10.5 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 M_Sun)
  • High mass: stars with a mass more than about 1.4 times the mass of the Sun (> 1.4 M_Sun)

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

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 continues 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

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