The Life Cycle of Stars
- The life cycle of a star follows 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 whether the star is:
- Low mass: stars with a core mass of less than about 1.4 times the mass of the Sun (< 1.4 MSun)
- High mass: stars with a core mass of 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
- 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
3. 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
Forces acting within a main sequence star. The balanced inward and outward forces will remain that way for millions, or even billions of years
- A star will spend most of its life cycle 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
- A star is classed as a low-mass star if it has a total mass less than 4 times the mass of the Sun
- A low-mass star will become a red giant before turning into a white dwarf
4. Red Giant
- Hydrogen supplies in the core 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 hydrogen fusion decreases, but it continues in the shell around the core
- The star initially shrinks which causes the core to become hotter
- When the temperature is high enough, helium fusion begins
- This releases massive amounts of energy which causes the outer layers to swell and cool to form a red giant
5. Planetary Nebula
- The helium supply in the core begins to run out
- The core contracts, but it does not get hot enough for further fusion reactions
- The outer layers of the star are released
6. White Dwarf
- The solid core collapses under gravity
- The remnant left behind is a very hot, dense core called a white dwarf
The lifecycle of a low mass star
Next Stages for Massive Stars
- A star is classed as a high-mass star if it has a total mass greater than 4 times the mass of the Sun
- A high-mass star will become a red supergiant before exploding as a supernova
- The remnant of the core will either be a neutron star or black hole
4. 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
5. Supernova
- The iron core collapses
- The outer shell is blown out in an explosive supernova
6. Neutron Star (or Black Hole)
- After the supernova explosion, the collapsed neutron core can remain intact having formed a neutron star
- If the remnant core has a mass greater than 3 times the solar mass, the pressure becomes so great that it 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: Plan your answer
- Make a list of the remaining stages in the evolution of a low-mass star adding any important points or keywords
| Red giant | Planetary nebula | White dwarf |
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Step 2: 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. 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: 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 | Main sequence | Red supergiant | Supernova | Neutron star/black hole |
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Step 2: 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 and gas pressure balance the inward gravitational pressure allowing the star to become stable as it enters the main sequence stage.
When the hydrogen runs out, the outer layers of the star expand and cool to form a red supergiant. The core becomes hot enough for helium fusion. Once helium fusion ends, successive cycles of expansion and collapse occur as heavier elements are fused in the core, up to iron.
Eventually, once iron has formed in the core and fusion reactions can no longer continue, the outward layers of the star collapse and the star undergoes a shockwave explosion known as a supernova.
The remnant of the core collapses further and forms either a neutron star or a black hole.
