Homework 4 PDF

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Homework 4 short answers...

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1) Briefly describe how a star is formed 1) Star formation is a common process in which gravity brings together matter in cold, dense molecular clouds and as gas moves inward, it converts the gravitational potential energy to thermal energy. This only happens when gravity is strong enough to overpower the outward push of thermal pressure. Gravity causes the molecular cloud to contract. It contracts until the cloud becomes too dense for thermal radiation to escape, the temperature begins to rise and the dense core becomes a protostar with the ability to conduct nuclear fusion. The cloud collapses from a large size to a small size and as this happens, it must spin very fast to conserve angular momentum. As a result, a proto-stellar disk forms around the protostar. Planets may form in the disk as the star continues to grow but eventually the stellar winds clear away the surrounding gas and a newly formed star emerges. 2) Do you think that it is possible that a 10-solar-mass main-sequence star could harbor an advanced civilization? Explain your reasoning. 1) No. This is because a 10-solar-mass has a very short life-time and the fact that it emits so much ultraviolet radiation means that its environment is not conducive to harboring living organisms. 3) Lithium, beryllium, and boron are elements with the atomic number 3, 4, and 5 respectively. Even though they are simpler elements than carbon (atomic number 6), why are they so rare compared to carbon and to many heavier elements? 1) The fusion process within the cores of stars bypasses these elements due to the fact that their atomic numbers are 3 through 5. Atomic number signifies how many protons are in an atom and the fusion that takes place in a star’s core involves the combination of three helium nuclei into one carbon nucleus. Helium has the atomic number 2 and carbon has the atomic number 6, which does not include the three elements whose atomic numbers are situated in between these two atomic numbers. 4) Briefly summarize the stages of life for a low-mass star 1) According to our textbook, a protostar begins to form when molecular clouds heat up from gravitational contraction and hydrogen fusion begins in its core. Once the star settles on to the main sequence, it fuses hydrogen in its core for10 billion years. Eventually, the core hydrogen is up and the core contracts until it is degenerate. Hydrogen fusion continues in the shell outside the core while the outer layers expand and cool. This is when the star becomes a red giant. A helium flash occurs and spreads throughout the core because the core is degenerate. Helium fusion stabilizes and this causes the star to move left on the H-R diagram. Helium fusion continues again outside in the shell outside the core, and when helium runs out again, the outer layers expand and the star becomes a red giant again. The star undergoes thermal pulses and loses its outer layers due to stellar wind. The core shrinks, heats up, yet can no longer fuse any more elements. This is the moment the star becomes a planetary nebula because heat from the core blows away and heats up the gas left over from the red giant phase. Then, the naked degenerate core is left which is a white dwarf. 5) Briefly summarize the stages of life for a high-mass star 1) According to our textbook, the beginning stages of a high-mass star are similar to those of a lowmass star except they happen much faster. A high-mass star on the main sequence fuses hydrogen to helium and stays at this stage for only several million years. They also undergo alpha-capture which creates heavier elements by fusing a helium nucleus with an existing atom. Once helium runs out in the core, the core contracts while helium and hydrogen fusion continue in the outer shells. Carbon ignition stops the core from contracting and the star moves left on the H-R diagram as carbon fusion is occurring in the core. Heavier stars continue this process and “zigzag” across the H-R diagram while heavier elements are fused in the core and used up as fuel. It isn’t until iron is produced that the core collapses because born cannot be fused to produce energy. Pressure increases causing electrons and protons to be converted into neutrons. A lot of neutrinos are released which forces the outer layers outward and causes an explosion known as a supernova. The supernova creates elements that are heavier than iron and the outer layers move away from the core very fast. Only a neutron star or black hole is left to replace the star.

6) Could our Sun ever undergo a nova or a white-dwarf supernova event? Why or why not? 1) Both novas and white-dwarf supernovas occur on white dwarfs in close binary systems. Our Sun is not a white dwarf, yet even when it becomes one the fact that it is not part of a close binary means that this could never occur. 7) Why do white-dwarf supernovae all have the same maximum luminosity? What makes whitedwarf supernovae useful to astronomy? 1) White dwarf supernovae occur when the mass of the white dwarf is more than 1.4 MSun and since the mass is the same and the entire object, which is made of degenerate matter, explodes at once, the maximum luminosity is the same. 2) They are useful in astronomy because they act as “standard candles.” A standard candle is an object for which we are likely to know the true luminosity. They are good standard candles for distance measurements because white dwarf supernova involve the explosion of stars of nearly the same mass, they are so bright they can be detected in very distant galaxies, they are common enough to detect every year, and they have similar light curves which makes them easy to distinguish from massive-star supernova. Basically, they make it easier to measure cosmic distances....