What is a Spectrum?

Light carries electromagnetic energy. Light, that comes from a natural source like a fire, or lightning, or a star, contains many different energies mixed together. For example, light from the Sun encompasses many different wavelengths or energies, corresponding to the colors of the rainbow - red, orange, yellow, green, blue, violet, etc. When light from the Sun is dispersed by the droplets in a cloud into a rainbow, or by a prism into its many colors, a spectrum has been created. The Sun is a typical star, except that it is much (280,000 times) closer to us than any other star. The light from any star can be spread out into a spectrum and studied. Normally, astronomers use a prism or a grating near the focus of a telescope to disperse starlight into a spectrum. A spectrum can be displayed as a plot of the brightness (the energy flux) of the star versus frequency or (more commonly) wavelength. A spectrum of a blue star has stronger energy flux at the blue, or short wavelength end, of the visible spectrum, normally on the left side of a visual plot. A red star will have more flux at the red, or longer wavelength side, normally on the right side of a visual plot. A star like the Sun is in between, so its spectrum will be relatively flat in flux versus wavelength. A video playing the spectrum of a sun-like star is available below.

In this project, we make available spectra of different types of stars in video format using sonoUno, where the stellar spectrum is scanned from the short wavelength (higher energy, bluer) part of a spectrum towards the longer wavelength (lower energy, redder) part. A red vertical line moves smoothly along the spectrum to visibly illustrate the part of the spectrum being played. A higher pitch is used to indicate greater flux intensity or brightness at each wavelength.

The spectrum of a star provides more detail about its overall energy distribution. In other words, at what wavelengths, frequencies or colors is the star bright, and where is it dim? If a spectrum is low resolution, it is not dispersed much, and we can only learn about its overall energy distribution. A higher resolution spectrum will also begin to reveal the special spectral lines that are signatures of different species of well-known atoms or molecules. For example, the simplest and most common atomic element hydrogen (made up of one electron and one proton) produces spectral lines in the optical region of the electromagnetic spectrum at very specific wavelengths 4102, 4340, 4861, and 6563 Angstroms. When astronomers see those lines, they know that the outer layers of the star are rich in hydrogen. In most normal stars like the Sun, hydrogen lines are seen in absorption. That means that the spectrum has sharp lines of lower flux at those specific wavelengths, where hydrogen atoms in the outer atmosphere of the star have absorbed the light coming from hotter, deeper layers. Other atoms or molecules form absorption lines at different wavelengths, which helps astronomers understand what they call the “elemental abundances” of each star. Sometimes these sharp lines appear to be brighter than the surrounding spectrum, and are called emission lines, which come from hot diffuse gas around the star.

The spectrum of a star is also useful for measuring the velocity of the star along our line of sight. The pitch - also called the frequency - of a car horn sounds higher when it is approaching us, and then becomes lower as it passes us and moves away. The change in frequency caused by shifts in relative velocity between two objects is known as the Doppler effect. The frequency of light coming to us from a star is also subject to the Doppler effect. Thus, we can use a spectrum of the star to measure its velocity relative to us.

The spectrum of any celestial object (a star, a quasar, a supernova, etc.) can thus provide us with at least three important types of information: (1) the broadband color of the object (2) the atomic and molecular elements in the outer atmosphere of the object and (3) the velocity of the object relative to observers here on Earth.

Please send us your comments, feedback or suggestions! CONTACT US: sdu@cfa.harvard.edu