The Life Cycle of Stars (Edexcel International A Level Physics)
Revision Note
The Life Cycle of Stars
The life cycle of stars go in predictable stages
The exact route a star's development takes depends on its initial mass
Initial Stages for All Masses
The first four stages in the life cycle of stars are the same for stars of all masses
After these stages, the life-cycle branches depending on the whether the star is:
Low mass: stars with a mass less than about 1.4 times the mass of the Sun (< 1.4 MSun)
High mass: stars with a mass more than about 1.4 times the mass of the Sun (> 1.4 MSun)
1. Nebula
All stars form from a giant cloud of hydrogen gas and dust called a nebula
Gravitational attraction between individual atoms forms denser clumps of matter
This inward movement of matter is called gravitational collapse
2. Protostar
The gravitational collapse causes the gas to heat up and glow, forming a protostar
Work done on the particles of gas and dust by collisions between the particles causes an increase in their kinetic energy, resulting in an increase in temperature
Protostars can be detected by telescopes that can observe infrared radiation
3. Nuclear Fusion
Eventually the temperature will reach millions of degrees Kelvin and the fusion of hydrogen nuclei to helium nuclei begins
The protostar’s gravitational field continues to attract more gas and dust, increasing the temperature and pressure of the core
With more frequent collisions, the kinetic energy of the particles increases, increasing the probability that fusion will occur
4. Main Sequence Star
The star reaches a stable state where the inward and outward forces are in equilibrium
As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases
A star will spend most of its life on the main sequence
90% of stars are on the main sequence
Main sequence stars can vary in mass from ~10% of the mass of the Sun to 200 times the mass of the Sun
The Sun has been on the main sequence for 4.6 billion years and will remain there for an estimated 6.5 billion years
Next Stages for Low Mass Stars
The fate of a star beyond the main sequence depends on its mass
The cut-off point is 1.4 times the mass of the Sun
A star is classed as a low-mass star if it has a mass less than 1.4 times the mass of the Sun
A low-mass star will become a red giant before turning into a white dwarf
5. Red Giant
Hydrogen fuelling the star begins to run out
Most of the hydrogen nuclei in the core of the star have been fused into helium
Nuclear fusion slows
Energy released by fusion decreases
The star initially shrinks and then swells and cools to form a red giant
Fusion continues in the shell around the core
6. Planetary Nebula
The outer layers of the star are released
Core helium burning releases massive amounts of energy in the fusion reactions
7. White Dwarf
The solid core collapses under its own mass, leaving a very hot, dense core called a white dwarf
The lifecycle of a low mass star
Next Stages for Massive Stars
5. Red Super Giant
The star follows the same process as the formation of a red giant
The shell burning and core burning cycle in massive stars goes beyond that of low-mass stars, fusing elements up to iron
6. Supernova
The iron core collapses
The outer shell is blown out in an explosive supernova
7. Neutron Star (or Black Hole)
After the supernova explosion, the collapsed neutron core can remain intact having formed a neutron star
If the neutron core mass is greater than 3 times the solar mass, the pressure on the core becomes so great that the core collapses and produces a black hole
Lifecycle of massive stars
Worked Example
Stars less massive than our Sun will leave the main sequence and become red giants.
Describe and explain the next stages of evolution for such stars.
Answer:
Step 1: Underline the command words
Describe questions require details of the processes occurring
Explain questions require details of how and why those processes occur
This question requires both
Step 2: Identify the mass of the star
The stars in the question are less massive than the Sun, therefore it is referring to low-mass stars
Step 3: Identify the stage of life cycle being asked about
The question asks for the next stage of evolution after becoming a red giant
It requires an explanation of the processes during the red giant phase
Step 4: Plan the answer
Make a list of the remaining stages in the evolution of a low-mass star
Red giant
Planetary nebula
White dwarf
Add to the list any important points or key words that need to be included in the answer
Red giant
Fuel runs out
Forces no longer balanced
Expands and cools
Fusion coninues in shell
Planetary nebula
Carbon-oxygen core not hot enough for further fusion
Outer layers released
White dwarf
Core collapses leaving remnant core
Step 5: Begin writing the answer using words from the question stem
Low-mass stars will leave the main sequence and become red giants…
Step 6: Use the plan to keep the answer concise and logically sequenced
Low mass stars leave the main sequence and become red giants when the hydrogen in the core runs out.
Reduced energy released by fusion leads to radiation pressure decreasing
Radiation pressure and gas pressure no longer balance the gravitational pressure and the core collapses
Fusion no longer takes place inside the core.
The outer layers expand and cool to form a red giant
Temperatures generated by the collapsing core are high enough for fusion to occur in the shell around the core
Contraction of the core produces temperatures great enough for the fusion of helium into carbon and oxygen inside the core
The carbon-oxygen core is not hot enough for further fusion, so the core collapses
The outer layers are ejected forming a planetary nebula
The remnant core remains intact leaving a hot, dense, solid core called a white dwarf
Worked Example
Describe the evolution of a star much more massive than our Sun from its formation to its eventual death.
Answer:
Step 1: Underline the command word
Underline the command word ‘describe’
A describe question does not need you to explain why the processes happen, but you do need to go into detail about what happens in each stage
Step 2: Identify the mass of the star
The star in the question is more massive than the Sun, therefore it is referring to high-mass stars
Step 3: Identify the stage of life cycle being asked about
This question is about the whole life cycle from formation to death
The common first five steps are needed, and then the steps which only apply to massive stars
Step 4: Plan the answer
Use the white space around the question to plan your answer
List the stages that a massive star goes through, this will help you form your answer in a logical sequence of events
Nebula
Protostar
Nuclear fusion
Main sequence
Red super giant
Supernova
Neutron star/black hole
Step 5: Add to the list any important points or key words that need to be included for each stage
Nebula – gravitational collapse
Protostar – heats up and glows
Nuclear fusion – H to He generates energy
Main sequence – stable, forces balanced
Red supergiant – expands and cools
Supernova – core collapses
Neutron star/black hole – remnants
Step 6: Begin writing the answer using words from the question stem to begin
A star more massive than our Sun will form from…
Step 7: Use the plan to keep the answer concise and logically sequenced
A star more massive than our Sun will form from clouds of gas and dust called a nebula
The gravitational collapse of matter increases the temperature of the cloud causing it to glow - this is a protostar
Nuclear fusion of hydrogen nuclei to helium nuclei generates massive amounts of energy
The outward radiation pressure and gas pressure balances the inward gravitational pressure and the star become stable entering the main sequence stage
When the hydrogen runs out, the outer layers of the star expand and cool forming a red super giant
Eventually, the core collapses and the star explodes in a supernova
The remnant core either remains intact forming a neutron star, or the core collapses further resulting in a black hole
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