A star can be described as a huge autogravitating sphere made of very
hot gas (mainly Hydrogen and Helium), which produces energy through a nuclear
fusion process, and then emits such energy in the form of radiation.
The stars are very far from our solar system, so they appear as small
bright spots in the sky. They constitute the main component of galaxies,
which are agglomerates of billions of large and small stars, of clouds
of gas and dust.
The stars appear, in the sky, gathered in sets, called constellations.
The astronomers have called many stars with names of Greek, Arab or Latin
origin. Others are classified with the name of the constellation to which
they belong and a letter of the Greek alphabet that indicates the luminosity
in relation to that of the other stars of the same constellation. For example,
Alpha Tauri is the brightest star of the Taurus constellation, Beta
Tauri the second brightest, and so on. Others received their names from
the particular catalogues in which they were classified. The most modern
catalogues, compiled with the aid of the observations of artificial satellites,
contain up to millions of stars, besides other galactic or extra galactic
objects.
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A map of Orion, one of the most famous and beautiful constellations of the boreal hemisphere. (SEDS) |
The stars are formed thanks to the gravitational collapse of an interstellar
cloud of gas (prevalently Hydrogen, with traces of other gases), and dust.
The clouds of interstellar gas are very large, with a mass of gas that
can reach a million times that of the Sun, and have very low temperatures,
ranging between ten to some hundred degrees above absolute zero (that is
from -263 to some degrees centigrade below zero).
These clouds are usually in a state of equilibrium, that is to say
that the gravitational force which would make them collapse is equilibrated
by the pressure created by the motion of the particles within. Sometimes
though such pressure is not sufficient, in some points the density increases
and the cloud spontaneously and slowly contracts under the action of its
own gravity. It is probably through this mechanism that the stars with
a smaller mass are formed, within dense and dark clouds. The most massive
stars, on the contrary, are apparently formed during the collapse of less
dense clouds, caused by external factors. One of these factors could be
the compression of the cloud by material that was expelled at high speed
by evolved stars (planetary nebulae
or supernovae). Or else, the casual collision
between two clouds during their motion within the galaxy, and the subsequent
collapse of part of them.
Actually, the clouds of interstellar gas are very large and their collapse
does not originate one only star, but a set of stars (that is a star
cluster ), after the fragmentation into smaller clouds. The fragments
then can give rise to a single star or a set of stars that orbit around
a common barycentre. In our galaxy, for example, the single stars are about
50% of the total. The remaining stars are gathered in binary systems
(the majority) or even multiple systems: multiple systems of 6 stars have
been observed! the binary stars are also
called binary systems.
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A spectacular region of stellar formation in M16, in the Orion constellation.
The "pillars" are formations of Hydrogen and dust in which new stars are
originating.
(HST) |
When the cloud contracts, its gas particles move faster and the nucleus
becomes hotter. In this phase, which is called protostar, the cloud emits
energy in the form of radiation, although much weakly than a star; this
happens at the expense of its own gravitational energy, which is converted
into radiation. During this phase, the protostar has a superficial temperature
of 2-3,000 degrees and is still immersed in the cloud of gas and dust from
which it originated. In general, a disc of gas is formed around the protostar,
and such gas gradually falls upon it. The star, in its turn, emits gaseous
spurts from the polar regions, along the symmetry axis of the disc. The
structure disc + spurts is very common in the early stages of the life
of a star.
In this phase the protostar is obscured by the surrounding material,
and therefore it is not very bright; the dust of the surrounding cloud
absorbs the radiation emitted by the object, and then re-emits the energy
at lower frequencies, in the infrared region of the spectrum, therefore
protostars can be detected in this band of wavelengths.
During the protostar phase, also called principal pre-sequence phase, the star goes through some instability phases, along with sporadic luminosity variations. Therefore, the so-called T Tauri variables can occur, from the name of a star of this type of the Taurus constellation. The gas and the dust that surround the star are gradually swept away by the spurts of gas and by the wind emitted by the star.
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Illustration of the disc of gas and dust and of the spurts typical of the forming stars. (HST) |
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The spurts of gas emitted by three originating stars. These spurts are frequent in the dynamics of stellar formation, they are emitted by a disc of gas and dust that falls upon a protostar. (HST) |
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Two spectacular spurts of gas emitted by the disc of gas and dust that surrounds a young star, in the Orion constellation, 1,500 light years from us. From one extremity to the other they measure approximately one light year. The central star is hidden by a dark cloud of dust. Bottom left, in a region close to the star you can distinguish some agglomerates of bright gas, emitted by the same star; these too are very common in young stars and are called Herbig-Haro objects (H-H). Bottom right, a typical structure produced by a shock wave. It is produced when the gas of the spurts, at a high speed, encounters the interstellar gas, which travels at low speed. (HST) |
The life of the stars
The contraction of the protostar continues until the temperature within are high enough to trigger the nuclear fusion, which will be its means of sustenance for millions or billions of years; the protostar is already a star. At this point, the energy it emits is not produced at the expense of its gravitational energy any more, but at the expense of its mass. In fact, the thermonuclear reactions consist of the fusion of several atomic nuclei into one, with a mass that is slightly smaller with respect to the sum of the masses of the original nuclei. The mass that is lost in the process is that transformed into energy according to the law
The modern theories of stellar evolution, along with the observation of how the stars are distributed in the various mass intervals, have fixed the inferior limit at approximately 0.08 times the mass of the Sun. In the same way, there is a superior limit for the mass of a star, over which it becomes unstable and cannot maintain its equilibrium. This limit is probably between 100 and 120 times the mass of the Sun.
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Images in false colours of the least luminous object observed around a star. It was to first brown dwarf definitely identified, called GL229B. It orbits around a red dwarf star, at approximately 18 light years from us, and its mass is barely 20-50 times that of Jupiter. (HST) |
The number of stars with a certain mass with respect to the total of
stars that are formed, depends on the mechanisms by which the protosolar
clouds disintegrate before their collapse. The probability that a star
with a certain mass is formed is inversely proportional to the mass, in
other words more small stars are usually formed than large.
A star can be imagined as a stratified structure, like an "onion", where
each layer has a certain value of temperature, density and pressure. These
values increase from the surface of the star to the centre. This gas structure
is in equilibrium between two opposite forces: the gravitational force,
directed inwards, that is the "weight" of the outer layers on the inner
ones, and the pressure of the radiation produced in the nucleus of the
star, directed outwards. During the entire life of the star, which could
last even billions of years, this equilibrium is always maintained, through
mechanisms of auto regulation.
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Eta Carinae, one of the most massive stars known, and the spectacular
nebula that surrounds it. The star has a mass that is 100 times that of
the Sun, and underwent a consistent gas emission approximately 150 years
ago, which originated the nebula that can be observed today. This nebula
has two lobes and a large and thin disc. The lobes also contain large quantities
of dust, that absorbs the blue light, and re-emits it in the red.
(HST) |
In the conditions of very high temperature and pressure within the stars,
all the gas is ionized. The gas nuclei are very close to each other and
collide at high speed. The fusion of two or more nuclei occurs when pressure
and temperature are high enough to overcome the mutual electromagnetic
repulsion (due to the fact that they have an electric charge of the same
sign). The nuclear fusion reactions require therefore two conditions:
a sufficient abundance of the combustible element and a temperature high
enough to overcome the repulsion of the nuclei.
Each chemical element requires a different temperature for the fusion:
the heavier the element, the higher the required temperature.
The simplest nuclear reaction that takes place within a star is the
fusion of Hydrogen: four nuclei of Hydrogen are fused into one nucleus
of Helium, and the slight mass difference is converted into energy. Such
reaction can take place only at temperatures of at least ten million degrees,
and sustains the star for most of its life.
The star maintains its pressure equilibrium through a thermostatic
mechanism: when the energy production in the centre decreases, the star
contracts, the internal temperature increases and the fusion reactions,
which depend on the temperature of the gas, are accelerated. During this
phase the star becomes hotter and therefore emits a radiation with a lower
wavelength than before.
Vice versa, when the energy produced is excessive, the star expands
to increase the surface through which it can disperse it. The expansion
cause the decrease of the pressure and temperature in the centre, and therefore
the fusion reactions slow down. During this phase, the star becomes brighter
because the emitting surface is increased, but the outer layers are colder,
and therefore they emit a radiation with a higher wavelength.
When Hydrogen, which is the main constituent of the star, begins to
run out in the centre, the energy production by nuclear fusion decreases;
the star is compelled to increase its inner temperature in order to trigger
the fusion of a heavier combustible, and therefore survive. Otherwise,
it would be crushed under its own weight and it would collapse.
After the Hydrogen, the star triggers the fusion of Helium. Three nuclei
of Helium unite to form one nucleus of Carbon, thus releasing energy. After
Helium, the Carbon is fused to form heavier elements, and so on. Thus,
Oxygen, Neon, Magnesium, Silicon, Sulphur, Argon, etc. are formed.
The stars are therefore very important sources of chemical evolution:
from Hydrogen, which is the most abundant element of the Universe, the
heaviest elements are synthesized in the stars. During its evolution, a
star returns part of this material to interstellar space, either through
slow and continuous processes such as stellar
wind, or in the course of explosive phenomena (planetary nebulae, supernovae);
then, clouds will be formed from this gas, then new stars, and possibly,
planets. The stars that are formed from this gas have a chemical composition
different from that of stars that are formed from gas that was not enriched.
On the basis of such difference, the astronomers classify the stars in
two groups: the "first generation" stars are called stars of population
II, while those that were subsequently formed from gas enriched
in heavy elements, are stars of population I.
The energy of the stars
During a thermonuclear fusion reaction, as we have seen before, the atomic nuclei unite to form a more complex nucleus, and the difference between the final mass and the sum of the original masses is converted in neutrinos and in radiation. Neutrinos are sub-atomic particles with no electric charge, that do not interact with matter and therefore can easily escape to a star, dispersing in space. They are produced in very large quantities: just think that every second, the Earth is hit by a flux of 107 solar neutrinos per cm2 and per second.
The radiation travels outwards in "packets" of energy called photons,
which are continuously absorbed and re-emitted by the atoms of the gas.
Before they reach the surface of the star and are released in space, the
photons must travel along a zigzag path through the atoms, which lasts
some million years!
At a certain stage, while travelling towards the surface, the photons
move so slowly that the energy must find some other way to flow outside:
convective motions of the gas are thus developed, in other words bubbles
of hot gas that move towards the surface and become colder, that act as
vehicle for the energy. If such energy were trapped within the star, the
equilibrium would end.
Once it has reached the surface, the radiation is emitted towards all
directions of space. Due to the fact that until this moment it has been
in equilibrium with the gas of the star, its characteristics depend on
those of the gas. That is to say, the higher the temperature of the gas
in the superficial layers of the star, the higher the frequency of the
emitted radiation. Actually, the star emits light at all wavelengths, but
with different intensity; the wavelength which had the maximum intensity
characterizes the "colour" of the star, and the hotter the star, the smaller
the wavelength.
Such fact is expressed by the Wien law, which states that the ration
between the superficial temperature of a star and the frequency at which
the intensity of the radiation is maximum, is equal to a constant.
The properties of the stars
The properties that characterize a star are its mass, its size, the superficial temperature (which determines the "colour" of the star) and the luminosity, which is described by a quantity called magnitude.
The mass of a star, as we already know, can range between one tenth and a hundred times the mass of the Sun, that is from 2 1029 to 2 10 32 Kg. On the other hand, the size can vary within a larger interval; the diameter of a star is always quite difficult to determine, and can be measured only for near stars. It can range between a few Km for a white dwarf to a hundred million Km for a red supergiant.
The colour, the luminosity and the temperature of the stars are studied by spectroscopy and by astronomical photometry. The analysis of a large number of stars allowed to recognize the common characteristics and to subdivide them into classes, called spectral types and in classes of luminosity.
Some stars show luminosity variations in time: some have regular, periodic
and relatively small variations, and are called variable
stars, others have huge and sudden variations of brightness, due to
explosive phenomena that modify their structure: novae
and supernovae.
The stellar motions
In ancient times people thought that the stars were fixed on the sky,
while we know that they have a relative motion, as a consequence of both
their rotation motion around the centre of our galaxy, and of the motion
of the Sun itself (and therefore of the Solar System). The motion of the
stars, although it is relatively fast, appears to us as being very slow,
due to the very large distances involved. The star nearest to us besides
the sun, called Proxima Centauri, is in fact 4.2 light years far (which
equals approximately 38,000 billion kilometres!). The motions of the stars
in the sky, called "proper motions", are therefore imperceptible if observed
during times that are much smaller than the life of a star, as is the length
of human life. They travel angular distances in the sky that rarely exceed
5 arc seconds each year.
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One light year equals approximately 9,460 billion Km.
Scientists think that, within a sphere centred on the Sun with a radius
of 300 light years, there could be 500,000 stars.
The star
Beta Pictoris with the disc, which maybe contains a planet. MPEG,
534 Kb. (STcI)
The protoplanetary
discs of the Orion nebula,
