IB Physics SL E.5 Fusion and stars Questions

Practise IB Physics SL E.5 by connecting fusion reactions, mass defect, stellar spectra and energy output.

50 questions in this syllabus scope, with question parts counted together.

Exam points & syllabus

Exam points

Explain main-sequence stability using outward radiation or thermal pressure balanced by inward gravitational force, with both directions explicit.
Link core fusion and its emitted photons to the outward support when explaining why the Sun or a main-sequence star does not collapse.
Distinguish white-dwarf support from main-sequence fusion support: electron degeneracy prevents further compression in the assessed remnant.
Define fusion as combining small nuclei into a larger nucleus and identify it as a stellar energy source, rather than merely saying “burning”.
Identify hydrogen as the main-sequence fusion fuel using the specified star’s HR position; do not confuse accumulated helium with active helium fusion.
For a fuel-limited stellar lifetime, combine the usable hydrogen fraction, luminosity and energy per complete reaction, accounting for four consumed protons.
Compare stellar fusion through what fuses, the rate or core conditions, and the location of different fusion regions or shells.
Link higher core temperature to greater nuclear kinetic energy and close approach against electrostatic repulsion, where strong attraction can act.
Link high core density to collision frequency, and distinguish core temperature and density from irrelevant surface temperature or radius in the data list.
In the supplied comparison, explain why high temperature favors fusion while ordinary radioactive decay is not controlled by that temperature change.
Compare low- and high-mass pathways in matched stages: giant or supergiant, envelope loss or supernova, and the resulting remnant.
Include mass loss and the mass left in the core when explaining a white-dwarf or neutron-star/black-hole outcome.
Explain the Sun’s red-giant transition through core hydrogen depletion or contraction, shell fusion and outer expansion with surface cooling.
Predict a cooling white dwarf’s luminosity to decrease, and give supported remnant properties when the prompt asks for the final stage.
Use qualitative main-sequence positions for mass comparisons, explicitly comparing the stated stars without importing an unsupplied power law.
Plot main-sequence stars using temperature and luminosity, read the actual logarithmic scales and use the Sun’s position as a reference where appropriate.
Identify white dwarfs as hot but faint, distinguishing them from cool, faint main-sequence stars; justify giant classifications using both relevant properties.
Respect the requested star-type precision and labels; some current items require “red supergiant” and a two-star plot may require both labels for one mark.
To reject main-sequence membership by comparison, combine the much higher luminosity with comparable temperature before stating the conclusion.
Extend the constant-radius trend through the specified white-dwarf points, retaining the approximate gradient of the supplied diagram.
Draw the requested evolutionary path on the HR diagram with direction and region labels: via giants to white dwarfs for the low-mass examples.
Measure stellar parallax relative to distant background stars at observations six months apart, and define p as half the total angular shift.
Keep orbit radius and diameter consistent with the parallax angle and total shift; for small angles in radians use the matching baseline divided by distance.
Apply d(pc)=1/p(arcsec), then convert parsecs to light years, metres or AU as requested, keeping powers of ten and the final unit correct.
When an observed six-month separation is supplied, halve it before taking the reciprocal; invert a known distance only after converting it to parsecs.
Explain that larger parallax corresponds to a nearer star and a larger apparent shift; distant stars become difficult to measure because their angles are small.
Use the inverse-square relation between luminosity and apparent brightness to determine a distance when both are supplied.
Use L=4πR²σT⁴ for spherical stars, taking the square root when solving for radius and keeping radius in metres distinct from solar-radius multiples.
For radius ratios, combine luminosity ratio with the inverse temperature ratio squared; at equal temperature radius scales as the square root of luminosity.
For binary stars at the same distance use brightness ratio as luminosity ratio; if distances differ first include the distance-squared factor.
At equal temperature, greater luminosity implies greater area and radius; preserve the actual assumptions when comparing similar temperatures.
A forward Stefan–Boltzmann calculation can determine luminosity from a known radius and temperature; show the required working in a show-that task.

Covered syllabus points

E.5.1—Stellar equilibrium E.5.2—Fusion in stars E.5.3—Fusion conditions E.5.4—Stellar mass and evolution E.5.5—HR diagram regions E.5.6—Stellar parallax E.5.7—Stellar radii

Exam points

Explain main-sequence stability as outward radiation or thermal pressure balancing inward gravitational force, with both directions explicit.
Link fusion in the core to the energy or photons producing outward pressure when explaining why a star does not collapse.
For a fuller pressure explanation, describe radiation exerting outward forces on particles and gravity attracting the star’s particles or layers inward.
Use pressure and force consistently and explain the balance rather than giving the unexplained label “hydrostatic equilibrium”.

[Maximum number: 2]

The diagram shows a simplified energy-balance model for the Earth surface–atmosphere system.

The following data are given:

\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

The primary energy source of the Sun is the proton-proton (p-p) chain of fusion reactions. Four protons and two electrons produce a helium nucleus together with neutrinos and gamma photons. The overall reaction is:

4{ }_{1}^{1} \mathrm{p}+2{ }_{-1}^{0} \mathrm{e} \rightarrow{ }_{2}^{4} \mathrm{He}+2{ }_{0}^{0} v_{e}+4 \gamma

Outline the role of fusion reactions in maintaining a stable radius of the Sun.

Exam points

Define nuclear fusion using the joining of small nuclei into a larger nucleus, distinguishing nuclei from whole atoms.
Identify nuclear fusion as the source of a star’s emitted energy; “burning” alone does not name the required mechanism.
Use a star’s position on the main sequence of an HR diagram to identify hydrogen as the element undergoing fusion.
For a fuel-limited lifetime, use the stated usable fraction of hydrogen rather than the star’s entire mass.
Convert luminosity to reaction rate using energy per complete reaction, include four hydrogen nuclei per net solar reaction and convert seconds to years.
In a full Sun–supergiant fusion comparison, address the fusing elements, the reaction rate or core conditions, and the location of fusion.
Distinguish solar hydrogen fusion from heavier-element fusion in a late-stage massive star, including different fusion regions or shells.

[Maximum number: 1]

Three stars A, B and C are labelled on the Hertzsprung-Russell (HR) diagram. $L_{\odot}$ is the Luminosity of the sun.

State the main element that is undergoing nuclear fusion in star C .

Exam points

Link higher core temperature to greater nuclear kinetic energy or speed, rather than merely stating that fusion needs heat.
Explain that positively charged nuclei repel and must approach closely enough for the short-range attractive nuclear force to act.
Link high core density to a greater frequency or probability of nuclear collisions.
Select core temperature and density from a stellar data list; surface temperature and radius do not answer the core-fusion condition question.
In the supplied reaction comparison, distinguish fusion’s greater likelihood at high temperature from ordinary radioactive decay’s temperature independence.
In the legacy plasma-condition choice, identify maintaining high temperature as the assessed difficulty without claiming a modern engineering status.

[Maximum number: 2]

Star A is a main sequence star.

The data for Star A and the Sun are given in a table.

Explain why nuclear fusion in the core of the star occurs when the temperature and density exceed threshold values.

Exam points

Compare low- and high-mass evolution in matched stages: giant or supergiant, envelope loss or supernova, and the corresponding remnant.
For a low-mass star, give the sequence red giant, planetary nebula and white dwarf, including mass loss when the prompt asks for it.
Distinguish initial stellar mass from the mass left in the core when explaining a white dwarf or a neutron-star/black-hole outcome.
For the massive main-sequence star identified on the HR diagram, give red supergiant, supernova and neutron star or black hole in order.
On an HR diagram, draw the low-mass evolution path towards the giant region and then towards the white-dwarf region, with direction arrows.
Explain the onset of the Sun’s red-giant phase through core hydrogen depletion or contraction, shell fusion and outer expansion with surface cooling.
Predict decreasing white-dwarf luminosity as it cools, rather than treating a stable remnant as a constant-luminosity fusion star.
For a qualitative main-sequence mass comparison, relate the hotter star’s HR position to greater mass and explicitly compare the given stars.
When describing a white-dwarf endpoint, include supported properties such as small size, high density, high temperature or low luminosity as required.

[Maximum number: 3]

The graph shows an incomplete Hertzsprung-Russell (HR) diagram. The position of the Sun and three other stars, A, B and C , are shown.

$M_{\odot}, R_{\odot}$ and $L_{\odot}$ are the mass, radius and luminosity of the Sun.

Describe the main stages in the evolution of star A after leaving the main sequence.

[Maximum number: 1]

The diagram shows a simplified energy-balance model for the Earth surface–atmosphere system.

The following data are given:

\begin{aligned} \text { Average albedo of Earth } & =0.30 \\ \text { Average global temperature of the surface } & =288 \mathrm{~K} \\ \text { Average Earth-Sun distance } & =1.5 \times 10^{11} \mathrm{~m} \end{aligned}

The positions of the Sun and the star Antares are shown in the Hertzsprung-Russell (HR) diagram.

State the star type of Antares.

[Maximum number: 1]

The graph shows an incomplete Hertzsprung-Russell (HR) diagram. The position of the Sun and three other stars, A, B and C , are shown.

$M_{\odot}, R_{\odot}$ and $L_{\odot}$ are the mass, radius and luminosity of the Sun.

Suggest whether the method of stellar parallax can be used to measure the distance to star C.

[Maximum number: 3]

The graph shows an incomplete Hertzsprung-Russell (HR) diagram. The position of the Sun and three other stars, A, B and C , are shown.

$M_{\odot}, R_{\odot}$ and $L_{\odot}$ are the mass, radius and luminosity of the Sun.

Determine the radius of star B in terms of $R_{\odot}$ .

$L_B=L_A=10^6L_{\odot}$ .
$10^6=(R_B/R_{\odot})^2(3.0/6.0)^4$ .
$R_B/R_{\odot}=\sqrt{10^6\times16}=4\times10^3$ .

Award [3] for BCA. Do not allow ECF from MP2 to MP3.

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