Star Light, Star Bright

Interpreting the messages in starlight. 1945 Dover Publications edition of the work first published by Prentice-Hall Inc in 1937.

Spectroscopy is fundamental to astronomy, and can tell us an amazing number of things about stars – and about many other objects scattered around the universe.  The reason we can talk so freely about the chemical composition of a star, its temperature or many other factors regarding its structure and location, is because the spectrum of radiation from it can be measured.  Similarly, the atmospheres of planets – even those orbiting distant stars – can be analysed in considerable detail.

It all goes back to when Isaac Newton first observed the spectrum of the Sun using a simple prism.  In his venerable article The Atom in Astronomy (The Telescope, May 1937), Leo Goldberg, then at the Harvard College Observatory, commented that Newton’s discovery of this band of colours was “one of the most significant in the history of physics and astronomy”.  But it was not until 1802 that William Hyde Wollaston observed the dark lines in the solar spectrum which were later found to indicate the elements present in the Sun.  Subsequent laboratory investigations identified the spectra of individual elements, and confirmed that no two elements exhibit the same set of lines.

In his classic work Atomic Spectra and Atomic Structure, also first published in 1937, Gerhard Herzberg, then Professor of Physics at the University of Saskatchewan and who was awarded the Nobel Prize for chemistry in 1971, showed how the investigation of atomic spectra provided information about the arrangement and motion of electrons in an atom.  The data that were obtained on the fundamental aspects of different atoms formed “a basis for an understanding of molecule formation and the chemical and physical properties of the elements.”

A range of light sources, mainly temperature radiation of gases and all kinds of luminescence (from, say, electrical discharges or when chemical reactions produce light), can produce the characteristic line spectrum for each chemical element.  So this spectrum, in Professor Herzberg’s words “can be used as an analytical test for the presence of an element – a test which has the advantage that extraordinarily small amounts of an element can be detected.”  Very helpful in astrophysics.

And so the spectroscope became, as Dr Goldberg put it, “an exceedingly valuable detective” in analyzing the light emitted by stars and identifying the kinds of atoms that are supplying it.  With a knowledge of atomic structure, astrophysicists were able to ascertain the physical conditions within stellar atmospheres.  He described it as “a triumph of human ingenuity” that understanding something as small as the atom “paved the way to a knowledge of the nature of stars.”

In due course, stars were categorised according to the similarity of their spectra, a process begun by Williamina Fleming at the Harvard Observatory and first published in 1890.  In From Scotch maid to innovative astronomer (Astronomy & Geophysics, June 2016) science writer Sue Nelson describes how this exceptional woman – an “astrophysics pioneer” – rose from humble origins to establish the first photographic standards of stellar magnitude and the alphabetical classification system for stars that is so familiar today.  Our Sun, a yellow dwarf, is now well known as a G-type star, a white main-sequence star such as Sirius is A-class, the red supergiant Betelgeuse is M-class, and so on.

This classification of stars leads to thoughts of classifying other celestial objects.  Galaxies have a fairly well-defined system – for example, elliptical galaxies are defined according to how elliptical they are – and there is a spectral classification for asteroids.  The (fictional) classification of planets in Star Trek stories explains the environments that are found there – M-class planets are Earth-like, and various other letters of the alphabet designate worlds of varying levels of habitability.  We need these definitions; it all helps us to make sense of an otherwise potentially bewildering universe.

Richard Hayes, Assistant Editor (Odyssey)

With a lifetime’s interest in science, history and human behaviour, Richard Hayes focuses his writing on how the imagination has created the world in which we live, and where it may lead us in the future.  Odyssey, the e-magazine of the British Interplanetary Society, draws on the rich treasury of science fiction to explore many fields of speculation, enabling us to glimpse what might yet be, both here on Earth and out amongst the stars.

For related Odyssey posts, please click here: Empty Space | Restricted Vision

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