ASTR 1020 HW Assignment 4CHAPTER 212. Why is star formation more likely to occur in cold molecular clouds than in regions where the temperature of the interstellar medium is several hundred thousand degrees? Stars can form when gravity exceeds the local pressure. Pressure is higher in hot material and lower in cold material, so colder clouds put up less resistance to gravity and can collapse more readily. Because molecules are heavier than individual atoms, they move around more slowly and can congregate in a small volume of space, increasing the local matter density and thereby the local gravity.4. Describe what happens when a star forms. Begin with a dense core of material in a molecularcloud and trace the evolution up to the time the newly formed star reaches the main sequence. Formation of a dense core from the dust and gas present in the molecular cloud, Overpowering of the pressure produced by the cold materials in the core from infalling gas and gravitational pull whixh causes rapid collapse and density increase causing the core to contract. After protostar, stellar wind breaks the storing of additional particles and the new star becomes observable.7. The evolutionary track for a star of 1 solar mass remains nearly vertical in the H–R diagram for a while (see Figure 21.12). How is its luminosity changing during this time? It's temperature? It's radius? In this vertical region on the diagram, luminosity is dropping while the surface temperature remains constant. This is a stage where the material of the star is falling inward without any hindrance and since the star has less and less surface area with which to give off radiation, its luminosity is decreasing together with its radius.8. Two protostars, one 10 times the mass of the Sun and one half the mass of the Sun are born at the same time in a molecular cloud. Which one will be first to reach the main sequence stage,where it is stable and getting energy from fusion?Objects with a low mass are not able to achieve a central temperature high enough to start a nuclear reaction while high mass objects can easily achieve the core temperature needed to start the birth of a star. The object having the mass of 10 times that of the Sun will reach the main sequence before the object having one half the massof the Sun after getting energy from fusion.CHAPTER 223. Astronomers find that 90% of the stars observed in the sky are on the main sequence of an H–R diagram; why does this make sense? Why are there far fewer stars in the giant and supergiant region?A star on the main sequence means that the star is converting hydrogen to helium in thecore. Since stars are made mostly of hydrogen, this process takes approximately 90% of a star's life. Being a red giant star is a brief stage in the life of each star when the staris readjusting to the loss of energy from the fusion of hydrogen.CHAPTER 231. How does a white dwarf differ from a neutron star? How does each form? What keeps eachfrom collapsing under its own weight?A white dwarf is an electron-degenerate object, while a neutron star is a neutron degenerate object. A white dwarf would form after the planetary nebula phase of a star, while a neutron star forms just before the rest of the star experiences a type II supernova detonation.2. Describe the evolution of a star with a mass like that of the Sun, from the main-sequence phase of its evolution until it becomes a white dwarf.After the main-sequence phase ends, the star will evolve toward the upper-right area of The hr diagram as its core contracts and the outer layers expand. Then becomes a red giant and will continue to expand its outer layers. When the star stabilizes, it will lose some of its outer layers as it becomes smaller, and moves back toward the main sequence region of the hr diagram. The core will be depleted of helium, and the star once more evolves toward the upper-right area of the hr diagram as the core contracts and the outer layers expand to an even greater extent. The outer layers of the star will be gradually blown out into space by the strong winds during this period. This leads to the formation of a planetary nebula out of the outer layers of the star. The remaining mass of the star in the core continues to contract and will become a white dwarf.3. Describe the evolution of a massive star (say, 20 times the mass of the Sun) up to the point atwhich it becomes a supernova. How does the evolution of a massive star differ from that of the Sun? Why?A massive star will leave the main sequence once its core hydrogen is depleted. After the innercore becomes iron, which requires energy to fuse, the star will experience a core collapse and create a neutron star from its core. The rest of the star explodes into a supernova. The evolution differs from the fusion history a massive star can fuse more elements and create more fusion products. A solar-mass star takes a long time for each stage of its evolution compared to the massive star. A solar-mass star losing mass via the planetary nebula phase, while a massive star undergoes a supernova eventCHAPTER 244. If a black hole itself emits no radiation, what evidence do astronomers and physicists today have that the theory of black holes is correct?Black holes in binary star systems leave signs of their presence on neighbor stars that have been detected. These signs include X-ray emissions, accretion disks, and large orbit perturbations.7. What is an event horizon? Does our Sun have an event horizon around it?An event horizon is a spherical surface that encompasses the black hole. This name expresses the fact that everything within the black hole is beyond our ability to see. The Sun does not havean event horizon around it.14. Look elsewhere in this book for necessary data, and indicate what the final stage of evolution— white dwarf, neutron star, or black hole—will be for each of these kinds of stars.A. Spectral type-O main-sequence star: black holeB. Spectral type-B main-sequence star: neutron star or white dwarfC. Spectral type-A main-sequence star: white dwarfD. Spectral type-G main-sequence star: white dwarfE. Spectral type-M main-sequence star: white