Star Colors & Spectroscopy

Stars like Betelgeuse, in Orion, are red to the naked eye, while Vega, in Lyra, is a brilliant white color. Why are they different? What about those odd nomenclatures for star colors, like "Type-O" or a "G-3 Sun like star"? What do these letter designations mean?

The story begins with the discovery that light is not usually made up of one single color; that is to say that light is not usually monochromatic. Rather, light is composed of many separate colors. White light is in fact a combination of all the colors in the spectrum, and we can see this when we hold a prism up to a white light. The prism has the ability to separate the various wavelengths from long (red) to short (violet) into a long string of colors called a spectrum (figure 1). This was first discovered by Sir Isaac Newton in 1672.

Figure 1: White Light Passing through a Prism. This is what Isaac Newton saw during his first experiments with optics, light and color.

By the late 1700's, every scientist had toyed with a prism at one point or another. Interesting is the fact that some, including Sir William Herschel actually employed the fundamentals of spectroscopy to starlight as early as 1798, when he followed the advice of two of his contemporaries, Thomas Collinson and Sir William Watson (Figure 2). Herschel did not think that much scientific worth was to be gained from this process, but his studies did show a correlation between what would later be called a star's spectral type and its visible spectrum.

Some question the timing of this next piece of historic discovery. In either 1792 or 1802, William Wallaston discovered thin dark lines running through the spectrum of the Sun when he passed the light through a thin slit and a prism (MacRobert 48, Malin 9). Wallaston did not think much about these dark lines, leaving the true glory of discovery to Fraunhofer, who in 1814 determined these to be spectral absorption lines of various elements. When a gas is in front of a light source, the gas absorbs certain areas of the light source's spectrum corresponding to the gas' chemical composition. Fraunhofer logged over 500 individual lines in the Sun's spectrum alone.

Later, in 1859, Kirchhoff determined that if a gas was glowing, it created bright (emission) lines in a spectrum. These lines corresponded to the same dark absorbtion lines visible in the solar spectrum, and therefore the same chemical elements. It was now possible to determine, just by looking at a star's spectrum whether it had a gaseous shell or not, and what chemical elements were in that stellar atmosphere. The same could now be done with nebulae with their glowing, excited gasses, causing emission spectra.

Throughout the late 19th century, two scientists, Secchi and Huggins, began observing stellar spectra in an attempt to classify them. They soon found that a system could be created by noting the strength of various spectral lines and the overall complexity of the stellar spectrum. In 1880 Huggins concluded that this measure of spectrum complexity was a measure of a star's life sequence: the older the star, the more lines it had in its spectrum.

In 1886, Edward Pickering created the first catalog of the stars by their spectral type using the technique created by Secchi and Huggins. He assigned each spectral type a letter (from A to Q omitting J) according to the complexity of the star's spectra. The result was a memorial catalog to Henry Draper. This method was a brave step which turned out to be somewhat flawed.

It was soon recognized that the spectral order could be reorganized to better show the progression of spectral lines other than just hydrogen. Between 1918 and 1924, the Draper Catalog was rewritten and a new version was published. This new catalog featured the new spectral order (Figure 2): O, B, A, F, G, K, M. Some values were redundant and thus removed (C and D for example). Also, this new system allowed for divisions of 10 between the star types: O-0, O-1, O-2...O-9, B-0, B-1...etc.

Figure 2: Stellar Spectral Types. When describing the color of a star, scientists refer to their letter designation for spectral type: W, O, B, A, F, G, K, M, (R,N)C, S

It was not long before scientists at Harvard realized that there might be a link between the spectral qualities of a star and its actual color. They suggested that not only did the spectrum show the actual physical makeup of a star, but also its temperature and age. In 1920, M. N. Saha confirmed that the new spectral sequence was indeed a measure of the star's temperature, from hot W and O type stars to cooler type M, R, S stars.

So, where does this leave you, the amateur astronomer? Well, when viewing a blue to blue-white star, you now know that it is a relatively young star that is hot and bright. Red stars are typically older, colder and have a lower absolute magnitude or brightness. Let us look at the Hertzsprung-Russell Diagram below (Figure 3). Astronomers made an interesting discovery when they plotted a star's spectral type versus its overall brightness on a graph. The majority of data plotted forms a curve starting from very bright blue and leads its way to dim and red. This line is called the "Main Sequence" because of the large percentage of observed stars that fit this curve.

Figure 3: Hertzsprung-Russell Diagram. This diagram shows the relationship between stars' brightnesses and their spectral types. Note the Sun is a "Main Sequence" star

Even though astronomers have come to name this the "Main Sequence" in stellar evolution, stars do not usually progress along this line during their existence. Stars tend to spend a great deal of their time as a main sequence star: massive stars stay towards the blue end, while less massive stars reside in the red, low brightness areas.

The hot blue stars live their lives quickly. After almost ten million years they will leave the realm of the main sequence as they use up their remaining nuclear fuel. They edge right into the realm of supergiants then begin to oscillate back and forth between spectral types before ending their lives as a supernovae.

Less massive stars, like our own sun, live on the main sequence for billions of years then migrate up to the red giant stage then to the lower left, becoming white dwarf stars, eventually fading into the vastness of space.

So the next time you are looking up on a clear night, find a star and look for its color. Some are not that obvious, while others will stand out as being an obvious red or blue. Some stars show their color better in a telescopic field, for example the double star Albireo (Beta Cyg), which has a striking pair of stars colored yellow (K3 II, red giant) and blue (B8 V, main sequence).

Figure 4: Albireo as Seen in a Telescope. Albireo is one of the most striking examples of a double star. Its components are one yellow and one blue star.

Works Cited:

Burnham, Jr., Robert MacRobert, Alan M. "The Spectral Types of Stars." Sky & Telescope Magazine. October 1996. Vol.92, No.4. Sky Publishing, Cambridge.

Malin, David and Paul Murdin. Colours of Stars. Cambridge University, London: 1984.

Last Modified: 3/19/03 8:55p
This page:© Copyright 2005 by John A. Blackwell