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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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