Spectroscopy in 1881

[/caption]

Presently, I’ve been reading a lot of very old papers and books in astronomy. The work I’m currently reading a portion of, is from 1881, and is a summary of all the findings of the year in all fields of Science. For those that aren’t familiar with that time period in astronomy, the big thing was spectroscopy. It was only ~30 years earlier that chemists and astronomers had begun to work out methods by which to investigate spectra and with the newly developed tools in hand, astronomers were pointing them at anything they could find sufficiently bright to get a spectra. Obviously, this meant the first target was the Sun. This work provides an interesting snapshot at a developing era in astronomical history.

The article describes a brief bit of background, noting that the pioneering work of spectroscopy was done by Fraunhofer, Kirchoff, Angstrom, and Thalen (but manages to leave out Kirchoff’s colleague, Robert Bunsen!). These early explorers noted that, although spectral lines may appear unique, several had lines that would appear in very nearly the same positions.

Another discovery around that time was the phenomenon of emission lines from the Sun’s corona. This had officially been discovered in 1868 during a solar eclipse, but now that astronomers knew about the occurrence, they began to explore it further and discovered that many of the features had no apparent explanation as the chemicals causing them had yet to be discovered on Earth. Incidentally, it would be a year following this publication that helium, one of the chief components of the Sun, would be found and isolated on Earth.

As the astronomers explored the corona, they inspected the various layers and found a bizarre thing: Magnesium appeared higher in the corona than sodium despite magnesium having a higher atomic weight which astronomers realized, should cause it to sink. While this is not explained, I should note that spectra often play tricks like this. It may well have been that magnesium simply emits better at the temperatures in that region given an overestimation of the abundance. This odd behavior, as well as the inconstant nature of the spectra on various portions of the Sun was described as “a great screw loose”.

Another portion of the paper provides another somewhat humorous snapshot of this moment in history as the writer remarks just how different the Sun is from the Earth. He states, “It was difficult to imagine a stronger difference to exist between any two masses of matter than the chemical constitution of the incandescent sun, and of the earth, which is now cooling.” He wonders if perhaps planets evolved from failed stars in which the Sun’s “immense temperature had not allowed a complex evolution of higher complex forms of chemical matter to take place”. While this may seem quaint, the periodic table had only been developed 12 years prior and the creation of heavy elements would not be well understood until the 1950’s.

Similarly, the confusion on the varying spectral lines between stars is apparent although the author shows that the answers were already being developed, although still not fully fleshed out. He cites Angstrom stating: “In increasing successively the temperature I have found that the lines of the spectra vary in intensity in an exceedingly complicated way, and consequently new lines even may present themselves if the temperature is raised sufficiently high.”

In this single flash of insight, Angstrom had predicted a methodology by which astronomers could have begun to classify stars. Unfortunately, the standard of classification had already been set and it would take until the next century for astronomers to begin classifying stars by temperature (thanks to the work of Annie Jump Cannon). However, the author demonstrates that investigation was underway as to the relationship between temperature and line intensity. This work would eventually connect to our modern understanding of stellar temperatures.

Happy Birthday Johannes Kepler!

smallkepler.thumbnail.jpg

December 27 is a day to celebrate the life of astronomer Johannes Kepler, who was born on this date in 1571, and is best known for his three laws of planetary motion. But also, coming up in 2009, The International Year of Astronomy (IYA) will celebrate the work of Kepler as well. Not only did Galileo begin his observations with a telescope almost 400 years ago in 1609, but also in that year Kepler published his book New Astronomy or Astronomia Nova. This was the first published work that documented the scientific method.

Kepler’s primary reason for writing Astronomia Nova was to attempt to calculate the orbit of Mars. Previous astronomers used geometric models to explain the positions of the planets, but Kepler sought for and discovered physical causes for planetary motion. Kepler was the first astronomer to prove that the planets orbited the sun in elliptical paths and he did so with rigorous scientific arguments.

An offshoot of Astronomia Nova was the formulation of concepts that eventually became the first two of Kepler’s Laws:

First Law: The orbit of a planet about the Sun is an ellipse with the Sun’s center of mass at one focus.

Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.

And Kepler’s third Law: The squares of the periods of the planets are proportional to the cubes of their semi-major axes.

Kepler was also instrumental in the development of early telescopes. He invented the convex eyepiece, which allowed an expanded field of vision, and discovered a means of determining the magnifying power of lenses. He was the first to explain that the tides are caused by the Moon and the first to suggest that the Sun rotates about its axis. He also was the first to use stellar parallax caused by the Earth’s orbit to try to measure the distance to the stars.

While Kepler remains one of the greatest figures in astronomy, his endeavors were not just limited to this field. He was the first person to develop eyeglasses designed for nearsightedness and farsightedness, the first to investigate the formation of pictures with a pin hole camera, and the first to use planetary cycles to calculate the birth year of Christ. He also formed the basis of integral calculus.

Kepler’s many books provided strong support for Galileo’s discoveries, and Galileo wrote to him, “I thank you because you were the first one, and practically the only one, to have complete faith in my assertions.”

Original News Source: The Writer’s Almanac