by Pio Passalacqua
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
THE 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.
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
The emission intensity
of the black body in the various wave lengths at a given
temperature is represented by a graph named
For every temperature exists a different
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
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 LINES OF THE
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
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
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
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.
OF THE STARS
The Harward classification
distinguishes seven spectral classes marked in the
order from the letters O, B, A, F, G, K,
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
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
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
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
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
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