The Spectrum of light and what it tells us
by Ludwig Scheibe (TU Berlin), July 2024
One fundamental and essential tool in the study of exoplanets is the study of light spectra. It is useful to have an understanding of the essential concepts behind spectroscopy and its application in astronomy.
Light as an electromagnetic wave: frequency & wavelength
Light is an electromagnetic wave. It is composed of rapidly changing electric and magnetic fields that propagate through space. As a wave, light has a frequency, measured in Hz, and a wavelength. The latter is basically the length between two “high” points, similar to two wave-crests in water waves. Frequency and wavelength are firmly and inescapably linked: The higher the frequency, the shorter the wavelength.
The light’s wavelength determines its color. Violet light has a particularly short wavelength, while red light’s wavelength is especially long. Electromagnetic (EM) waves with wavelengths of about 400 to 700 nanometers (nm) are visible to the human eye. Shorter than that, and we call it ultraviolet, and light with longer wavelength is called infrared. Both of these are beyond human perception, but can be measured by our instruments.
This is an illustration of the range of wavelengths in visible light, and the associated colors. The wavelength values are given above the color band and are in nanometers (nm). Credit: Gringer via Wikimedia Commons
The spectrum of light
Most light that we encounter in the universe is a mixture of waves with different wavelengths. White light, for example, is a mixture of all the visible wavelengths of light. Crucially, different wavelengths can have different intensity. These intensities for different wavelengths are what we call the light’s intensity spectrum.
The spectrum is primarily determined by the light’s source, but can be strongly influenced by things happening to the light on the way to the observer. For example, if the light passes through a material, it is affected by that. Therefore, measuring the spectrum of incoming starlight, we can learn a lot about the properties of the star as the light source, as well as about the processes affecting the starlight on its way to us. The method of measuring the spectrum and drawing conclusions from it is called spectroscopy. Some examples in astronomy are:
- Starlight has a continuous spectrum over a range of wavelengths with varying intensities. This overall function has a peak, a wavelength with the highest intensity, and that is determined by the star’s temperature (s. Wien’s law). Thus, we can measure the temperature by measuring the spectrum.
The appearance of different types of stars depending on their temperature. The temperatures are given in Kelvin. Colder stars have their maximum intensity at long wavelengths and thus look red or orange. Very hot stars, conversely, look blue. Credit: Harre & Heller (2021) under CC BY 4.0, with the label “sun” added.
The intensity spectrum of the Sun; basically the brightness of different wavelengths. Dotted lines indicate the part of the spectrum visible to the human eye. The high point at about 480 nanometers corresponds to the sun’s outer temperature of about 5770 Kelvin. Data source: Meftah et al. (2017)
- Within the continuous stellar spectrum, there are certain wavelengths missing, called absorption lines. They are caused by the gas in the star’s photosphere, and so they can give us an idea of the star’s composition. This effect was first discovered for our sun by Joseph Fraunhofer, which is why they are called “Fraunhofer lines”.
The Solar light spectrum showing the absorption lines from the Sun’s own atmosphere. This should be a continuous line, as in the previous image, but because the high detail in this image would make that line impractically long, it is shown with artificial “line breaks”. Credit: N.A. Sharp/KPNO/NOIRLab/NSO/NSF/AURA under CC BY 4.0
- During the transit of a planet in front of a star, part of the starlight passes through the planet’s atmosphere, which changes the overall spectrum, similar to the absorption. This gives us information about a planet’s atmosphere composition.
- The lines in a stellar spectrum tend to stay fixed for a long time, so if they shift periodically, that can be an indication for the star wobbling around due to a planetary companion. This is one of the methods used to detect extrasolar planets and more information on it can be found here.
These applications are incredibly important for exoplanet research. They allow us to characterize planets by the way they influence their star’s light. One of the major tasks in astronomy today and of the recent past is to find new and better ways to accurately and reliably measure these spectra – be that through sophisticated data-analysis or the advent of new technology.