Star Light, Star Bright

Stellar Spectrum

Credit: FORS Team, 8.2-meter VLT Antu, ESO Source: NASA

You may have notices that the brighter stars are slightly colored. Every star is of some color, but the fainter the star, the less sensitive the eye is to its color. The stars color is an indication of many things to an astronomer. The overall color of a star is an indication of its temperature, just as the dull-red glow of a rod of metal in a fire indicates relatively low temperature: brighter red, a hotter temperature; and orange, yellow and white, progressively hotter temperatures still.

Do you remember holding a prism up to catch the rays of sunlight? The resulting rainbow is the visible part of the suns stellar spectrum. With the invention of spectroscope 150 years ago, it has become possible to divide the light of any star into its component colors, forming stellar spectrum. Light is split into spectral colors, corresponding to wavelength. Red light has long wavelengths, blue light has short wavelengths. Spectra from hot objects are continuous, like a rainbow.

Atoms only emit light of specific colors, revealing their fingerprints as a line spectrum If you burn a chemical like sodium and look at its flame with a spectroscope, you would see not see a bright band of light but a yellow line where the yellow part of colored spectrum would be. You may, in fact, see two yellow sodium lines close together. Every chemical element has its own "bright line" spectrum, just as sodium has. The line which a glowing gas emits are characteristic of that gas--they are its fingerprint. so the presence of certain lines on a spectrum will indicate that certain chemicals are present.



If you have a hot glowing source, like the Sun, giving a continuous spectrum, and the sunlight passes through some cooler gas like our atmosphere, there are dark lines just in the same position as the bright lines occur if the gas were glowing on its own. Thus the dark lines are caused by gases round the Sun. By examining those lines we can tell what chemical substances are present in the Sun or in any other star. These dark lines differ in stars of different temperatures because the different elements absorb light as different temperatures. They also vary according to the color of the star.


Now here comes the fun part. In star guides and other astronomy books, you will find that a star may have a set of letters and numbers behind its name, like our Sun G2V. These really do mean something. Astronomers use a somewhat confusing system for classifying stars. First they assign a spectral type, an indicator of the star's surface temperature as revealed by its spectrum.


The O stars are blue and they are the hottest.

Above 11,000 degrees Kelvin.


The B stars are bluish-white.


The A stars are white.


The F stars are slightly yellow.


TThe G stars are yellowish-white.

5000 degrees Kelvin to 6000 degrees


The K stars are orange.

3500 degrees Kelvin to 5000 degrees


The M stars are red and they are the coolest.

At or below 3500 degrees Kelvin.

The seven spectral classes are further subdivided into tenths fro greater precision--00, 01, 02, 03, 04, 05, 06, 07, 08, 09.

To distinguish types of stars like dwarfs, and giants, astronomers assign each star a luminosity class.

Ia, I ab, Ib



Bright giants





Main sequence




White Dwarf

According to these classifications, the Sun is a G2V star. The blue supergiant Rigel is classified as B8Ia and the red supergiant Betelgeuse as M2Iab. Sirius, a main-sequence star somewhat hotter than the Sun is A1V, and the red dwarf Proxima Centauri is M5.5V.



Updated Dec. 3, 2002

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