|موضوع: دوره حياه النجوم 26/11/2010, 00:01|| |
في هذه الموضوع عرض لحياة النجوم من التكوين حتى نهاية حياتها , و يحتوي بعض التفصيلات الجيده التي من الممكن ان تكون مفيده في مساعده الطالب على فهم ماده علم الفلك في الجامعه.
هذا الموضوع هو عباره عن تجميع للمعلومات من مرجعين جيدين و معتمدين دراسيا :
2) Astronomy: a journey to the cosmic frontiers
ارجو ان تستفيدو من هذه المعلومات و انا جاهز لأي سؤال يخطر في بالكم و ارجو من الله ان يقدرني على الأجابه.
2.0 ……..The Formation of the stars
2.1……Stellar Cloud Stage
2.2…… Protostar Stage
2.3…… Young Star Stage
3.0…… The Evolution of the Star
4.0…… The End of Life of the Star
4.1 ……. The white Dwarf
4.2 …. . The Neutron Star
4.3 …… The Black Holes
5.0 …… Summary
6.0 ……. References
I want to thank Dr.Nizar Hamory for giving me this chance to do this seminar, and I want to thank Dr. Tareq Al Abdallah, the Astrophysics doctor in the Physics Department for helping me.
The stars are basic part of out universe specially for our dear planet earth, the very beginning of the stars still unknown, but there are some theories that try to explain the source of the initial material that form the first generation stars .
The first generation stars their life from the moment the nuclear fuel start to work inside the star, the star will go through its life by using its nuclear fuel until it runs out of its fuel.
When the star relatively deplete its fuel, it starts to die in three ways which in turn depend on the mass or the presence of the companion star (binary system), the previous factors determines the way the star will die, but eventually the star will explode producing the initial materials for other stars to form by forming stellar clouds form these materials, these clouds give the birth to the new stars to form and to stars the previous cycle again.
2.0 The formation of the stars:
The formation of the star comprises three stages:
1) Stellar cloud stage.
2) Protostar stage.
3) Young star stage.
2.1 stellar cloud stage:
The stellar cloud is the cradle for the newly born stars called protostars, the stellar cloud composed of H and CO and large amount of dust and gases like NH3, we better call the stellar cloud molecular cloud, because its cool and dense enough that the gasses inside the cloud are molecules rather than ions or atoms.
There are range of sizes for the molecular clouds, and the most likely to form stars are the type called Giant molecular clouds, which contain 10^6 times the mass of the sun, these clouds comprises smaller areas where the density are the most, which represent the cloud cores ;the areas where the contraction occurs as the result of the attraction that each molecule produce and effect on the other molecules , but this process is not enough for the contraction to form the protostar , so there must be a supply of heat that increase the motion of the molecules and helps the formation of the protostar inside these cores, the source of this heat comes from a companion star that give heat to the cores and aid in the formation of Protostar.
The cloud cores occur through out the molecular cloud, but the most likely to form a protostar is the ones at the center of the clouds; because the mass density is the highest in the middle, which in turn gives the highest friction between the molecules to form the protestar.
The size of the protostar depend on the size of the core that form the protostar, here we have to notice that the cores are two types:
1) Warm cores: the more hot and larger in size and form more massive protostar, and even more number of protostars,
2) Cold cores: the more cold and smaller in size, and form less massive protestars.
2.2 The protostar stage:
The protostar is the a body form inside the cloud core as the result of the contraction of the molecules under the influence of their gravity in the center of the cloud forming initial mass inside the core represent the initial mass of the star that will grow to develop the star that we know now.
The protostar will grow and become more massive as more matter fall into it , on the other hand, more massive it become, the faster in rotate around it self, as the result of the angular momentum that is proportional to the diameter of the body .
When the matte falls into the protostar is small amount in the beginning , the protostar become transparent, so the IR radiation result from the friction of the molecules will go through it and escape, leaving the core cold , as the matter increase in the protostar increase- specially dust- the protestar become opaque and will block the radiation causing the temperature and pressure to increase, this process will continue until the pressure become enough to balance the mass of the protostar.
At this level the protostar will become a considerable mass rather than a cloud, when this level is reached, the rapid contraction stops and the protostas will produce a strong winds that will stop the infalling process and form rotating disk of dust and gasses.
It’s important to talk about the energy source of the star in this stage, especially that there are no nuclear reactions inside it because it’s too cold for this reactions, so the source of energy is the contraction that produce the energy radiate outward to support this initial mass, this contraction called Kelvin-Holmtz contraction.
2.3 the young star stage:
In this stage, the star will produce strong winds that stop the matter to infall and than gradually work to blow away the whole disk of dust to produce the visible star; these stages are:
1) The winds blow in the polar axis direction because there is no resistance in this direction.
2) The winds start to spread and remove part of the disk.
3) Finally, the winds will splash in all directions causing the whole disk to vanish leaving the star visible and it will enter the main sequence star series.
In this stage the nuclear fusions which represented in the H burning to produce He, and then the protostar become a star in the main sequence as the first nuclear reaction took place.
The protostar will become a star when the temperature of it increase to 10^6 K , so the H burning will start the core of the star.
3.0 the evolution of the star :
The evolution of the star starts from the moment the H burning start to support the mass of the star.
The star will use nuclear fuels like (H, He, N, P) respectively, now let’s show the stage represented by each type of the fuel used:
1)The first nuclear fuel use is the H by H-burning at the temperature of 10^6 K, at this stage the mass of the star is well supported against the pressure of the gravity that pulls inward, this stage is the most important in the evolution because it enters the star into the main sequence stars.
2) The second nuclear fuel used is the He by He (alpha chain process and triple alpha chain process). At this stage, the support is little bit lesser than the support of previous
3) The third nuclear fuel is used is the Neutrons by the processes (R-process and S-process). This stage represent the period after the relative depletion of the H and He-the main nuclear fuels – and the support represented by this type is used after the supernova of the envelop of the star.
4) The fourth nuclear fuel is the protons which represented by the processes (Proton capture process). This stage become active after the supernova of envelop of star.
4.0 The end of life of the star:
There are three ways the star can end his life with, after the depletion of the nuclear fuel;
These ways are:
4.1 The White Dwarf:
this type is basically related to the binary system stars , where the transfer of material is critical to form such a star, and also this type is the result of the less massive stars (less than the 1.5 the mass of the sun) .
the star with the small mass will be coated with the material that transferred to its top from the companion red giant star, the result of this addition of material will increase the temperature and pressure until it become more than the gravitational force, then the star will explode away the envelop of the star leaving the star’s core alone , in a process called(SUPERNOVA TYPE 1), now the core will contract because there is no H or He, until it become so dense that the electrons become close together, and will repulse forming a new force against the gravitational inward pull.
The white dwarf is in the size of a planet and very dense and luminous. When the white dwarf runs out of its energy, it become dim with no luminosity and called Black dwarf.
4.2 The Neutron Star:
This type of stars is related to the isolated Giant stars –more massive than 1.5 the mass of the sun- the continuous contraction of the core and the continuous expanding of the envelop as the result of the high percentage of H in the envelop which will produce IR that will make it expand until this expansion become out of control relative to the gravity of the core, so the envelop will explode away in process called (SUPERNOVA type 2)
The core will continue expanding until it become so dense and smaller in size of the White dwarf , then the Neutrons become too close so the repulsion starts and this will create anew force against the gravitational inward pull.
Note: the repulsion occurs between the Neutron-Neutron and Electron –Electron is called (Pauli Exclusion Principle).
4.3 the Black Holes :
This type of the stars is similar to the Neutron star but it happened in more massive stars and the remnant of the supernova is smaller and denser. The star`s envelop will expand and the core will contract until the envelop become out of control to the gravity of the core, then the envelope will explode away producing Supernova type 2 and the remnant is in the size of the baseball but it has a very strong gravitational attraction as the result of very high density, even the massive stars or the light cannot escape from its gravitational field.
1) The stars start their life form the contraction of the cores of the stellar cloud.
2) The protostar formed at the core will grow and its temperature and pressure will increase until it become a young star.
3) From the moment the star start’s the first nuclear reaction when its temperature reach 10^6 K, the star produce winds that remove the rotating nebula disk and the star become a main sequence star.
4) As the star use it s nuclear fuels, it shrinks more and more until the nuclear fuel depletes.
5) After the depletion of the nuclear fuel, the star will die in three ways depending on the mass and the presence of the companion giant stars.
6) The force that hold the star’s core after the supernova occur is the Pauli exclusion principle (N-N and e-e).
- Astronomy: A Journey to the Cosmic Frontier ;
- chapter 18 (the formation of stars and planetary system),
- chapter 19 (the evolution of the stars),
- Chapter 20 (White Dwarf, Neutron Star, Black Holes).
|موضوع: رد: دوره حياه النجوم 26/11/2010, 04:59|| |
|موضوع: رد: دوره حياه النجوم 18/2/2011, 08:15|| |