Stellar evolution extension IB DP Physics Study Notes - 2025 Syllabus
Stellar evolution extension IB DP Physics Study Notes
Stellar evolution extension IB DP Physics Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand
the conditions leading to fusion in stars in terms of density and temperature
the effect of stellar mass on the evolution of a star
Standard level and higher level: 4 hours
Additional higher level: There is no additional higher level content.
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Our Solar System
The Sun
The Sun, a small and relatively insignificant star, classified as a G2V
The Sun sits at one of the foci of the elliptical orbits of the objects moving around it.
332900 x the mass of Earth
108 x the diameter of Earth
Nuclear Fusion in Stars
As we have seen in topic 7, the combination of two lighter elements to form a heavier element causes a liberation of energy (Nuclear Fusion)
In stars, we see the conversion of hydrogen into helium. Later in the development of the star, helium and other atoms are fused into heavier elements.
Fusion requires extremely high temperatures on the order of 107K
Balance is Key
The fusion will produce radiation, gamma photons and neutrinos, that will in turn collide with surrounding protons and electrons, thus transferring energy.
This radiation pressure acts to stabilize the Sun against gravitational collapse, creating a kind of equilibrium.
Beyond Our Solar System
Stellar Cluster (aka globular cluster)
- A group of stars that are physically close to each other in space
- These are created by the collapse of the attraction
- The stars are held close by gravitational attraction
- Can contain thousands or possibly millions of stars
Other Stuff in the Universe
- Galaxies – vast collections of billions of stars
- The core of most galaxies is thought to be a supermassive black hole
- A cluster of galaxies can contain thousands of galaxies, though ours contains very few
- Nebulae – large amounts of gas and dust that are thought to be the birthplaces and/or remnants of stars
Did you know…
- The only object that naturally orbits the Earth is our single moon… despite the way things look in the sky?
- Indeed, the Sun appears to orbit us because the Earth rotates
- This is also why the stars in the night sky seem to move
Stuff We See
- The length of the “day” changes based on our axial tilt and position relative to the Sun
- Certain “planets” seem to move relative to the fixed stars behind them. Some even experience “retrograde motion” because they orbit the sun as we do but at a different rate.
- The moon, Venus, and Mercury all experience phases because of their position relative to the Sun and to Earth
The Light Year (ly)
- A measure of distance, not of time
- Defined as the distance that light travels in one year (~3.2 x 107 seconds) at a speed of ~3.0 x 108 m/s
- This translates to about 9.46 x 1015m
- This is the unit that is most often used when dealing with interstellar distances
Wow, that’s far…
- Light takes only 328 minutes (.0006 ly) to reach the Kuiper Belt area from the Sun
- Light takes 4.2 ly to reach the closest star to our Sun
- Generally, the separation between stars in our galaxy is on the order of 1017m
- The separation between galaxies in a cluster is on the order of 1023m
- The separation between clusters is on the order of 1024m
- Despite only changing the exponent by a small number it is important to note that this translates to a huge increase in distance!
It’s the ship that made the Kessel Run in less than 12 parsecs
Types of Stars
- Main Sequence Stars – ordinary stars, similar to our sun, make up 90% of all stars
- Red Giants – very large, cooler temp than our sun
- White Dwarfs – much smaller than the sun (earth size) but much hotter
The Fate of Stars
- Neutron stars – have undergone gravitational collapse and are now mostly neutrons at their core. These will generally remain neutron stars after a supernova when the mass of the collapsed star is between 1.5 and 3 solar masses.
- Supernovae – when a neutron star’s core cannot collapse further, the outer layers get reflected back outward causing a huge shock wave, blowing the outer layers of the star away.
FYI:
In a supernova, the star will tear apart (mostly) and send out a shock wave. This can cause a flash of brightness over 100x greater than the whole universe!
Black Holes – the mass of the core of the dying star is so great that the gravitational force is strong enough to overcome the neutron degeneracy pressure, causing it to collapse, entirely. Their gravity is sufficiently massive to prevent even EM radiation from leaving the surface
The Hertzsprung-Russell (HR) Diagram
- A plot dealing with Spectral Class, Luminosity, Temperature, and Absolute Magnitude, and star type…or at least a few of these at a time
- Notice that the Scales on the axes are non-linear and that temperature is plotted from high to low
Estimating Luminosity
- For the stars beyond the distance at which parallax is useful, we can use the measureable brightness and the spectral type of the star to estimate luminosity using Wien’s Law and the HR diagram
It’s Better to Burn Out than to Fade Away
- The mass of a star also has a great impact on how long its fuel source (the hydrogen in the core) will last.
- Our Sun has been burning for roughly 5 Billion years and will continue to burn for another 5 Billion years.
- A star of 25 MSun would exhaust its supply of hydrogen in just about 1 Million years. This is because the higher mass results in the core temperature being hotter and so more fusion can occur.
Our Sun Can’t Live Forever
- Our Sun represents a typical main sequence star that is destined to become a red giant star.
- This is a multi-phase process that occurs at different layers within the star.
- Our Sun will one day exhaust the supply of Hydrogen at its core. However, throughout its life it radiates energy into its outer layers.
- As such, when hydrogen fusion ceases in the core, it can continue in the surrounding material.
Core Values
- Without fusion in the core, gravitational forces do not encounter resistance and so the core will contract.
- This will raise the core temperature and in turn increase the energy flow to the outer layers.
- This will mean that the core will contract but the outer layers will expand.
- At this point, the surface will cool and the luminosity will go up (2000x its present value).
- The Sun’s new size will cause it to engulf the orbits of Mercury and Venus.
It’s Not Over Yet
- Fusion of Hydrogen in the outer layers will produce Helium which in turn increases the mass of the core, causing further contraction.
- This will yield a temperature sufficient enough to fuse Helium and create products like Carbon-12 and Oxygen-16, which is where most living tissue on Earth gets its Carbon from.
- Helium burning at the core will eventually cease and the energy radiated by the next core contraction will allow for Helium fusion in the outer layers, thus further expanding the Sun.
Just Die Already…
- This second red giant phase will engulf the Earth’s orbit and produce a luminosty 10000x greater than the Sun had presently.
- This phase is also associated with periodic burst of luminosity in which the outer layers of the Sun are ejected into space leaving an exposed core with a temperature of 100,000 K
- The exposed core, which is now the size of the Earth, will have no fusion occurring and thus will simply cool down. It will no longer contract because the gravitational force is not enough to exceed the outward electron degeneracy pressure (electron repulsion because they can’t be in the same quantum state)
- This White Dwarf star will eventually just fade from sight.
Limits
- A star like our Sun will not have a core that contracts forever. At some point, the electrons won’t be able to be packed any closer.
- A White Dwarf has an upper limit for mass at which point any greater mass than this limit will mean there is too much gravity for the electron degeneracy pressure to be enough to keep it from collapsing.
- This limit, which is 1.4 MSun is known as the Chandrasekhar Limit.
- Generally speaking, stars of mass 8 MSun or less end up as white dwarf stars.
- Stars with mass between 4 MSun and 8 MSun are able to fuse carbon and thus produce heavier elements like neon, sodium, magnesium, and oxygen during their red giant phase.
- Neutron stars have a special upper limit to their mass, just as White Dwarves do. This is known as the Oppenheimer-Volkoff Limit. We will come to this soon.
Giant vs. Supergiant
- Stars of 8 Solar Masses or more are able to fuse even heavier elements and thus yield elements all the way up Iron.
- This fusion happens in layers within the star just as we saw in our own Sun with each layer representing another red giant phase.
- Fusion ceases with Iron because to fuse iron actually absorbs more energy than it produces due to the massive coulomb forces that must be overcome.

What’s Next For Our Giant Stars?
- When the fusion at the core has ceased, the core contracts very rapidly. This high temperature releases gamma-photons which collide with the iron nuclei and break them into alpha particles.
- Almost instantly, there is so much density in the core that the electrons begin to combine with protons to make neutrons and neutrinos.
- This results in a mass release of energy from the core but also a dramatic contraction.
Regular Novas are Lame by Comparison
- Here we see a huge pressure wave going outward while the stellar material of the outer layers is trying to collapse inward. The pressure wave wins and accelerates outward ripping the star to pieces, resulting in a Supernova!
- The energy release is sufficient enough to allow for the fusion of other elements and thus we see that all elements heavier than iron are in fact generated in supernova explosions.
What’s Left Over?
- The core of the star is now made up entirely of neutrons and is packed together as tightly as in a nucleus. (a cubic cm of a neutron star would have a mass of roughly 300 – 400 billion kilograms!)
- The force that keeps the neutron star from collapsing even further is the neutron degeneracy pressure. If the mass of the neutron star is less than the Oppenheimer – Volkoff limit (currently estimated to be 1.5 – 3 solar masses) then it will remain a neutron star.
- If the mass exceeds the Oppenheimer – Volkoff limit, the gravitational force will overcome the neutron degeneracy pressure, further collapsing into itself and becoming a black hole.
The Evolution of a Star
IB Physics Stellar evolution extension Exam Style Worked Out Questions
Question
What is the sequence for the evolution of a main sequence star of about 2 solar masses?
A. Red super giant $\rightarrow$ supernova $\rightarrow$ neutron star
B. Red giant $\rightarrow$ planetary nebula $\rightarrow$ white dwarf
C. Red giant $\rightarrow$ supernova $\rightarrow$ white dwarf
D. Red super giant $\rightarrow$ planetary nebula $\rightarrow$ neutron star
▶️Answer/Explanation
Ans:B