ASTRO WEEK 3 Mastering Astronomy Chapter 16

ASTRO WEEK 3 mastering astronomy chapter 16 The interstellar clouds called molecular clouds are _______ the cool clouds in which stars form Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds? infared Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud. thermal pressure What kind of gas cloud is most likely to give birth to stars? a cold, dense gas cloud Which of the following statements is probably true about the very first stars in the universe? They were made only from hydrogen and helium. Which of these colors of light passes most easily through interstellar clouds? red light What law explains why a collapsing cloud usually forms a protostellar disk around a protostar? conservation of angular momentum What can we learn about a star from a life track on an H-R diagram? The surface temperature and luminosity the star will have at each stage of its life. Which star spends the longest time in the protostellar phase of life? a 1 solar mass star What happens within a contracting cloud in which gravity is stronger than pressure and temperature remains constant? It breaks into smaller fragments. Why are the very first stars thought to have been much more massive than the Sun? The temperatures of the clouds that made them were higher because they consisted entirely of hydrogen and helium. What slows down the contraction of a star-forming cloud when it makes a protostar? trapping of thermal energy inside the protostar What effects are jets and magnetic fields thought to have on a protostar? They carry away angular momentum, helping the protostar grow in mass. Which kind of pressure prevents stars of extremely large mass from forming? radiation pressure Which kinds of stars are most common in a newly formed star cluster? M stars The five colored curves on the diagram have arrows pointing to the left. Each of these five curves represents a star of a different __________. mass The arrows on each protostar's curve on the diagram's indicate that ______ protostars change in surface temperature and luminosity as they develop Which protostars maintain nearly the same luminosity throughout the time that they are protostars? protostars with masses about 10 or more times that of the Sun Based on the protostar tracks on the diagram, which statement must be true about the Sun? The Sun was much more luminous when it was a protostar than it is today. Suppose two protostars form at the same time, one with a mass of 0.5MSun and the other with a mass of 15MSun. Which of the following statements are true? *The 15MSun protostar will be much more luminous than the 0.5MSun protostar. *The 15MSun star will end its main-sequence life before the 0.5MSun star even completes its protostar stage. The life tracks shown on the diagram for different mass protostars are based on computer models. Observationally, how can astronomers test whether these models are correct? By observing and comparing protostars and stars of different masses within a single star cluster. The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the order in which they occur, from first to last. First to last: molecular-cloud fragment, contracting cloud trapping infrared light, protostar with jets, main-sequence star The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the central temperature, from highest to lowest. highest temp to lowest temp main-sequence star, protostar with jets, contracting cloud trapping infrared light, molecular-cloud fragment Notice that the central temperature increases as the formation process progresses. This is a consequence of the law of conservation of energy: The cloud loses gravitational potential energy as it shrinks, and some of this energy is converted to thermal energy within the cloud (the rest is radiated away as light). The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on their rotation rate, from fastest to slowest. (Assume that the angular momentum of the forming star is conserved throughout the formation process, though in fact it may shed some angular momentum by ejecting material into interstellar space.) fastest rotation to slowest rotation main-sequence star, protostar with jets, contracting cloud trapping infrared light, molecular-cloud fragment Notice that the rotation rate increases as the formation process progresses. This is a consequence of the law of conservation of angular momentum: The cloud inevitably starts with some small net rotation, so as it contracts (reducing the radius of the rotation) the rotation must speed up to keep the total angular momentum the same. Each item following is a characteristic of a one-solar-mass star either during its protostar phase or during its main-sequence phase. Match the items to the appropriate phase. protostar phase: *luminosity much greater than the Sun *radius much larger than the Sun *pressure and gravity are not precisely balanced *energy generated by gravitational contraction main-sequence phase: *energy generated by nuclear fusion *lasts about 10 billion years *surface radiates energy at same rate that core generates energy As a clump of interstellar gas contracts to become a main-sequence star, its changing position on the H-R diagram tells us __________. how its outward appearance is changing Watch the red dot representing the protostar in the video. After it reaches its highest point on the diagram, how do the protostar's surface temperature and luminosity change as it approaches the main sequence? Its surface temperature increases, but its luminosity decreases. When does a newly forming star have the greatest luminosity? when it is a shrinking protostar with no internal fusion When a newly forming star is at its greatest luminosity, what is its energy source? gravitational contraction What is the approximate range of masses that newborn main sequence stars can have? 0.1 to 150 solar masses Which of the following statements about brown dwarfs is NOT true? Brown dwarfs eventually collapse to become white dwarfs. The following figures show the spectral types of four main-sequence stars. Rank them based on the time each takes, from longest to shortest, to go from a protostar to a main-sequence star during the formation process. Longest time to shortest time M6, G2, A5, O9 Notice that the correct ranking has the stars in order from lowest to highest mass (remember that for main-sequence stars, mass follows the spectral sequence order OBAFGKM). Less massive stars take longer to go through all stages of life than do more massive stars, so they also take longer to reach the main sequence. Provided following are the spectral types of four different main-sequence stars. Rank the stars based on the strength of the radiation pressure that pushes outward as they are forming, from highest pressure to lowest pressure. highest radiation pressure to lowest radiation pressure O9, A5, G2, M6 Notice that the correct ranking has the stars in order from highest to lowest mass (remember that for main-sequence stars, mass follows the spectral sequence order OBAFGKM), because radiation pressure is stronger in more massive stars. In fact, it is so strong for very massive stars that it prevents stars with masses above about 150 solar masses from forming at all. Provided following are four different ranges of stellar masses. Rank the stellar mass ranges based on how many stars in each range you would expect to be born in a star cluster, from highest number to lowest number. highest number to lowest number less than 1 solar mass between 1 and 10 solar masses between 10 and 30 solar masses between 30 and 60 solar masses Low-mass stars are born in far greater numbers than higher-mass stars, so clusters almost inevitably contain mostly low-mass stars and fewer and fewer stars of higher masses. Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once. SEE FOLLOWING QUESTIONS The Sun formed, probably along with other stars, within a large ______ molecular cloud A ______ consists of two stars that orbit each other every few days. close binary A ______ is a "star" so small in mass that its core never gets hot enough to sustain nuclear fusion reactions. brown dwarf Most of the gas remaining from the process of star formation is swept into interstellar space by a ________. protostellar wind As a protostar's internal temperature increases, its growing _______ helps slow its contraction due to gravity. thermal pressure Planets may form within the _____ that surrounds a forming star. protostellar disk Mass can be lost through a ______ of material ejected along a protostar's axis of rotation jet A "star" with mass below 0.08 solar mass has its gravitational contraction halted by ______. degeneracy pressure