Astronomy Assignment #9: Stellar Evolution I
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Complete the assigned problems from the text listed below and address the Instructor Assigned Topic. Mathematical problems may be hand written. Write out the problem, show your work in solving the problem and state your answer in a complete sentence. Failure to complete all three of these tasks will result in less than full credit awarded. The Instructor assigned topic must be typed.
Review Questions from the first half of Chapter 13: Lives and Deaths of Stars
- What fundamental property of stars determines their evolution?
Mass is the fundamental property of stars that determines their evolution because mass sets the central pressure, temperature and density that controls the fusion rates and fusion rates determine luminosity, and lifetime.
- Why do massive stars last for a short time as main sequence stars but low-mass stars last a long time in the main sequence stage?
Massive stars last for a short time as main sequence stars because their higher central pressures, temperatures and densities establish a higher fusion rate in their cores. The higher fusion rates (i.e. luminosities) burn through the core hydrogen faster; thus shortening the high mass star’s lifetime. Low-mass stars last a long time in the main sequence stage because their lower central pressures, temperatures and densities establish a lower fusion rate in their cores. The lower fusion rates (i.e. luminosities) burn through the core hydrogen slower; thus extending the low mass star’s lifetime.
- How can you detect protostars if the surrounding gas and dust blocks visible light?
Protostars emit mostly IR thermal radiation as they generate energy by converting gravitational potential energy into heat during collapse. The IR thermal radiation can pass through significant amounts of dust without attenuation. Thus, the dust is transparent to IR radiation and we can “see” the stars within or behind the dust clouds in the IR.
- How do T-Tauri stars get rid of the surrounding gas and dust from which they formed?
T-Tauri stars are a class of very young (not quite main sequence) protostars that exhibit a very strong stellar wind that is believed to be an effect of the young star’s magnetic field. The effect is to propel material away from the star’s photosphere at speeds up to 100 km/s. It is this strong stellar wind the sweeps away the surrounding gas and dust from which the star formed.
- What is happening in the core of a main sequence star and why is it so stable?
In the core of a main sequence stars core h-burning is happening…that is the fusion of hydrogen into helium through the p-p chain in the core. The main sequence stars are so stable, only very slowly changing their luminosity, radius and temperature while on the main sequence, because of the natural thermostat mechanism in main sequence stars. The thermostat mechanism acts to return the core fusion rates back to an equilibrium rate in the event of fluctuations in the core fusion rate. This is known as a negative feedback cycle. For example, if core fusion rates momentarily increase, then the excess energy generated will increase the temperature of the core and cause the core to expand slightly. The resulting expansion then acts to reduce core fusion rates because of a drop in core density that lowers the chances of the nuclear collisions needed to maintain the fusion rate. Thus a small departure from the equilibrium fusion rate results in tiny changes in the cores physical characteristics that act to restore the equilibrium fusion rate.
- What happens to a main sequence star that has stopped fusing hydrogen in its core?
When a main sequence stars has stopped fusing hydrogen in this core then the balance maintained by hydrostatic equilibrium between the outward thermal pressure from the core and the inward gravitational pressure from the envelope cannot be maintained. The unbalanced gravitational pressure causes the core of the star to collapse and heat. However, even though no hydrogen fusion is possible in the collapsing core (since there is no hydrogen in the core anymore, it being all converted into helium) a thin shell of hydrogen in a shell around the collapsing core is pushed deeper into the star as the core collapses and can now fuse for the first time. Shell H-burning begins. The shell H-burning releases gamma rays that do not have to thermalize out of the core so they hit the envelope with more energy that core gamma rays would and, in effect, cause the envelope to swell to many times its previous radius. Thus when a main sequence star that has stopped fusing hydrogen in its core, energy production shifts to a shell around the collapsing core and causes the star to become a giant star.
- Are all red giants or supergiants very massive stars? Why are red giants so big and red? What is going on inside the giants?
All red giants or supergiants are NOT very massive stars. In fact, our own Sun will become a red giant and a red supergiant as it evolves through its final sequence of energy production mechanisms. Giant stars are not necessarily giants in mass, but are giants in radius. Inside all giant stars energy is being produced in shells and it is the shell gamma rays that inflate the envelope of the stars. The inflated envelope cools more efficiently due to its lower density and thus appears redder in color (corresponding to the cooler temperatures).
- What is the evolution sequence for stars around the mass of our Sun? How long is the Sun's main sequence lifetime?
The evolution sequence for stars around the mass of our Sun is a follows; GMC, Bok Globule, Protostar, Main Sequence Star, Red Giant, Horizontal Branch Star, Red Super Giant, Planetary Nebula, White Dwarf Stellar Remnant. The Sun’s main sequence lifetime is about 10 Billion years (10×109 years)
Instructor Assigned Topic
Complete an evolutionary track for the Sun on the blank HR diagram attached using the data in the table attached. Label each section of the sun’s post-main sequence evolutionary track with the sun’s current method of energy production as well as the name of the phase at the endpoint of each segment.
Post-Main Sequence Evolution of the Sun
Stage / Energy Production Method / Spectral Type / M / Surface Temper-ature / Radius in Solar Radii / Core Temperature / LifetimeMain
Sequence / Core Hydrogen Burning / G2 / +4.8
(1 L) / 5,800 K / 1 / 15 Million K / 10 Billion Years
Red Giant / Shell Hydrogen Burning / M3 / -3.6
(2,350 L) / 3,500 K / 166 / 50 Million K / 100 Million Years
Horizontal Branch / Core Helium Burning / K1 / 0
(100 L) / 5,000 K / 10 / 200 Million K / 50 Million Years
Red Super Giant / Shell Helium Burning / M3 / -3.9
(3,000 L) / 3,000 K / 180 / 250 Million K / 10,000 years
Planetary Nebula / None / 3,000 K / 300 Million K / Short
White Dwarf / None / 50,000 K / 0.01 / 100 Million K / Very Long
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