The
Functioning of a Star
Structure and
Energetic Genesis
The Nuclear Furnace of the Stars' Cores
The energy of a star is generated in its core (in the case of the Sun,
this zone is about 280 000 km wide) through the nuclear combustion that fuse
the light materials into heavier elements.
During most of a star's life, the element that feeds this combustion is
the hydrogen. Two hydrogen nuclei (in other words, two protons) fuse and give birth to a deuterium
nucleus (1 proton and 1 neutron). 1 neutrino and 1 positron (carrying with them
the positive electric charge not used by the newly formed
nucleus) are released during the process, being also released energy under the
form of a gamma photon. The deuterium nucleus joins another hydrogen nucleus,
while another gamma photon is again released. Finally, the resulting nucleus
(Helium 3) joins another Helium 3 nucleus, which gives birth to the formation
of a Helium 4 nucleus (2 protons and 2 neutrons) and two hydrogen nuclei. A
gamma photon is emitted again. During all the process, 0,7%
of the mass is converted into energy, responsible for the brightness of the star.
During periods ranging between 10 million (107) years (for
the biggest stars) and 100 billion (1011) years or more (for the smallest
stars), the
stars will keep burning the hydrogen until its exhaustion. The biggest stars
have shorter life expectancies, because they need to incur in a much higher
activity rhythm in order to counterbalance the more intense gravity. Therefore, they consume the raw
materials used in the nuclear fusion much more quickly. This activity is the
responsible factor for the extreme luminosity of these stars.
Zone of Transport through Radiation
In average dimension stars, the layer above the central region where the
nuclear reactions are carried on is called the zone of transport through
radiation (with a thickness of 360.000 to 410.000 km in the case of the Sun). There,
the photons generated in the deep interior of the star are continuously
absorbed and re-emitted by the atoms of transparent gas that are present there.
The gamma radiation gradually loses energy during this slow transportation
process (one photon moves only 1 cm each 10 minutes). The process gradually
enlarges the wavelength of those protons.
Zone of Transport through Convection
Due to the decrease of the temperature, the gas of the star becomes
opaque in the regions that are closer to the surface. When the temperature
reaches 2 million degrees (at a deepness of about 150 000 / 200 000 km, in the
case of the Sun) there it starts the zone of transport through convection.
In an analogue way to what happens in a boiling water pot, the ascending
movement of the hot gas masses and the descending one of the cold gas masses
becomes, in this zone, the phenomenon that is responsible for the energy
transmission to the outskirts of the star.
Already close to the star's surface, at a deepness of 20 000 km, the
temperature (150 000 ºK) finally allows the helium nuclei to join the electrons
in order to form neutral atoms (absent of electric charge).
The Stratification in the Massive Stars and in Mini-Stars
As it was said, in a normal star, the core is overlaid by a zone of
transport through radiation and this one by a zone of transport through
convection. However, it is noteworthy the fact that in very big and massive
stars, like Rigel, the stratification is reversed: in the regions that are
closer to the centre, the energy is transported through convection while close
to the surface it is transported through radiation. On the other hand, in small
and low-mass stars, the zone of transport through radiation doesn't exist.
Above: stratification of the
interior of an average sized star, below: stratification of the interior of a
massive star (Ediciones Orbis - Astronomia)
Photosphere
The visible layer of the star is called photosphere that, in the case of
the Sun, is about 300 km thick. It's here, after a trip that lasted for several
million years, that the gamma photons proceeding from the centre of the star are finally
emitted into the space under the form of visible
radiation
(6000 ºK).
The photosphere is a transparent layer and displays a grainy appearance
that is caused by the convective movements. The hotter zones, where the gas is
rising, display a brighter appearance, opposite to the darker one shown by the
colder regions, where the gas descends.
This layer is also the dwelling place for the starspots, where a strong
magnetic field prevents the convective movements, forcing the transport of the
energy through the radiation process, which is an inefficient mechanism at the
star's surface. For this reason, the starspots are regions of less energetic
emission and, therefore, darker zones.
Surrounding the starspots there are the photospheric faculae, which are
zones characterized by an intense convective activity that compensates the low
activity in the interior of the spot. For this reason these regions are
brighter than the rest of the photosphere.
Photosphere: solar spots and a dim
facula, to the left (National Solar Observatory)
Chromosphere
Above the photosphere there is a chromosphere, with a thickness and a
temperature that, in the case of the Sun are about 15 000 km and 100 000 ºK. It
is a rarefied atmosphere where one may observe spicules, which are short-living
fire streams moving at very high speeds.
Other interesting phenomena are the flashings, with heights that, in the
case of the Sun, reach 50 000 km. The flashings result from the explosive
liberation of energy previously stored in the magnetic fields of the spots.
At last, this region is also the harbour place for the prominences,
which are bright gas jets displaying the shape of arches. In the case of the
Sun, they may reach a height of 40 000 to 400 000 km, therefore extending up
into the inferior layers of the corona. The shape of the prominences is
determined by the magnetic fields.
Solar eclipse: close to the edge -
chromospheric spicules, farther away - helmet streamers, in the corona (High
Altitude Observatory)
One of the biggest solar prominences
ever observed, attaining a height of 588 000 km (NASA)
Corona
The stellar corona, on the other hand, is the most exterior layer of a
star like the Sun. It has a density that is millions of times inferior the
photosphere's and a temperature that, in the case of the Sun, is about 1
million ºK. It's thought that the high temperature of the corona results from
the difficulty it has on dissipating the heat coming from the photosphere.
The most relevant features of the corona are the helmet streamers
(sustained by the prominences), the high density coronal arches, whose shape is
determined by the magnetic fields, and the coronal holes, which are dark zones
of low density.
The Magnetic Field
In order to understand all of this range of phenomena, it's unavoidable
to refer their most determining cause: the magnetic field of a star. The magnetic
field is generated by the hot gas plasma existing in the interior of the star. This
gas is ionized and a very good electric conductor. It behaves like a fluid and
moves around the centre of the star, following its rotation. As this rotation
is fluid and not rigid, there are moving electron currents generate electric
currents, which afterwards originate the magnetic field.
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