Smartworks Ch 13

Label the places on the HR diagram where one would find the different phases of stellar evolution for a solar-mass star.

A solar-mass star starts at around 1 LSun and 6,000 K. As it evolves along the red giant branch, it swells and so becomes brighter and cooler. This stops when the star reaches the helium flash, and then it quickly moves to the horizontal branch, which is hotter but somewhat dimmer. The star then grows cooler and brighter along the asymptotic giant branch. From here, the star is surrounded by a planetary nebula as it shrinks, growing hotter. Eventually a small and dim but initially very hot white dwarf is all that remains.

Correct. As a star goes through different stages at the end of its life, its energy production and size change drastically. Higher thermal pressure from nuclear fusion reactions cause the star to expand, and a drop in energy production causes it to collapse.

After the main sequence, a star blows up to a large size as a red giant, then decreases in size to become a horizontal branch star after the helium flash. During the AGB phase, it then increases in size more than ever before.

Correct. The star rises slightly up the graph, increasing its luminosity (although not as drastically as evolved low-mass stars do). It also moves to the right, which results in a larger radius/diameter and a cooler surface temperature, which leads to a redder color and thus a different spectral type.

After they leave the main sequence, high-mass stars increase in luminosity and get larger and cooler, which gives them a red color and changes their spectral type.

Rank the following stages of evolution or events in the life of a solar mass star like the Sun from earliest to latest.

All stars begin in the protostar phase while forming. They then spend the majority of their lifetime on the main sequence. Solar-mass stars evolve along the red giant branch until they experience a helium flash. This sends them to the horizontal branch until they evolve along the asymptotic giant branch (AGB). The winds during the giant phases eventually build a planetary nebula around the star. Finally, once all fusion is completed, the star collapses to a white dwarf.

Correct. Carbon serves as a catalyst in the CNO cycle, turning into nitrogen and oxygen along the way, then reverting back to carbon. As such, no new carbon, nitrogen, or oxygen is created.

Although the intermediate steps in the fusion process are different, the end result for both the p-p chain and the CNO cycle is the same: 4 hydrogen converted to 1 helium, 2 neutrinos, and light.

Correct. If the star gets large enough, the gravitational force could get so weak it would lose hold of its outer layers, which would then expand out into space.

As r increases in the equation above, F decreases—the gravitational force on the surface of the star gets weaker as it expands.

Correct. Only high-mass stars have large enough self-gravity to achieve the high densities and temperatures necessary for carbon fusion to occur.

Higher mass means higher temperatures.

Correct. This is a very useful way to get relatively accurate distances to other galaxies, if we are lucky enough to observe a supernova explosion in them.

If we know the luminosity the supernova occurs at, we can measure the apparent brightness of it and compare the two to find distance (more distant objects appear fainter).

Nuclear fusion stops at iron in the core of the star, and the iron ash core collapses under its own gravity. Electron degeneracy pressure stops the core collapse in a low-mass star, but the self-gravity of high-mass stars is large enough to overcome this, and the electrons slam into protons to form neutrons. Like electrons, neutrons are not able to exist within a certain distance of one another. As the core collapses, the neutrons hit this limit in density, and a new, extremely strong outward pressure called neutron degeneracy pressure appears. What effect might this have on the material outside the neutron degenerate core, which was previously collapsing under gravity?

It will expand outward. Neutron degeneracy pressure creates an outward force so strong that the collapsing material reacts as if it has hit a brick wall.

Correct. Even though high-mass stars have more total fuel in their cores than low-mass stars, they burn through it much more quickly because of both these factors.

More efficient reactions means more of them happening, and higher-temperature (higher-velocity) protons are more likely to overcome the repulsive electric force between them and fuse together.

Correct. Even as the thermal pressure drops, electron degeneracy pressure (which is much stronger) will stay the same, preventing the white dwarf from collapsing under its own gravity.

Since electron degeneracy pressure remains the same, it prevents the star from collapsing, and the star will stay constant in size.

Correct. This event is called a nova. It makes the white dwarf glow very brightly for a short time, and the pressure ejects material outward into a nebula. When the white dwarf runs out of hydrogen to burn, the nova stops, and material eventually falls back onto it from its companion, potentially repeating the whole nova process.

Since hydrogen burns at the lowest temperature, it will be the first to ignite in nuclear fusion on the surface.

Label the Messier objects by the type of object to the left of the label.

The planetary nebula has very bright colors and is pretty symmetric. They are so named because Messier thought the colors reminded him of planets in our Solar System. The spiral galaxy has a bulge and spiral arms. The globular cluster is a collective of stars concentrated toward the center. The star-forming nebula is colorful but less symmetric. In this case, the star-forming nebula is the very famous Orion Nebula.

Which of the following statements about white dwarfs is true?

They first appear at the center of a planetary nebula. Explanation: White dwarfs are produced during the death of a Sun-like star. When the star ejects its outer layers, they form a planetary nebula. The core of the star is left at the center of the nebula and is the white dwarf.

Correct. Since new white dwarfs are the leftover cores of stars, they are very hot. Hotter objects are brighter than cooler ones of the same size, but white dwarfs are relatively tiny.

White dwarfs have a small surface area to radiate from, making them fainter than stars in other stages that are cooler but much larger in size.

A pulsar is also

a neutron star. Explanation: A pulsar is a rapidly rotating neutron star.

A star is determined to be burning helium into carbon in a shell above its core. What mass star can it be?

either intermediate-mass or massive Explanation: Only stars with masses greater than 0.85 solar masses will have core temperatures high enough to fuse helium into carbon.

The huge amount of energy released in supernova explosions is strong enough to fuse nuclei together into elements heavier than iron. Because these elements cannot be created in ordinary stellar nuclear fusion, supernovae are the only natural source of them in the universe. Locate a periodic table of the elements such as the one in your book, and use it to determine which of the following elements are produced only in supernova explosions.

gold, copper, and silver All elements with an atomic number higher than that of iron (26) are created only in a supernova.

Which force or process is primarily responsible for creating a black hole?

gravity Explanation: The gravitational collapse of a high-mass star will form a black hole.

A star is on the horizontal branch of the HR diagram. Which of the following describes nuclear fusion within the star?

helium to carbon in the core; hydrogen to helium in the first shell Explanation: Horizontal branch stars are evolved stars and are not on the main sequence. Their cores have become hot enough that they can fuse helium to carbon. Nuclear burning also occurs in shells—a hydrogen-burning shell surrounds the core.

Which of the following accurately describes changes to a star when it first moves off the main sequence of the HR diagram?

increased radius and decreased surface temperature Explanation: When a star moves off the main sequence, radiation from the hot core causes the outer layers to expand and cool. The star becomes a red giant.

A star is 10 billion years old. What final form may it take when it dies?

only a white dwarf Explanation: A star that lives 10 billion years is a low-mass star. Its final stage of evolution is a white dwarf.