Saturday, June 18, 2011

THE STARS AND TYPES

One of the main characteristics of a star is its mass, ie the amount of matter that is the star and determine its fate.


According to the mass of the star is going to put in a certain position of the main sequence, the mass also depends on the temperature of the star, the color and spectral type.
In addition, a massive star soon will have a certain kind of development stage, the other will travel any other way.
If you take as a yardstick by the solar mass stars are more massive stars, and up to 100 times to 100 times less massive.
Calculate the mass of the star for the binary stars is simple if you know their movement.

For those of main sequence but enough to know that their brightness is closely related to the mass. For stars more massive than the brightness is equal to the high mass to the cube, for less massive than the mass raised to the fifth power.
The stars also differ in density. The main sequence stars have densities such as solar and thus more than one.

Red giants are not very dense, for example, because it very bulky, while the white dwarfs and neutron stars are very dense.
The temperature depends on the mass. Inside, the temperature must be at least that to start the fusion reactions.
The surface temperature is rather higher in more massive stars, up to 25000 ° K, but is lower in less massive ones up to 3000 ° K.
From the surface temperature depends on the spectrum. According to the spectrum of the star can be classified into spectral classes. The main ones are O, B, A, F, G, K, M, from the most massive, hottest, brightest blue stars that is up to the less massive.


Each class is divided into 10 subclasses ranging from 0 to 9. The Sun is a G2.
Brightness is another factor, which depends on the mass. What we see is the apparent brightness of stars, which depends on the absolute luminosity and distance.
The instrumental measurements are made with photometers that measure the amount of energy that comes from the star, but it is always apparent brightness.

The absolute luminosity is the amount of energy radiated by the star into space.

For the sun, the measurement is simplified by the fact that you know the distance and the solar constant.

The brightness can be found by making comparisons between the various stars. Already in the second century BC ^ classifications were made based on the apparent brightness of the second order of magnitude, the order number 1 occupied by the most luminous.

Today the measure of apparent brightness is done with the tools and the relevant comparison is the lodestar. The order of magnitude in brightness is called magnitudes.

Between a class and one of magnitude 2.5 are magnitudes.
The negative magnitudes indicate bright stars, the less than positive light.
In order to compare the various brightness values ​​must be comparable: one tries to measure the absolute luminosity, ie the brightness of the stars that would have placed at a distance of 10 parsecs from the observer on the celestial sphere. At this point the distance is the same so what varies is the amount of light radiated.

There are stars that have variable brightness (variable stars) or for intrinsic reasons, namely because of the characteristics of the star or extrinsic causes.

Many of these stars are binary systems formed by a bigger star and a smaller one.
When the two stars are both visible brightness is maximum, but if you are less bright eclipse. (Variables in eclipse).
The movement of the stars is split into two movements: the long-sight between the star and the observer and the proper motion, ie moving on the celestial sphere.

All stars have a magnetic field whose intensity varies over time and whose polarity is reversed constantly.

Among the other star of the interstellar material is studied by spectrographic analysis to know its composition. And 'present hydrogen, helium, calcium, water, ammonia, hydrogen sulfide, calcium oxide, radicals (unpaired electrons with the atomic groups such as OH), organic molecules like formic acid and ethanol. From here the group of theories that believe that life started out of the ground.

There is also cosmic dust consists of silicates and ice crystals.

In some places the material thickens to form interstellar nebulae that can be of two types: dark nebulae, regions of space where new stars are forming, and bright reflection nebulae or emission regions of space where there are young stars that illuminate the residual material that reflects or absorbs radiation of the stars.

The nebulae can be either in hydrostatic equilibrium and therefore the gravitational energy is compensated by the internal gas pressure, or collapsing or expanding nebulae.

The collapse of a nebula is for an external cause. What he did to start the collapse of the nebula that gave rise to the solar system was favored by a supernova explosion whose shock waves have caused the collapse.

The nebula, which can give rise to a star system must have a low temperature (10 ° K) and a low density (10-24 g/cm3) and a mass of 1000, 10000 times the solar mass. From a cloud of this size will form a family of stars during the collapse because the material is fragmented.
The collapsing cloud becomes denser, but still too low because the particles exchange energy between them and increase the temperature of the nebula. Exchanges are made only with the outside world.

If the body gets hot there is no energy that opposes the collapse. This phase is that of the isothermal collapse of the duration of which depends on the initial density.
From this time the nebula becomes increasingly dense and rotates faster and faster to maintain its angular momentum. At this point, the individual fragments and portions continue to collapse, giving rise to individual bodies.

The cloud continues to collapse until a critical density value when the particles begin to exchange energy between them. And 'the adiabatic phase of collapse. The nebula begins to heat up and a force opposed to the internal collapse.
They form a body called the increasingly rich and luminous protostars, which is a fairly stable body.

The collapse continues slowly, the temperature increases until it reaches the values ​​required for fusion reactions. The body becomes a main-sequence star.

The equator has of protostar residual material that will create the planets.

It has the star when they start the reactions in the nucleus.
The stars can be placed in a Cartesian diagram called HR has the absolute brightness of the stars ordered and as the abscissa or the surface temperature or spectral color or index.

The stars in the diagram have not randomly but according to the diagonal. These are the main sequence. To the left are the most massive stars, brighter and less massive than down.
As long as the core has enough hydrogen in the star remains the main sequence. The most massive stars are less massive ones are not more.

Above the main sequence are the giants and red supergiants that are in the final phase of their evolution.
Under the main sequence are the white dwarfs are the end result of massive stars shortly.

The diagram is then to study the evolution of stars.
Important are the globular clusters, families present in the halo of ancient stars polattico. They have the same age, the same composition but different masses. The study of these stars we see the influence of the mass on the lives of stars.
The energy released in the nucleus around the core is transported by radiation and by convention in the surface layers.
In the main sequence stars the merger takes place only in the nucleus, for small stars through the proton proton chain.
The composition of the nucleus is always changing, hydrogen decreases the star is enriched with helium, which have larger hydrogen atoms.

For helium is needed to melt temperatures and higher pressures.

The longer the stay in the main sequence as the core is enriched with helium until the hydrogen fusion no longer provides the energy that promotes stability.

The star then collapses and heats up. In the core you create those temperatures and pressures needed to fuse helium into carbon atoms 3 give a helium atom of carbon.
Around the core temperature and pressure are such as to start the fusion of hydrogen.

The layered structure of the star: the reactions occur both in the nucleus around the core.

These new reactions free an enormous amount of energy that dilates the star becomes a red giant. At this point the star leaves the main sequence.

Red giants are not intrinsically very bright (because their surface temperature is low), because they are very bulky. For this reason, energy dissipates quickly.
All the stars begin to contract and become red giants at some point in the main sequence.

The next steps depend on the mass. If you are like the sun or a little higher, up to 1.44 solar masses, which is the limit Ciandraseca.
At this point the red giant quickly loses energy produced by fusion reactions that take place around the nucleus. The star collapses at this point and heat, but the temperature, which depends on the amount of mass, it is not so modest as to trigger fusion reactions.

The collapse continues until the degenerate atoms, the electrons give rise to an electronic medium in which the nuclei are immersed that you reject.

This repulsion force, electromotive force, the collapse stops and the nuclei are arranged to reach equilibrium.
The forces in the Universe are four: the strong nuclear force that binds nucleons, the weak nuclear force, which causes radioactive decay, the electromagnetic force and gravity.
The object that is formed is very bright (intrinsic), but the size of a planet, a white dwarf.

The surface temperature is very high and is due to gravity that is converted into heat.
Initially the body is very hot and very bright, but being in cold and dark energy become: brown dwarfs.

The density is very high because the enormous mass of the star is surrounded by the size of a planet.
There are red giants that are derived from very massive stars with masses up to 3.5 solar masses, are the super red giants.
Since larger mass of the central portion collapses more quickly heats up and radiates energy by exciting the surface layers of gas that give off light and increase their kinetic energy away from the white dwarf. These gases then form a planetary nebula, which has this name because it is around the white dwarf as the planets around stars.

The star gets rid of the rest mass and can collapse to become a white dwarf.

The more massive a star is longer assumes a layered structure due to repeated contractions and expansions. The graph then the star continues to move to the area from the main sequence of red giants and vice versa.
In the nucleus comes to synthesize materials that are heavier.
These are intrinsic variable stars.

This continuous movement of contraction and expansion occurs in the nucleus until the iron is not present.

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