SES4U1: Star Structure, Formation, and Evolution
Star Structure and Formation: The Big Picture
 Stars are ______________________________.
 Stars exist because of ______________.
 They shine because _______________________.
 They continuously ________________________________________.
 They _____________________________________.
 They form because gravity makes ________________________________________.
 They die when they have no more fuel, and ______________________________.
Stellar Structure – Main Sequence (MS) Stars
 ________________________________  _________________________________.
 Temperature and pressure are related  __________________________________________________.
Hydrostatic Equilibrium – Stellar Structure
 Stars _______________________________________________.
 It should cool ___________________________________________________.
 BUT... Sun has been stable for 5 billion years!
 What’s the energy source to maintain equilibrium? _________________________!
Energy Generation in the Sun
 Energy is generated through nuclear fusion in the CORE of the Sun.
 What are the conditions there?
 _______________________________.
 How hot are stars?
 Sun  ____________
 But... this is a _____________________________.
FUSION
 Fusion  ___________________________________________________________.
 Unlike chemical reaction – not just the electrons involved.
 Centre of Sun  only nuclei and electrons, separated.
 _____________________________________________.
 Why don’t positive protons repel one another by electrostatic repulsion?
 ___________________________________________________.
Fusion and Sun
 Sun transforms 5000 LBS of matter into energy EVERY SECOND!
 Over lifetime of Sun, 1/10 of Sun’s H is converted to He, so 0.07% of Sun’s mass will keep it shining 10
billion years  ___________________________________.
 Why isn’t fusion happening all the time?
 ONLY happens when temp ______________________________________________________________
_________________________________.
 Remember: NOT a chemical reaction! TRANSFORMS the elements.
Stellar Structure
 How do we figure out the structure of a star when we can only see the surface?
 Scientific Models:
 BUILD a model based on our “rulebook”.
 COMPARE calculated properties with observed properties.
 IF they DIFFER, adjust the “rulebook”.
 What’s the “rulebook”?
 Imagine a star is divided into a series of shells.
 Apply _______________________________________________________________________________.
Stellar Structure - Ingredients
 Total mass = _______________________________________.
 Amount of energy flowing out of a shell = ____________________________________________.
 Hydrostatic equilibrium (__________________________________________).
 Energy travels from hot to cool regions by ____________________________________________.
Pressure-Temperature Thermostat
 In a star, inward pull of gravity balances the internal pressure.
 As the star loses energy, _______________________________________________________________________
______________________________________________.
 If reactions begin to produce too much energy:
 ____________________________________________________________________________________.
 If reactions slow:
 _____________________________________________________________________________________
_______________________________________________________.
Mass-Luminosity Relation Explained
 Remember that most massive MS stars are also the most luminous?
 Explained by _____________________________________________________.
 More MASSIVE star:
 More _____________________________________________.
 ______________________________ at centre.
 ______________________________ at centre.
 Higher T + higher P = ______________________________.
 More energy generation = __________________________.
Stellar Births
 Stars form from material between the stars – the ______________________________.
 ISM ___________________________________ other elements.
 This is ~ the same composition as Sun!
 Any cloud of dust/gas is a ___________________________.
Interstellar Medium
 How do we know there’s stuff between the stars?
 We see it!
 Emission from ________________________________________________.
 ______________________  block starlight.
 ______________________  excited by starlight, but emits its own light.
 ______________________  reflects starlight.
From Nebula to Protostar
 Stars form from the collapse of “dense” (~1000 atoms/cm3) ________________________.
 Cloud has few _______________________________________ of material.
 Temperature ~ 10K (____________).
 Why do they collapse? __________________
 Sitting in gravitational equilibrium, compressed slightly, gravity takes over!
 Converts _________________________________ to ____________________ (infalling material heats up).
 Cloud fragments as it collapses – each ______________________________, emitting radiation because it is hot.
From Protostar to Star
 What slows and eventually stops the collapse? _____________________
 Gas falls in, heats up – _____________________________!
 Three kinds of pressure:
 _____________________________________________________________
 _____________________________________________________________
 _____________________________________________________________
 When the temperature rises high enough, FUSION begins, and ___________________________!
 Surrounding gas/dust ______________________________.
Formation Timescales
 Stars with the ___________________________________.
 Gravity collapses the cloud ________________________________.
Stellar Evolution
 “Evolution” means what happens to a star ________________________________________________________.
 How can we see this, since we don’t see any single star evolve significantly during our lifetime?
 Observe ___________________________________ and try to piece together the story.
 Like taking a snapshot of the human population and figuring out how humans age.
Rules of Stellar Evolution
 Births of stars governed by _______________________________________.
 Structure of MS stars governed by the same gravitational equilibrium.
 EVERYTHING that happens to a star, from birth to death, is governed by _________________________________
________________________________!
Main Sequence Lifetimes
 Once a star is born, how long does it live on the Main Sequence?
 Stays on the main sequence _______________________________.
 Eventually, _____________________________________.
 Energy generation changes  ________________________.
 Stars spend ______________________________ on MS.
 More massive stars use up __________________________, so run out of _________________!
 They spend ______________________________!
Post-MS Stars
 What about the other stars on the H-R diagram?
 These stars have ___________________________.
 Core out of H  ______________________  no energy generated there  ___________________.
 H is still fusing (“burning”) in a shell.
 ________________________________________________________________________________________
Post-MS: He Fusion
 Core collapses, heats, and _______________________________________.
 Luminosity  (____________________________________________).
 _____________________________________________.
 Since force of gravity (mass) hasn't changed, _________________________________________.
 Expansion causes outer layers to cool.
 Star gets _____________________________!
Red Giants – Now What?
 Eventually, RGs will run out of He in core!
 He burning won’t last as long as H “burning” because _______________________________________________.
 _______________________ controls what happens next!
Fate of “Low-Mass” Stars
 Stars like Sun are considered ___________________.
 Left with ___________________.
 _______________________.
 Outer When they run out of He in core:
 Layers blown away _______________________________________________.
 Collapsed core is called a __________________________________________________.
 Outer layers called a __________________________.
 NOTHING to do with planets!
“Low-Mass” Star: Evolutionary Track
 Stars run out of H at centre, ____________________________________.
 Run out of He at centre, _____________________________________________________________.
White Dwarf Stars
 Continuing the story of “low-mass” stars like Sun – what is this white dwarf that remains?
 ___________________________________________________________________________________________
____________________________________________.
 Degeneracy Pressure  ___________________________________________________.
Degeneracy Pressure
 Quantum Mechanical Effects – important for high density
 Pauli Exclusion Principle  ____________________________________________________.
 Properties:
 Degenerate material ______________________________.
 ____________________________________________ (unlike normal gas) – only depends on E levels.
 Add mass  _________________________________________________________________________
White Dwarf Stars
 Supported against gravity by pressure of “degenerate electrons”.
 Can never be __________________________________________.
 Shining because _________________________________________________.
 Generate ___________________________________________.
 DENSE: ~ mass of Sun in object ~ size of Earth!
 ~1 cm3 would weigh ~1000 lbs on Earth.
Fate of “High-Mass” Stars
 Remember, evolution of stars depends on MASS – we’ve been discussing the fate of stars like Sun.
 High-mass stars go through similar initial stages, but _________________ (remember M-L relation!).
 Runs out of H in core, ______________________________________, meanwhile outer layers expand and cool
 red SUPERgiant
 Core becomes hot enough to fuse _____________________________________.
 As the core runs out of each, it __________________________________________________________________,
resulting in an “onion-like” structure.
 Finally, _______________________________________.
 __________ is most tightly bound nucleus.
 No reaction (either fission or fusion) results in energy generation.
 ________________________________________________________!
Death of a High-Mass Star
 Core still supported by _________________________________.
 BUT matter is raining down from above.
 Eventually, _________________________________________________.
 Collapsing core becomes a _____________________________________________________________________
____________________________________________________________________.
 Envelope blasted apart in a ________________________.
Neutron Star
 Core (_______________________) supported by _______________________.
 Young ones have ________________________________________________.
 A beam of light comes out of the magnetic poles if we're in the beam, we see pulsing. (Pulsar)
Black Hole
 ________________________________________.
 Star collapses to a single point? (Singularity)
 Event horizon  ________________________________________________.
 The only things we can find out about a BH are ________________________.