From the February 2013 issue

Have astronomers seen any spectral lines they can’t identify?

John Wendler, Stow, Massachusetts
By | Published: February 25, 2013 | Last updated on May 18, 2023
Astronomers study the Sun’s radiation spectrum to learn what type of gas make up our star; each element is responsible for characteristic lines. Researchers have observed a few thousand lines in the Sun’s spectrum that they have not yet identified. N. A. Sharp, NOAO/NSO/KITT PEAK/AURA/NSF
Yes. Each spectral line corresponds to a specific energy transition related to a particular atom — and thus, element. Astronomers analyze such lines to tell us what celestial objects are made of.
In the spectrum of the Sun, for example, our spectrographs (the instruments we use to study the spectrum) can resolve more than 15,000 lines just in the range that our eyes can see. If we look at the entire electromagnetic range, the Sun has at least three times as many lines, and perhaps even more. We can identify many of them, but at least a few thousand remain unknown today. Many of the unidentified ones lie at ultraviolet wavelengths (shorter than what our eyes can see); these lines blend together, making it hard to distinguish one from the next, even with our best instruments.

The study of spectral lines is really just the study of the complex configurations and behaviors of electrons in atoms. As astronomers, we owe a large part of our success in identifying spectral lines in astronomical objects to laboratory spectroscopists. These scientists dedicate years, and sometimes entire careers, to studying the detailed spectroscopic signatures of atoms, molecules, and plasmas (hot ionized gases) in laboratories on Earth.
Thanks to this help on the ground, astronomers have been able to successfully identify tens of thousands of lines in all sorts of objects in space: stars, galaxies, nebulae, quasars, supernovae, comets, atmospheres of planets, and even the gas and dust between them all.

Ian Roederer
Carnegie Observatories,
Pasadena, California