Astronomy Notes PDF

Astronomy  Chapter22 Chapter Stars and the Milky Way Stars and the Milky Way Part I  Lec. 2 2.1 Stars Formation 2.2 The Life Cycle of the Stars  2.1 The Star Formation The event of the birth and formation of stars is a daily event like the...

Astronomy  Chapter22 Chapter Stars and the Milky Way Stars and the Milky Way Part I  Lec. 2 2.1 Stars Formation 2.2 The Life Cycle of the Stars  2.1 The Star Formation The event of the birth and formation of stars is a daily event like the event of their annihilation, and this is what is called the life cycle of stars, which is about the formation of stars from dusty and gaseous gatherings in space called nebulae, and the huge gaseous gatherings that have not yet formed are described as generating stars, they are also called “incubators”. they are incubators for the nascent stars, parts of that huge cloud condense under the influence of its gravity, which leads to the emergence and formation of a star or several stars. The star birth process went through several stages: it is a process, during which part of the molecular clouds condenses under the action of self-gravity and takes a spherical shape, and that huge ball of gas and dust continues to contract, and this contraction is accompanied by a rise in the temperature of the gas. Gas usually consists of hydrogen and helium, which are the lightest elements. The high temperature of the gas continues to contract, so the atoms turn into ions and free electrons at the high temperature. This state is called plasma. The plasma ball continues to shrink under the action of its gravity and its temperature increases until it is sufficient to start the reaction of the ionized hydrogen element to form the helium element.  This interaction is called nuclear fusion, and a lot of energy is produced from it, so the star begins to shine. And then it becomes a star and this is its birth, which birth occurs when the temperature of the star's core reaches about 12 million degrees Celsius - where the nuclear fusion reaction begins. Fig. 2.1: Carina nebula (NASA, ESA, and M. Livio, The Hubble Heritage Team and the Hubble 20th Anniversary Team (STScI))  1- Molecular Cloud Shrinkage. 1- Molecular Cloud Shrinkage. These gases and dust gather under the influence of gravity (the force of gravity) that pulls them together, and this cloud shrinks to each other and continues to shrink and its density increases. This shrinkage leads to an increase in its temperature due to the transformation of the potential energy resulting from the The Operation of gravitational force into thermal energy, so the hydrogen and Star Formation dust cloud becomes spherical, or part of it becomes spherical. The period of gravitational contraction lasts approximately for a period of 10-15 million years. These newly born stars emit jets of gas along their axis of rotation. The collision of these jets with the interstellar medium results in patches of small clouds known as the "HERBIG-HARO clouds". The jets are sometimes about 1000 light-years across. Fig.2.2: Herbig–Haro object diagram Fig. 2.3:Three-color composite of the young object Herbig-Haro Example What is the peak wavelength of the emission from cool (100 K) dust? Solution Using Wien’s law, which relates the peak wavelength and the temperature max  k , T 2.9 10 3 m K max  , 100 K max  2.9105 m. The peak wavelength of 100K dust is about 30×10-6 m, or 30 microns. This is in the infrared part of the spectrum.  2- Nuclear Fusion Begins and Protostar Formation: Under the influence of intense internal pressure, nuclear fusion occurs between the nuclei of hydrogen atoms, and the first ray of heat, light and pressure is released to resist gravity. This newborn star is called the "proto-star". Soon, the process of hydrogen fusion (proton- proton chain reaction) increases, forming helium gas with heat and light resulting from the reaction waste, and the energy generated from nuclear reactions is directed towards the Fig. 2.4: A young protostar in the Orion star-formation complex surface of the star to radiate its light, announcing the birth of a new star.  everybody attracts another body in the universe with force across the line connecting the centers of the two bodies. Their intensity is directly proportional to their masses and inversely proportional to the square of the Newton's distance between them. concept of classical gravity Where, F is the force due to gravity. G is the universal gravitational constant between masses. m1 is the mass of the first particle. m2 is the mass of the second particle. r is the distance between the two particles. New Star Equilibrium: 1- Gravity It is the tendency of masses and bodies to move and gravitate toward each other, as in the gravitational pull between the earth and the sun, or between celestial bodies and each other. Gravity or the force of gravity is one of the forces affecting the universe, as it forms stars and galaxies, and it is a property of all bodies and particles. Gravity has been defined by two concepts:  Firstly, Newton's concept of classical gravity.  Secondly, Einstein defined it in the general theory of relativity.  The presence of any form of matter, energy, or torque causes a Einstein defined it in curvature in space-time, and it is a definition of relativity that the general theory of considers that spatial space and time are threads of one fabric that bend together, and because of this curvature, the paths objects can relativity deviate or change direction within this curvature of space-time. 2- Nuclear Fusion Forces Nuclear fusion is a process in which two atomic nuclei come together to form a single nucleus. The fusion of light nuclei such as the proton, which is the nucleus of the hydrogen atom, and the deuteron, the nucleus of heavy hydrogen and tritium, plays a huge role in the universe. During this fusion, a huge amount of energy is released, which appears in the form of heat and radiation, as happens in the sun.  Inside the sun, for example, the ionizing hydrogen fusion reaction takes place through stages to generate helium, at an estimated temperature of 15 million degrees Celsius to 17 million degrees Celsius, and this occurs within several Fig. 2.7: proton-proton chain reaction different reactions that result Fig. 2.6: Nuclear fusion in the sun's heat.  Each star at its beginning has a certain size that depends entirely on its mass and the type of nuclear reactions inside it. 1. White Dwarfs (Small stars), these stars continue to burn for trillions of years, a slow burning of their inner matter. 2. Main Sequence Stars (Medium size stars), like the sun, and they have average ages, which are estimated at “only” Size of the billions of years!. The current age of the sun is estimated at 5 billion years, and it will remain the same, burning hydrogen and turning it into helium for another 5 billion Newborn years. 3. Giant stars (3 to 8 times the mass of the Sun) that are candidates for exploding as a supernova and leaving behind Star a neutron star or pulsar. These stars have a relatively short lifespan, estimated at millions of years, because they burn their fuel with extreme ferocity in order to equalize the gravitational forces that want to crush the star inwards. 4. Supergiants are the more massive stars that have begun to evolve after finishing their time as a main sequence star. These stars grow in radius, and can change temperature dramatically, but they do not change much in luminosity  2.2 The Life Cycle of the Stars Every star has a life cycle: a beginning, middle, and an end. The life of a star may last billions of years. Our Sun, for example, has been around for almost five billion years and is not yet near the end of its life cycle. Using modern instruments, scientists have A Star’s Life Begins been able to study stars at different stages of their life cycle. Our knowledge of gravitational forces has also greatly contributed to understanding the life cycle of stars. A star has its beginning deep inside a massive cloud of interstellar gases and dust called a nebula. A nebula consists primarily of hydrogen and helium. The Carina Nebula is another star-forming region  The smaller a star is the longer it will live. Larger stars have more fuel, but they have to burn (fuse) it faster in order to maintain equilibrium. Because fusion occurs at a faster rate in massive stars, large stars use all their fuel in a shorter length of time. So…A smaller star has less fuel, but its rate of fusion is not as fast. Therefore, smaller stars live longer than larger stars because their rate of fuel consumption is not as rapid.