the composition of distant stars.

Even before the discovery of spectra, scientists had tried to find ways to categorize stars.  By observing spectra, astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral lines.  This means of classifying stars by spectral features was quickly realized to be a powerful tool.  The primary spectral classification scheme was developed at Harvard Observatory in the early 20th century by Henry Draper (1837-1882), who in 1872 made the first photograph of a stellar spectrum, that of Vega.  After his death the bulk of the classification work was done by Annie Jump Cannon (1863-1941) from 1918 to 1924.  Draper's original scheme used capital letters running alphabetically followed by numeric subcategories.  Subsequent revisions have eliminated some lettered catagories as stellar evolution became better understood.  Cannon developed the famous mnemonic Oh! Be a fine girl: Kiss me! to help remember the order of the catagories from hottest to coldest.

• the Hydrogen Balmer lines,
• lines of neutral and singly ionized helium,
• iron lines,
• the H and K doublet of ionized calcium at 396.8 and 393.3 nm,
• the G band due to the covalent C-H molecular bond,
• the 422.7 nm neutral calcium line,
• several metal lines around 431 nm,
• and the lines of titanium oxide.

Experiment A

Huygens estimated the distance to the nearest stars by comparing their brightness to that of the Sun as seen through a hole drilled in a disk. He found that the hole needed to be 1/28,000 the apparent diameter of the Sun. The distance between the Earth and Sun is 1.5 x 10⁸ km. Based on Huygens' measurement, how far away is a typical nearby star?

Experiment B

References

• Jesse Allen, The Classification of Stellar Spectra, NASA
• J. J. O'Connor and E F Robertson, Christiaan Huygens, School of Mathematics and Statistics, University of St Andrews, Scotland
• Christof Plicht, Joseph von Fraunhofer, Red Hill Observatory,England
• Logan Hennessey, Annie Jump Cannon, Wellesley College
• Thanks to Richard Pogge of Ohio State University for graciously giving permission to use for experiment B materials developed for his Astronomy courses.