Mastering Astronomy: Star Death
The ____ is the process by which hydrogen fusion proceeds in high-mass stars.
CNO Cycle
The Crab Nebula is the result of a ____ that was witnessed on Earth in the year 1054.
supernova
The diagram indicates that the third most abundant element in the Milky Way Galaxy is _____. boron hydrogen helium lithium oxygen
Oxygen (The vertical axis represents relative abundance, so the fact that oxygen has the third-highest point on the diagram (after hydrogen and helium) means that it is the third most abundant element.)
____ actually occurred about 150,000 years ago in the Large Magellanic Cloud.
Supernova 1987A
The observational data for the element abundances agree quite well with what we expect based on our current understanding of nuclear fusion and stellar evolution. But imagine the data had turned out to be different. Which of the following differences, if it had actually been observed, would have forced us to rethink our entire picture of stellar evolution? •The abundances of elements heavier than iron turned out to be much smaller than we've actually observed. •All the abundances after hydrogen and helium turned out to be smaller by a factor of 2 than what we've actually observed. •The abundance of elements heavier than uranium turned out to be greater than the abundance of carbon.
The abundance of elements heavier than uranium turned out to be greater than the abundance of carbon. (We expect carbon to be relatively common because it is formed by the fusion of three helium nuclei, and helium is the second most abundant element. We expect uranium and heavier elements to be quite rare because they form only in rare reactions associated with supernova explosions. Therefore, if elements heavier than uranium turned out to be more common than carbon, we would be forced to conclude that our model of element creation is incorrect.)
The debris from the death of a high-mass star forms a ___ several light years across.
supernova remnant
According to the diagram, the approximate abundance of oxygen atoms in the galaxy is __________. 1/10 that of hydrogen 1/100 that of hydrogen 1/1000 that of hydrogen 1/10,000 that of hydrogen
1/1000 that of hydrogen (Looking along the vertical axis, you can see that the point for oxygen lies about halfway between the points labeled 10−4 and 10−2, which means about 10−3, which is 0.001 or 1/1000. Therefore, the abundance of oxygen atoms is about 1/1000 that of hydrogen.)
Provided following are various stages during the life of a high-mass star. Rank the stages based on when they occur, from first to last.
Contracting cloud of gas and dust protostar main-sequence O star red supergiant supernova neutron star (Remember also that high-mass stars progress through all these stages at a much faster rate than lower-mass stars. The highest-mass stars may be born, live, and die in only a few million years. Note also that while this particular high-mass star leaves behind a neutron star after its supernova, an even higher-mass star may instead leave behind a black hole)
Provided following are various elements that can be produced during fusion in the core of a high mass main sequence star. Rank these elements based on when they are produced, from first to last.
Helium Carbon Oxygen Iron (During their main-sequence lives, all stars fuse hydrogen into helium in their cores. During the late stages of their lives, massive stars fuse helium into carbon, and ongoing reactions create successively heavier elements, including oxygen. Iron is the last product of fusion in a massive star's core; iron fusion does not release energy, so the production of iron is the event that provokes the stellar crisis that ends (within seconds) in a supernova.)
Listed following are characteristics that describe either high-mass or low-mass stars. Match these characteristics to the appropriate category.
High-Mass Stars: •late in life fuse carbon into heavier elements. •end life as a supernova •have higher fusion rate during main sequence life Low-Mass Stars: •the Sun is an example •have longer lifetimes •end life as a planetary nebula •final corpse is a white dwarf (A long-lived star such as the Sun eventually ejects its outer layers as a planetary nebula, leaving behind a white dwarf.)
The following items describe observational characteristics that could indicate that an object is either a white dwarf or a neutron star. Match each characteristic to the correct object.
White Dwarf: •may be surrounded by a planetary nebula. •emits most strongly in visible and ultraviolet. •may be in a binary system that undergoes nova explosions. Neutron Star: •may be in a binary system that undergoes X-ray bursts. •can have a mass of 1.5 solar masses. •may be surrounded by a supernova remnant. •may repeatedly dim and brighten more than once per second.
Carbon can be converted into oxygen in the cores of high-mass stars if carbon nuclei undergo a _____ .
helium capture reaction
Betelgeuse is a supergiant star that will eventually supernova, which means that by mass it is classified as a ____ .
high-mass star
According to current understanding, the two most abundant elements in the universe were made __________. during supernova explosions in the Big Bang by nuclear fusion in the cores of stars by nuclear reactions on interstellar dust grains
in the Big Bang (Hydrogen and most of the helium in the universe formed during the first 5 minutes of the universe's birth in the Big Bang.)
According to the diagram, what is the most abundant element with an atomic number greater than or equal to 20? oxygen calcium hydrogen iron nickel
iron (In this question you were asked to consider only data points at or to the right of atomic number 20 along the horizontal axis. In this region, the data point for iron is the highest on the graph, which means it has the greatest relative abundance. Note that the iron abundance is only a little more than 10−6=1/1,000,000 that of hydrogen. In other words, for every atom of iron in the Milky Way Galaxy, there are nearly 1 million atoms of hydrogen.)
A _____ has a density higher than the density of a white dwarf.
neutron star
Based on the diagram, which of the following statements best describes the observed pattern of abundances for elements with an atomic number between 6 and 20? • All of the abundances are about the same. • There is a general trend of decreasing abundance with increasing atomic number, but elements with even atomic numbers tend to be more abundant than those with odd atomic numbers. • There is a general trend of increasing abundance with increasing atomic number, but elements with odd atomic numbers tend to be more abundant than those with even atomic numbers. • These abundances decrease smoothly from atomic number of 6 to atomic number of 20. These abundances fluctuate randomly.
• There is a general trend of decreasing abundance with increasing atomic number, but elements with even atomic numbers tend to be more abundant than those with odd atomic numbers. (The even-numbered peaks are higher than their neighboring points, for reasons that we will explore in the remaining questions.)
In Part D, you saw that elements with even atomic numbers tend to be more abundant than neighboring elements with odd atomic numbers. What nuclear process explains why this is the case? •Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional hydrogen nucleus. •Starting from lithium (atomic number is 3), the most common nuclear reactions involve the fusion of an additional hydrogen nucleus. •Nuclei with odd numbers of protons tend to be unstable and undergo radioactive decay. •Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional helium nucleus.
•Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional helium nucleus. (Helium nuclei have two protons—which means it has an atomic number of 2—so fusing a helium nucleus into some other element increases the atomic number by two. Carbon is formed by the fusion of three helium nuclei, which is why carbon has an atomic number of 6. Fusing another helium nucleus to carbon makes oxygen, with an atomic number of 8; fusing a helium nucleus to oxygen makes neon, with an atomic number of 10; and so on. That is why even-numbered elements tend to be more common.)
