ABSTRACT
In this study of the physics of stars and their astronomical identification, It is evident that stars undergo certain physical processes as they live through their life cycle. How gravity, magnetic field and nuclear fusion play a role in stellar evolution is explained extensively. This research describes the structure and evolution of stars. The structure and evolution of a star is determined by the laws of Hydrostatic equilibrium, energy transport and generation and conservation of energy while the mass of the star is the governing factor in the evolution and structure of the star and determines its properties. Also, stellar spectra is discussed in this work. Different elements absorb different wavelengths of light. The spectrum of a star lets us know what elements are in the star. Finally, stars and how they are identified astronomically is also discussed as different stars have different astronomical identification.
CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
The study of stars is of very crucial value in astronomy as the stars are a fundamental unit of galaxies, which are very important building blocks of the universe. The physics of this stars is a relevant area that needs to be covered in order to uncover what causes stars to behave the way they behave when they are formed, during their life time and when they die.
The structure and evolution of a star is determined by the laws of Hydrostatic equilibrium, energy transport and generation and conservation of energy. The mass of the star is the governing factor in the evolution and structure of the star and determines its properties.
This study will enable one to understand the physical science behind the life cycle of stars and their astronomical identification, as it covers the physical processes (Hydrostatic equilibrium, conservation of energy, energy transport) that occur during their lifetime.
The basic units of luminous matter in the whole universe are the stars. A star is a luminous globe of gas producing its own heat and light by nuclear reactions (nuclear fusion). It is therefore important to study and to know the astrophysics behind the formation, the life, the explosion or death of stars and also their astronomical identification and properties. With respect to the formation of stars, there have been some views about how stars are formed with relation to physics over the centuries. According to McNally, (1971), there are four major groups of star formation theory which include formation under the influence of gravity, formation by random accretion, formation by condensation, and formation by processes associated with the activity of galactic nuclei. More attention is given to the formation under the influence of gravity because its conceptual framework is considered better defined than that of the others.
The formation under the influence of gravity, which is the oldest, was coined out from the universal law of gravitational attraction proposed by Isaac Newton (Larson, 2003). It inferred that stars were formed by gravitational attraction and condensation of diffuse matter in space. This concept is very old but it is only in the past century that the evidence has become convincing that the stars are being formed by condensation of diffuse interstellar matter in our galaxy and others and it is only in recent decades that we have begun to gain some physical understanding of how this happens. Observations at many wavelengths, especially radio and infrared have led to great advances in the knowledge of the physics of stars. On the intermediate scale of stars forming giant molecular clouds, turbulence and magnetic field now come into play as the most important effects counteracting gravity, which is the major force acting in space as a whole. Thermal pressure becomes very important in resisting gravity on the small scales of individual pre-stellar cloud cores (Larson, 2003). It sets a minimum mass that a cloud core must have in order to collapse under gravity to form stars. After the collapse of a cloud core begins, centrifugal force and angular momentum become in halting contraction and forming a binary or multiple system of stars. When a very small central region eventually attains stellar density, increase of thermal pressure halts the collapse and a protostar forms and continues to grow.
In this inception stage of the life cycle of a star, it is seen that there are some physical conditions that play important roles which include mainly gravitational attraction, electromagnetic waves, magnetic field, thermal pressure, centrifugal force and angular momentum, associated with it.
As a star matures it exhausts the supply of its main fuel which is hydrogen. Originally, the star undergoes a proton-proton chain reaction which allows the hydrogen to fuse, first to deuterium, then to helium. A Carbon-nitrogen-oxygen (C-N-O) reaction also contributes a large portion of the energy generation. This enormous energy generated by the core of the star exerts a radiation pressure, balancing the weight of the star’s matter thereby preventing further gravitational collapse. At this stage, the star starts becoming stable.
As time progresses, the core of the star exhausts its hydrogen supply, and the energy required to counter the effect of gravity drops until the core becomes hot enough helium fusion to begin. What happens after a star ceases to produce energy through helium fusion now depends on the mass of the star, whether it is a low mass star, a mid-sized star or a massive star.
For a low mass star, what happens has not been directly observed. But recent astrological models suggests that red dwarfs with mass 0.1 M☉ may stay on the main sequence for six to twelve trillion years, gradually increasing in both temperature and luminosity. And take several hundred billion years more to collapse slowly into a white dwarf (Sky and Telescope, 1997).
For the mid-sized star of about 0.5-10 M☉, they become red giants during two different phases of their post-main-sequence evolution: red-giant-branch stars, whose inert cores are made of helium, and asymptotic-giant-branch stars, whose inert cores are made of carbon. Asymptotic-giant-branch (AGB) stars have helium-burning shells inside the hydrogen-burning shells, whereas red-giant-branch stars have hydrogen-burning shells only (Hansen et al, 2004). These mid-range stars ultimately reach the tip of the asymptotic-giant-branch and run out of fuel for shell burning. They then contract and produce a planetary nebula with a central star. The central star then cools to a white dwarf.
The massive stars already have cores that are large enough hydrogen burning and helium ignition. They expand and cool and are much brighter than low mass stars, but do not brighten as much as the lower mass stars. These massive stars are unlikely to survive as red supergiants; instead, they will destroy themselves as typeII supernovas. Extremely massive stars lose mass so rapidly due to radiation pressure that they tend to strip off their own envelopes before they can expand to become red supergiants. If the core of the star is not too massiv0e (less than approximately 1.4 M☉, taking into account mass loss that has occurred by this time), it may then form a white dwarf.
1.2 Aims and Objectives of the Study
This study will enable one to explain the characteristics of stars at the different stages of their life cycle. It will aid one to know the elements involved in the different stages of their evolution.
The study is aimed at unveiling and summarizing the physical processes involved in the formation of stars, how they mature and live, and how they eventually die.
The study is aimed at exploring the astronomical identification of stars from different views.
1.3 Research Questions
- Stars undergo some physical processes during their evolution. What are this processes and how do they govern the evolution of stars?
- The mass of a star is determined by different factors in physics. What are the factors and how do they determine the mass of stars?
- Light and heat are emitted by stars continuously and spontaneously. How is this emission of light and heat by stars related to Physics?
- Stars generate large amount of energy from reactions that go on within the. What are these reactions and how do they lead to such massive generation of energy?
- How does the mass of a star affect its luminosity and structure when it exhausts its supply of fuel?
- What are the astronomical identification of stars?
1.4 Scope of the Study
The physics of stars is a major aspect in astrophysics. The stars are one of the major building blocks of the universe and therefore should be studied intensely from every aspect (Mathematics, Physics and Chemistry).
This study covers the physics of stars formation and the process they undergo to be born; how the stars go through different processes for continuous luminosity as they mature and the physics behind it; and the physics behind their explosion or collapse as they go into the final stage of their life cycle.
Being able to identify these stars astronomically is also very important in astrophysics. This work encompasses different identifications of stars in astronomy.
1.5 Significance of Study
Once we look beyond the solar system, most of what we can learn about the universe is based on observing stars. They emit photons profusely, and we have the means to study photons in detail. We can study their life cycles, how they are distributed in space, how they group into clusters and galaxies, their effects on the surrounding gas and dust.
Fortunately, stars are profoundly important for our universe, even though they do not dominate its total material (most is in a dark form that we know little about). Not only do they light up the sky, they produce the raw materials that make life possible, and if there is life out there, it is most likely orbiting a star on its planet. We compensate for the relatively little we can learn about a single, typical star by studying them in their diversity and trying to understand how the physical properties change in groups that may include a range of masses, ages, and so forth.
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