Starlight And What It Tells Us

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

In 1844 the French philosopher Auguste Comte wrote that the chemistry of the heavenly bodies would probably never be known. Within a generation, Comte's prediction was demolished, 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 the colors of the rainbow. When the colors are spread out by a prism or other means, they form a sequence called the spectrum (red, orange, yellow, green, blue, and violet).

The colors of visible light are actually only a tiny part of the electromagnetic spectrum. Light, which is just one form of electromagnetic radiation, consists of waves. The distance between waves is the wavelength of the light, and the 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, violet has 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 still longer wavelengths are called radio. Radiation with shorter wavelengths than violet light is called ultraviolet; still shorter wavelengths are called X-rays and 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 radiation is 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 from some other source and the intrinsic color of the material is not important. A bar of gold and a bar of black graphite at 3000 degrees C would emit the same color light. The Sun is very close to a theoretical black body in its light emission - all stars are. The Moon is not. The Moon absorbs sunlight and re-emits it in different forms, for example, a lot of visible 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, and sodium-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 the familiar "red hot" color appears. As the object gets still hotter, the peak shifts into the yellow part of the spectrum and the object glows orange, then yellow. But then the object becomes a paler yellow and finally white, not green. Why? When the emission peak is in the green (as it actually is for the Sun!), the object is still emitting copious red and yellow light, and these wavelengths combine to give a fairly pure white. But the very hottest stars have peaks in the violet and beyond, and their blue and violet emission is so much greater than their red and yellow that the stars appear blue-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 a star and its color we can calculate its apparent size in the sky, and if we know its distance, we can calculate its actual size. Dwarf stars are the size of the Earth or smaller. The Sun is 1.3 million kilometers in diameter, over 100 times the diameter of the Earth. 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 of the famous Bunsen burner) found that Fraunhofer's lines were produced when gases absorbed or 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 a year Kirchhoff had begun analyzing the Sun. In 1868 the element helium was discovered from its spectral signature in the Sun, the only element so discovered. Its name comes from the Greek helios for Sun.

Physical Conditions in Stars

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

After helium was discovered, astronomers began looking for other new elements in the cosmos, and a number were announced and named. All later turned out to be ordinary elements like oxygen under extreme and unfamiliar physical conditions, for example with many 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 Doppler Effect.

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

Light that is red- or blue-shifted merely changes color. There is no way to tell from color alone that a star is moving. Contrary to many popular illustrations, red-shifted stars 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 effect is that spectral lines change position. The change in position is easily measured on a photographic plate. The Doppler Effect allows astronomers to determine three important facts:

As an indication of the subtle details spectroscopy can reveal, it is possible to detect vertical movements of material on the Sun from its Doppler shift. Just as earthquakes on the Earth send waves through the Earth's interior, disturbances on the Sun send waves through its interior, and these waves can be detected from their effects on the surface of the Sun. Study of these disturbances, called helioseismology, is revealing 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 the motion of stars as proper motion, and measure the apparent motion of the stars on photographs taken years apart. Most stars would take centuries to move the apparent width of the Moon, but they do move.

If we know the distance to a star, we can calculate its velocity across our line of sight. The Doppler Effect enables us to determine the radial velocity of the star, or its speed along our line of sight. If we know the two velocities, we can determine 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 are in motion. Just as a driver in a snowstorm sees the snowflakes appear to radiate away from his direction of motion, astronomers also observe that most of the stars in the sky are moving away from a certain region of the sky. This direction, in the constellation Hercules (straight overhead for U.S. observers in the early evening in summer) is the direction 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, others yellow, still others bluish-white. When we determine distances to the stars still more differences appear: some very nearby stars are extremely faint, while very distant stars are sometimes very bright.

Spectroscopy provides a means of making sense out of the variety of the stars. The very hottest stars are blue-white and show only the spectral lines of helium. Somewhat cooler stars are white and show lines for hydrogen as well. Still cooler stars (yellow and orange) 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 form in their atmospheres.

Astronomers divide stars into spectral classes, labelled with letters. Originally the classes were designated A, B, C ..., but some classes were dropped and others rearranged in order. The major classes of stars, from hottest to coolest, are now designated O, B, A, F, G, K and M. There is a handy phrase for remembering the letter sequence: "Oh, be a fine girl (guy), kiss me". Four other classes are used for peculiar stars: W for very rare, hot stars, N, R, and S for chemically peculiar cool red stars. For stars that fall between the main classes, subdivisions are used: an F5 star is halfway between classes F and G, for example. Our own Sun is classed as G2.

The Hertzsprung-Russell Diagram

One of the most important tools in understanding the stars was devised in 1913 by Ejnar Herzsprung and Henry N. Russell. When they plotted a graph of absolute magnitudes of stars against spectral class, they found that most stars plotted on a diagonal line, with the O stars brightest and M stars faintest. Such a diagram is called a Hertzsprung-Russell Diagram, or sometimes H-R Diagram for short. Astronomers refer to the main band of stars as the Main Sequence. The Sun is a Main Sequence star. Some stars plotted well above the Main Sequence, meaning they were more luminous than average. These stars are called giants and supergiants. Still other stars plot well below 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 surface temperature, and all emit about the same amount of light per unit area, so their light output is a measure of their diameter.

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Created 26 March 1998, Last Update 20 January 2020