Chapter 11: Inferring Patterns in Star Life Cycles
What makes a nebula dark? What makes an emission nebula glow?
A dark nebula is when grains of dust are so opaque that they block out light from the stars. Emission nebula are found near hot, luminous stars of spectral types O and B. These stars emit ultraviolet radiation. When atoms in the nearby interstellar gas absorb these energetic ultraviolet photons, the atoms become ionized. These ionized atoms are the energy source of an emission nebula
What happens to the size of a star if its core shrinks, increasing in temperature and density?
A shrinking core will have a higher rate of nuclear reactions causing the outer layers to shine more brightly, heat, and expand.
From largest to smallest, what is the sequence of sizes of a white dwarf, a red dwarf, and a brown dwarf.
A white dwarf is the smallest, about the size of planet Earth; a brown dwarf is in between, being somewhat larger than a gas giant planet like Jupiter; and a red dwarf is largest, being one of the smallest stable stars.
Why does a red giant appear red if it is cool?
According to Wien's law, the most common wavelength of light emitted by a glowing object becomes longer, toward the red end of the spectrum, as temperature decreases
What is a white dwarf? Does it produce light in the same way as a star like the Sun?
After a low-mass star consumes all the hydrogen in its core, mass ejection strips away the star's outer layers and leaves behind the hot carbon-oxygen core. This is called the white dwarf. A white dwarf only emits light because it is still hot.
What physical process is needed to initiate the formation of a star?
Any compressive physical process that will cause an interstellar cloud of dust and gas to collapse can start the stellar formation process.
If no one has ever seen a star go through the complete formation process, how are we able to understand how stars form?
By studying different stars at different stages of their life cycle
Which physical interaction, or force, causes the atoms to behave this way?
Gravity
Explain how its is possible for the core of a red giant to contract at the same time that its outer layers expand.
Heat from the contracting core pushes the hydrogen-fusing shell outward
Where does the increasing amount of helium come from inside the core of a star?
Helium atoms are created during the nuclear fusion process of combining hydrogen atoms near the star's central core.
Are horizontal-branch red-giant stars becoming more or less luminous?
Horizontal-branch stars are moving to the left on the H-R diagram and are increasing their temperature, but not changing their luminosity.
What is different about the core of a red giant that allows helium to fuse when it cannot occur in the core of our present-day Sun?
In a red giant, the core is at a significantly higher temperature and density that allows helium atoms to combine and fuse together, releasing energy.
Why does helium fusion require much higher temperatures than hydrogen fusion?
It has twice the positive electric charge of a hydrogen nucleus, and there is a much stronger electric repulsion between two helium nuclei than between two hydrogen nuclei. For helium nuclei to overcome this repulsion and get close enough to fuse together, they must be moving at bery high speeds, which means that the temperature of the helium gas must be very high.
What would a reflection nebula become if it could be compressed to 3 times its typical density?
It would become a dark nebula because a dark nebula has the same composition as a reflection nebula, but the fine grains of dust comprising it would block out nearly all of the distant starlight if it were compressed to a higher density.
In which population of stars is Alpha Centauri, a nearby star with a size and chemical composition similar to our Sun?
Metal-rich stars, like the Sun, are classified as Population I stars.
Was our Sun formed at about the same time that the universe originated?
No. Our Sun is a metal-rich star, which means it was formed from the remains of previously existing stars.
When the Sun becomes an AGB star, will Earth's orbit be inside the core?
No. The core of an AGB star is quite small, about the diameter of planet Earth, not the diameter of Earth's orbit about the Sun.
If a core of degenerate electrons actually depended on temperature, how would it respond to an increase in temperature?
Normal gases expand when temperature increases, which does not occur in a core of degenerate electrons
How can an astronomer tell the difference between a planetary nebula and a planet?
Planets have absorption line spectrums and planetary nebulas have emission line spectrums
What is the difference between Population I and Population II stars? In what sense can the stars of one population be regarded as the children of the other population?
Population I stars are those are metal rich. Population II stars are those that are metal poor. Population I stars formed from expelled material of Population II stars.
Do protostars increasing in brightness gain mass or eject mass during formation?
Protostars both gain mass, through dust grain collisions that slow the material causing mass to migrate into the protostar, and simultaneously lose mass, through a T Tauri wind process, during their formation.
Why do convection cells in the smallest of main-sequence stars, red dwarfs, result in such long lifetimes when they have such small amounts of fuel?
Red dwarfs have convection cells that move fresh hydrogen from the surface into the core for fuel so that nearly all of the hydrogen in a red dwarf can be used for fuel.
As the red giant moves through the planetary nebula stage to the white dwarf star stage, in which directions on the H-R diagram are the star's graphed positions moving?
The evolutionary tracks of a red giant becoming a white dwarf move from right to left as the shrouding planetary nebula diffuses and from upper left to the lower right as the star dims.
If massive O and B stars have the most fuel, why do they have the shortest lifetimes?
The inward pull of gravity in the most massive of stars compresses the core, making it hotter and more dense, resulting in dramatically higher rates of thermonuclear reactions occurring in the core.
Describe the energy source that causes a protostar to shine.
The protostar collapses when the pressure inside it is too low to support all the cool gas the mutual gravitational attraction of its part. As the protostar collapses, gravitational energy is converted into thermal energy, making the gases glow.
Why is a star fueled by a hotter core of helium fusion less luminous?
The star is less luminous because a hotter core expands, reducing the temperature of the outer shell, which slows the shell hydrogen fusion reactions, reducing the overall energy output.
A red giant with a large mass produces a _____________, leaving behind a ___________ and a _______________.
Type II supernova; neutron star; black hole
How does the chemical composition of the present-day Sun's core compare to the core's composition when the Sun formed? What caused the change?
When the Sun formed, it's composition was about 74% hydrogen, 25% helium, and 1% heavy elements. Now, the Sun's core contains a greater mass of helium than of hydrogen. These changes are as a result of hydrogen fusion in the core.
How does the size of a white dwarf change as it cools?
White-dwarf stars do not change their size as they cool because the electrons are in the degenerate state and the white dwarf cannot contract any further, regardless of temperature.
For approximately how many years will an isolated planetary nebula continue to expand?
Without slowing down due to friction with nearby dust, a planetary nebula will continue to expand indefinitely.
What are thermal pulses in AGB stars? What causes them? What effect do they have on the luminosity of the star?
a relatively short-lived burst caused by a substantial increase in the luminosity of an AGB star caused by the reignition of the helium sheell
Would you expect the temperature at the center of the protostar to increase or decrease with time? Explain your reaosning.
increase because gravitational energy is converted to thermal energy
On what grounds are astronomers able to say that the Sun has about 5.4 x 10^9 years remaining in its main-sequence stage?
looking at the rate that hydrogen is depleted from the core
Imagine that you are observing the region of space where a cloud of gas and dust is beginning to collapse inward to form a star (the object that initially forms in this process is called a protostar). Will the atoms in the collapsing cloud move away from one another, move close to one another, or stay at the same location?
move close to one another
A red giant with a small mass produces ___________, leaving behind
planetary nebula; white dwarf
What is the chemical composition of the remaining stellar core after the planetary nebula formed?
the remaining core is composed of carbon and oxygen