by Pio Passalacqua 

 For millennia and millennia in the message carried by the light reaching us from the stars, only the part that points out the position of them in the sky has been read. Only the direction of origin of the light was considered andtherefore one could only establish that this or that star were found in this or that position, in comparison to determined fundamental references man had ingeniously succeeded in fixing; a purely geometric information. And on this single information the whole building of the classical Astronomy has been builtup to Copernico, Galileo, Keplero, Newton, Herschel.
It was only toward the end of the ‘700 that the intensity of the stars light started to become an object of systematic research (with Herschel). But above all when the way to decompose the light in its own elementary components wasdiscovered, getting the so-called spectrum, it was possible to begin to decipher the message coming from the Sun and the other stars (Fraunhofer - Kirchoff).


All of you have seen the rainbow, it is only the light of the Sun divided in its own components of different wave lengths. The water drops suspended in the air, making the rainbow, divide the radiations of various wave length one fromthe other, diverting them in different measure, so that the eye distinguishes them separated and this is what we name spectrum. The optic feeling is very complex: among the various forms it simultaneously assumes, one depends on the greatest or the smallest quantity of light penetrating in the pupil in the unitof time and it is the feeling of the " intensity "; another depends on the wave lenght of the light and this feeling is called " color ". The shortest waves that our eye can perceive give a feeling that we name violet, the longest onesperceptible give the feeling called red dark. Between these extremes we have in the order: the blue, the green, the yellow, the orange. Beyond the red there is the black, a black called "infrared " and beyond the violet there is also a black, a black called " ultraviolet ".
For studying the composition of the light and establishing how the intensity is divided among the varied colors are used special tools, called spectroscope andspectrograph.
With the spectroscope the spectrum is studied looking directly through the ocular of the tool; with the spectrograph the spectrum is instead recorded on a photographic plate.
The light to examine is let in the spectrograph through a narrow opening (hundredths of millimeter of width and some millimeters of length) called slit. From the inside of the tool, the slit so illuminated appears as if were it itself the source of light and the spectrograph produces on the photographic plate an image of the slit, that is a bright line, for every wave lengthpresent in the light.
The separation of the waves of various wave length is performed by a prism that takes advantage of the deviation the light suffers passing from the air to the glass and viceversa and that it is different for the different wave lengths.Besides there are two objectives, one to convey the light originating from the slit on the prism and the other to focusing, on the photographic plate, the light separated by the prism in the various colors.
The lines in the spectrum appear so more distant one from the other as greater is the difference of wave length.

If the source emits radiations of any wave length, then the various images of the slit will result attached one to the other and then the spectrum assumesthe appearance of a continuous strip from the violet to the red that is called continuous spectrum.


Whatever body emits electromagnetic radiations, always, however the emission quickly increases with the growth of the temperature.
As it is known the lowest possible temperature is 273,2 degrees centigrade below zero. In fact the temperature of a body depends on the average speed of the elementary particles making up it and at the temperature of -273,2° every speed of the particles is annulled, so it is clear that a lowest temperature of this is a nonsense. Considering a solid body, at a temperature near theabsolute zero the emitted radiation is practically null and little by little the temperature increases, the radiation starts to become notable first in the region of the radio waves, then also in the infrared one. With the rise of the temperature the maximum intensity of the radiated energy is shifted more and more toward the shortest wave lengths.
When the temperature reaches about 700° K (430° Cs) although the maximum of the intensity is still in the infrared, the body begins to become bright: at first a dark reddish light, then still climbing the temperature, red-orange, yellow,then white, then white-blue one.
If the light emitted by this incandescent body is observed with a spectroscope, a continuous spectrum is seen; and as we have already said it is noticed that with the temperature rising the maximum intensity is shifted more and moretoward the shortest wave lengths.
For an incandescent solid with the growth of the temperature the intensity increases and the color changes. However at the same temperature both the intensity and the color are different enough according to the nature of thebody heated: if it is iron or silver or coal etc.
For having a standard reference, the physicists resort to an ideal body that has the property to emit with an intensity superior to any other body one with the same temperature (black body). Regarding the black body in a certaintemperature, the distribution of the emitted radiation is only one and it can be only that, whatever the substance making it may be.
The emission intensity of the black body in the various wave lengths at a given temperature is represented by a graph named Planck curve. For every temperature exists a different Planck curve.

Observing the Planck curve it is noticed the total emission (measured by the area contained by every curve) quickly grows with the rise of the temperature and the maximum intensity is shifted toward the shortest wave lengths, that is in the direction from the red to the violet.
At a low temperature the emission occurs in the distant infrared almost exclusively; then, rising the temperature, the maximum little by little shiftsin the visible region of the spectrum and a large part of the radiation is emitted as light. At higher temperatures it falls in the ultraviolet.

Law of Wien: l m * T = const. 2880

Example for the Sun: temperature 5800° wave length of the maximum intensity (lm) = 2880/5800 = 0,496 microns, in the middle of the visible region of the spectrum.


The spectrum of a gas not excessively dense and sufficiently warm has an entirely different appearance from an incandescent solid or liquid one: instead of a continuous streak with the iris colors, looking in the ocular of aspectroscope is observed a succession of isolated bright lines of different color.
Each of these is an image of the slit in a specific wave lenght; the number, the position and the intensity of the lines are different according to the chemical nature and the temperature of the gas.
The group of lines that appears in the spectrum is characteristic and unmistakable for every chemical element.
If the light of an incandescent solid crosses a cold gas, the continuous spectrum of the source appears ploughed by dark lines occupying the same positions of the lines that would appear bright if that same gas had heated toa sufficient temperature. The dark lines are no other that light lacking in those particular wave lengths, light subtracted by the gas to the source: a gas absorbs those same radiations that it is able to emit (Kirchoff law).
The light of a lamp, being a source of continuous light, emits radiations (or photons with the corpuscolar terminology) of every possible wave length: thereis not any empty space of wave length between a radiation and the other. If this light passes through a cruet containing a gas, the atoms of this gas absorb, getting excited, a percentage of the photons with an energy equal to one of the possible level jumps (since according to the quantum theory theelectrons in an atom can move only on orbits well determined, for jumping from an orbit to the other one they need energy of a defined wave length).
The photons absorbed are again emitted within a billionth of second; however they are not emitted to the original direction but on the contrary each of them in a direction at random, so only a part of the photons takes back thedirection of the observer that is looking at the lamp through the gas. Therefore to this observer arrive less photons with the characteristic wave length of that gas than those ones the lamp has emitted toward the direction of it.
Consequently, if the spectrum of this light is observed, in coincidence with the wave lengths of the absorbed radiations some dark lines appear(light missing from the continuous emitted by the lamp). If instead the cruet is sideways observed, so that the light doesn't come to the observer directly from the lamp, only the photons emitted again by the gas will be seen after the missing excitation of the atoms, and in the spectrum the bright lines typicalof that gas will appear.


Also in the spectrum of the stars there is the presence of dark lines that plough the iris streak: a star results formed of an high temperature body, gaseous but enough dense to produce a continuous spectrum, as an incandescentsolid; it is from the surface of this body named photosphere that the whole light of the star really originates.
Around the photosphere there is a wrap of transparent gas called atmosphere of the star; the lowest layer of the atmosphere that comes into contact with thephotosphere, more cold and more dense, absorbs from the continuos light the radiations characteristic of the gas that is forming it causing the dark lines that plough the spectrum.

Beginning from the end of the last century the spectra of hundreds of thousand of stars were studied (Secchi, Donati, Huggins etc.) achieving, after a firstspectral classification been due to Secchi, the famous and modern Harward classification.


The Harward classification distinguishes seven spectral classes marked in the order from the letters O, B, A, F, G, K, M.
The O class includes the blue stars with the highest temperature, the M class the red stars with the lowest temperature.
Here are the essential characteristics of the different classes:

O Class - white blue stars with a very high temperature between 60.000° and 30.000°. Only few lines plough the continuous spectrum and they are lines ofthe neutral and ionized helium, as well as weak lines of the hydrogen.

B Class - white blue stars having 30.000° - 10.000°. They show the lines of the neutral helium while the ionized helium ones are missing; the lines of the hydrogen are more intense than those in the O class.

A Class - white stars with a temperature between 10.000° and 7.500°. The hydrogen lines have in this class the maximum intensity; weak lines of some metals as calcium and magnesium appear.

F Class - white stars having a temperature between 7.500° and 6.000°. The hydrogen lines, weaker than in the preceding class, are still very intense. The lines of the metals appear numerous.

G Class - white yellowish stars with a temperature between 6.000° and 5.000°. The hydrogen lines are even more weak than those in the F class, while thelines of the metals are very numerous and intense: neutral and ionized calcium, iron, magnesium, titanium, etc. The lines of the ionized calcium (CaII), known as Hand K lines, fall in the near ultraviolet and are the most intense of the spectrum.

K Class - cold stars with a red orange color. Since the temperature is between 5.000° and 3.500° the spectrum is mostly full with metals lines. The hydrogen lines are very weak.

M Class - stars even more cold with a 3.000° temperature and therefore have a reddish color. The atmosphere, the most external layers of these stars, containnot only elements but also chemical compounds, that is molecules producing bands in the spectrum.

Every class is divided in 10 subclasses or types, marked with the numbers from
0 to 9 in addition to the letters.

When an atom is ionized, the going out electron is the most external and therefore, in the ionized atom, the duty to emit or to absorb is up to the most external of the remained electrons. However this electron is more strongly tiedup to the nucleus, so with its jumps it gives rise to increasing differences of energy and therefore it absorbs or emits photons with more energy and that is of less wave length compared with the neutral atom.
The spectral lines of the ionized atom are therefore shifted to the ultraviolet in comparison to the similar lines of the neutral atom. If the atom is twice ionized, that is if it loses two electrons, the lines result even more shifted:with the increase of the ionization degree the lines are moved more and more toward the far ultraviolet.
Atthe very high temperatures of the O class stars, that are able to break the strong tie binding one of the two electrons to the helium nucleus, the lines of the either neutral that ionized helium appear.Obviously, all the other elements are also ionized: the hydrogen, having lost its one electron, has reduced to the only nucleus and it cannot produce spectral lines; only a small percentage of hydrogen atoms has preserved the electron, so the quantity of radiation absorbed is small and the hydrogen lines appear very weak.
The other elements are ionized more times and their lines fall for the most part in the distant ultraviolet that cannot observed from Earth.
In the stars of B spectral class, the temperature is lower so the percentage of helium is lower, rather beginning from the type B6 are practically zero. Thelines of the neutral helium predominate instead.
In the A spectral class the lines of the hydrogen prevailing, because by now at 8000° of temperatures, the hydrogen is practically all neutral and therefore it is able to absorb the radiation originated from the inside of the star.
In the following F and G classes begin to dominate the lines of the metals easily ionized as calcium, iron, magnesium, sodium, that only at so low temperatures they result neutral or ionized only one time; in the G class startappearing intense in the near ultraviolet a couple of calcium lines ionized once: they are the H and K lines.
In the spectral K and M classes the temperature is fallen to such a point that in the atmospheres of these stars start to appear a large number of moleculesalso, that is atoms tied up among them.
In the molecules the system of the possible energetic levels is much more complicated than in the atoms, because the tied up atoms forming the moleculeinteract between them in various ways; therefore the possible levels are divided in so dense and numerous sublevels that instead of the single lines of the spectrum there are groups of lines, connected the one to the other to produce wide zones of absorption (or of emission) named " bands ".
As we have seen, the effect of the difference in temperature explains in satisfactory way the difference in the spectral characteristics of the stars.
The intensity of the hydrogen lines and the weakness of the metals lines in the A spectral class don't mean at all that these stars are poor of metals, and that instead the G class stars where the metallic lines appear in thousands are rich of them.
So also it doesn't mean that the hydrogen is abundant in the T class stars and scarce in the M class stars.
As a matter of fact all the stars have almost equal chemical composition, their mass  is made of 80% hydrogen, 19% helium and the remaining 1% of otherelements.

by Pio Passalacqua
translated into English by Salvo Marino