Starlight And What It Tells Us

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay


In 1844 the French philosopher Auguste Comte wrote that the chemistry of the heavenlybodies would probably never be known. Within a generation, Comte's prediction wasdemolished, no doubt to his delight and astonishment.

The Electromagnetic Spectrum

Isaac Newton was the first to discover that white light is actually a mixture of thecolors of the rainbow. When the colors are spread out by a prism or other means, they forma sequence called the spectrum (red, orange, yellow, green, blue, and violet).

The colors of visible light are actually only a tiny part of the electromagneticspectrum. Light, which is just one form of electromagnetic radiation,consists of waves. The distance between waves is the wavelength of the light, andthe number of waves that pass a given point per second is the frequency of light.Red light has the longest wavelength and smallest frequency of any visible light, violethas the shortest wavelength and highest frequency. The shorter the wavelength of light,the higher its energy.

Radiation with longer wavelengths than red light is called infrared, and stilllonger wavelengths are called radio. Radiation with shorter wavelengths thanviolet light is called ultraviolet; still shorter wavelengths are called X-raysand gamma Rays.

Black-Body Radiation

Hot objects emit radiation. The hotter they are the more they emit, at higher energies.Thus hot objects show a characteristic pattern of light emission. This sort of radiationis called black-body radiation.

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How Can a Red-Hot Object be "Black?"

The term "black body" is used because none of the light is reflected fromsome other source and the intrinsic color of the material is not important. A bar of goldand a bar of black graphite at 3000 degrees C would emit the same color light. The Sun isvery close to a theoretical black body in its light emission - all stars are. The Moon isnot. The Moon absorbs sunlight and re-emits it in different forms, for example, a lot ofvisible light is absorbed by the surface and re-emitted in infrared. Incandescent lights,as befits their names, are pretty close to black-body emitters, fluorescent, neon, andsodium-vapor lights are not.

As temperature increases, two things happen:

As an object heats up, the peak of emission creeps into the visible range and thefamiliar "red hot" color appears. As the object gets still hotter, the peakshifts into the yellow part of the spectrum and the object glows orange, then yellow. Butthen the object becomes a paler yellow and finally white, not green. Why? When theemission peak is in the green (as it actually is for the Sun!), the object is stillemitting copious red and yellow light, and these wavelengths combine to give a fairly purewhite. But the very hottest stars have peaks in the violet and beyond, and their blue andviolet emission is so much greater than their red and yellow that the stars appearblue-white.

Whay Black Body Radiation is so Useful

Black Body Radiation is Directly Related to Temperature
Measure the color of a star, determine its temperature. It's that simple.
Black Body Radiation Depends Only on Temperature
A light bulb, a fire, a blast furnace, a star, and a nuclear fireball at 2000 C emit the same color light. It doesn't matter why the object is hot, only how hot it is. Most "red" stars are actually quite close in color to an incandescent light bulb, a fact that can be verified by comparing Antares or Betelgeuse to an incandescent light a kilometer or so away.
Black Bodies Emit Constant Radiation per Unit Area
If you were to punch a small hole in a piece of paper, then view, say, a lava flow or a blast furnace with it, you would not be able to tell what you were looking at, only how hot it was. But as long as the light filled the entire pinhole, you could not tell how far away it was, either. Here's why:

The bottom line: the color of a star tells us its temperature. From the brightness of astar and its color we can calculate its apparent size in the sky, and if we know itsdistance, we can calculate its actual size. Dwarf stars are the size of the Earth orsmaller. The Sun is 1.3 million kilometers in diameter, over 100 times the diameter of theEarth. Supergiant stars can be larger than the orbit of the Earth.

Spectroscopy - The Astronomer's Ultimate Weapon

In 18xx, the German chemist, xxxxxxx Fraunhofer, discovered that when he spread sunlight into a spectrum, the spectrum was crossed by great numbers of fine dark lines. In 1859, the German chemist G. Kirchhoff and the English chemist Robert Bunsen (inventor ofthe famous Bunsen burner) found that Fraunhofer's lines were produced when gases absorbedor emitted specific wavelengths of light. The science of spectroscopy was born.At first the new discovery was used for analyzing gases in the laboratory, but within ayear Kirchhoff had begun analyzing the Sun. In 1868 the element helium was discovered fromits spectral signature in the Sun, the only element so discovered. Its name comes from theGreek helios for Sun.

Physical Conditions in Stars

Not only do different elements have different spectral signatures, but the signaturesof atoms depend on whether or not tha atoms are ionized, and on how many of theirelectrons have been removed. Such information gives the astronomer valuable insight intothe temperatures and pressures in stars. Also, spectral lines are modified by electricalor magnetic fields. After 130 years, astronomers are still finding new ways to getinformation out of the spectra of stars.

After helium was discovered, astronomers began looking for other new elements in thecosmos, and a number were announced and named. All later turned out to be ordinaryelements like oxygen under extreme and unfamiliar physical conditions, for example withmany of their electrons stripped away.

The Doppler Effect

Besides allowing the astronomer to determine the chemical compositions of stars,spectroscopy provides the astronomer with another powerful technique: the DopplerEffect.

Most people have had the experience of hearing the pitch of a train whistle orambulance siren drop as the source moved past. As the sound source moves toward theobserver, the sound waves are compressed, making the pitch of the sound higher. As thesound source moves away from the observer, the sound waves are stretched out, making thepitch of the sound lower. In a similar way, light from an approaching star has itswavelengths shortened, or blue shifted, and light from a receding star has itswavelengths lengthened, or red-shifted.

Light that is red- or blue-shifted merely changes color. There is no way to tell fromcolor alone that a star is moving. Contrary to many popular illustrations, red-shiftedstars do not look red; as the visible spectrum of a star is shifted to longer wavelengths,its ultraviolet spectrum is shifted into the visible range. The key to the Doppler effectis that spectral lines change position. The change in position is easily measuredon a photographic plate. The Doppler Effect allows astronomers to determine threeimportant facts:

As an indication of the subtle details spectroscopy can reveal, it is possible todetect vertical movements of material on the Sun from its Doppler shift. Just asearthquakes on the Earth send waves through the Earth's interior, disturbances on the Sunsend waves through its interior, and these waves can be detected from their effects on thesurface of the Sun. Study of these disturbances, called helioseismology, isrevealing details of the Sun's interior in astonishing detail.

Motions Of The Stars

The stars are not fixed, but move measurably over the years. Astronomers refer to themotion of stars as proper motion, and measure the apparent motion of the stars onphotographs taken years apart. Most stars would take centuries to move the apparent widthof the Moon, but they do move.

If we know the distance to a star, we can calculate its velocity across our line ofsight. The Doppler Effect enables us to determine the radial velocity of thestar, or its speed along our line of sight. If we know the two velocities, we candetermine the true velocity and direction of the star's movement in space.

The Sun's Motion

The stars appear to move both because they are in motion and because we ourselves arein motion. Just as a driver in a snowstorm sees the snowflakes appear to radiate away fromhis direction of motion, astronomers also observe that most of the stars in the sky aremoving away from a certain region of the sky. This direction, in the constellationHercules (straight overhead for U.S. observers in the early evening in summer) is thedirection the Solar System is moving, at about xx miles (xx km) per second.

Spectral Classification

It is obvious even to the unaided eye that stars differ. Some are reddish, othersyellow, still others bluish-white. When we determine distances to the stars still moredifferences appear: some very nearby stars are extremely faint, while very distant starsare sometimes very bright.

Spectroscopy provides a means of making sense out of the variety of the stars. The veryhottest stars are blue-white and show only the spectral lines of helium. Somewhat coolerstars are white and show lines for hydrogen as well. Still cooler stars (yellow andorange) show the signatures of elements heavier than helium (what astronomers call metals),and the very coolest stars (red) are cool enough for a few very sturdy molecules to formin their atmospheres.

Astronomers divide stars into spectral classes, labelled with letters.Originally the classes were designated A, B, C ..., but some classes were dropped andothers rearranged in order. The major classes of stars, from hottest to coolest, are nowdesignated O, B, A, F, G, K and M. There is a handy phrase for remembering the lettersequence: "Oh, be a fine girl (guy), kiss me". For stars that fall between the main classes, subdivisions are used: an F5 star ishalfway between classes F and G, for example. Our own Sun is classed as G2. With the discovery of brown dwarfs, two new classes were recently added. Very cool dwarfs emitting mostly in the infrared are Class L, even cooler stars are class T. These stars are cool enough (only 700-1300 K) that methane can survive in their atmospheres. Since methane absorbs red and infrared light, these stars are very dim and would actually appear purplish. An even cooler class of star has been predicted and given the label Y.

Four other classes are used for peculiar stars: W for very rare, hot stars, N, R, and S for chemically peculiar cool red stars. Classes N and R were recently combined into a new Class C (it has been so long since the letter was originally applied that we can use it without fear of confusion). White Dwarfs have been assigned a class of their own, called D. The table below gives typical figures, but there are wide variations. Classes O through Y are "normal" stars in their stable, mature stages; classes W and D-C are stars in later stages of evolution.

Class Temperature Absolute Magnitude Luminosity (x Sun) Diameter (x Sun) Mass (x Sun) Expected Lifetime Notes
W 25,000-50,000 -5 20,000 5-10 >20 A few million Very hot stars expelling their outer atmospheres.
O 30,000-60,000 K -5 1,000,000 16 64 Less than a million Orion's Belt
B 10,000-30,000 K -3 20,000 7 18 10 million Spica
A 7500-10,000 K +0.5 40 2 3 600 million Vega, Sirius
F 6000-7500 K +2.5 6 1.5 1.7 2.5 x 109 Procyon
G 5000-6000 K +5 1 1 1 1010 Solar type stars
K 3500-5000 K +6 0.4 0.9 0.8 1011 Proxima Centauri
M 2000-3500 K +10 to +15 0.04 0.5 0.4 1013 Barnard's Star
L 1300-2000 K > + 20 10-4 0.1 0.2 Trillions to cool Borderline stars
T 700-1300 K > + 25 10-6 0.05 0.1 Trillions to cool Substellar
Y <700 K >> + 25 10-8 0.05 <0.1 Trillions to cool Substellar
D 10,000-60,000 +10 to +15 0.04 0.01 1 Trillions to cool White Dwarfs
N 4000-6000 -5 to 0 10-1000 10-100 1 109 as giant Orange carbon rich giants
R 2000-4000 -5 to 0 10-1000 10-100 0.5 1010as giant Red carbon rich giants
S 2000-5000 -5 to 0 10-1000 10-100 1 1010as giant Giant Stars with zirconium and titanium oxide
C 2000-6000 -5 to 0 10-1000 10-100 0.5-1 109-1010as giant Carbon rich giant stars. Combination of old classes R and N

The Hertzsprung-Russell Diagram

One of the most important tools in understanding the stars was devised in 1913 by EjnarHerzsprung and Henry N. Russell. When they plotted a graph of absolute magnitudes of starsagainst spectral class, they found that most stars plotted on a diagonal line, with the Ostars brightest and M stars faintest. Such a diagram is called a Hertzsprung-RussellDiagram, or sometimes H-R Diagram for short. Astronomers refer to the mainband of stars as the Main Sequence. The Sun is a Main Sequence star. Some starsplotted well above the Main Sequence, meaning they were more luminous than average. Thesestars are called giants and supergiants. Still other stars plot wellbelow the Main Sequence, meaning that they emit light very feebly. These stars are called dwarfs.The size terms are literal; stars of a given spectral type all have about the same surfacetemperature, and all emit about the same amount of light per unit area, so their lightoutput is a measure of their diameter.

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Luminosity Classes

Stars may have the same temperature, color, and spectral characteristics, but shine by quite different processes. Since widely different size stars can have the same temperature and spectral class, astronomers also employ Luminosity Classes. Eight classes are defined:

The full description of a star includes both its spectral and luminosity class. The Sun is G2V.


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Created 26 March 1998, Last Update 15 January 2020