E.5 Fusion and stars questions for IB Physics

74 questions on Fusion and stars, written by Augment in the style of IB exam papers. Six of them are below in full, to answer on paper.

IB Physics
Part of E Nuclear and quantum physics
SL and HL

Questions by paper and level

SL and HL questions are for students at either level. HL only questions cover what only HL students study.

Paper	SL and HL	HL only	All
Paper 1A	34	0	34
Paper 1B	4	0	4
Paper 2	28	8	36
Every paper	66	8	74

Counted on 10 November 2026

Questions to try

Written by Augment in the style of IB exams, not past papers. Answer them on paper, then check each one against its markscheme with a free account.

Question 1 · PHY-00259Paper 1A · 1 mark

A red giant star has a luminosity of $100 L_{⊙}$ and a surface temperature half that of the Sun. $L_{⊙}$ is the luminosity of the Sun and $R_{⊙}$ is the radius of the Sun.

What is the radius of the red giant?

A
$2.5 R_{⊙}$
B
$20 R_{⊙}$
C
$40 R_{⊙}$
D
$1600 R_{⊙}$

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Question 2 · PHY-01527Paper 1B · 11 marks

A group of students model the measurement of stellar parallax in the laboratory. A small sighting device can be placed at either of two positions, A and B, on a straight rail. The distance AB is the baseline $b$ . A vertical pin stands at a perpendicular distance $d$ from the midpoint of AB. A mark on a building about 200 m away, seen through an open window, plays the part of the distant background stars.

View from above, not to scale. Sighting positions A and B sit on a rail a baseline b apart. A pin is a distance d from the rail on the perpendicular bisector of AB. From A and from B, angle p lies between the line to the pin and a dashed line towards the distant marker.

From each of A and B the students measure the angle $p$ between the direction of the pin and the direction of the distant marker. They repeat this for six values of $d$ . The angles are measured with an uncertainty of $\pm 0.2 °$ . The graph shows the variation of $p$ with $\frac{1}{d}$ , with the line of best fit. The uncertainty in $\frac{1}{d}$ is negligible.

Graph of p in degrees against 1/d in m^-1. Six points with error bars of 0.2 degrees: (0.25, 2.2), (0.333, 2.8), (0.40, 3.5), (0.50, 4.2), (0.667, 5.8), (1.00, 8.4). A straight best-fit line runs very close to the origin.

(a)
Outline why the marker must be very far away compared with the pin.
1 mark
(b)
Calculate the largest percentage uncertainty in $p$ for these data.
2 marks
(c)
Explain why the students plot $p$ against $\frac{1}{d}$ .
2 marks
(d)
Determine $b$ .
3 marks
(e)
Outline how the graph supports the hypothesis that $p$ is proportional to $\frac{1}{d}$ .
1 mark
(f)
In astronomy the parallax angle of a star is measured using the orbit of the Earth as the baseline. A space telescope measures parallax angles with an uncertainty of $\pm 2 \times 10^{- 5}$ arc-second. Determine, in pc, the greatest distance of a star whose parallax angle can be measured with a percentage uncertainty of no more than 10 %.
2 marks

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Question 3 · PHY-00355Paper 2 · HL only · 20 marks

A small uncrewed probe is to travel from Earth to a star 4.4 ly away in the reference frame of Earth. The probe is accelerated by a powerful laser beam to a cruising speed of $0.20 c$ and then travels at this constant speed. Ignore the motion of the star relative to Earth.

(a)
The probe accelerates from rest to $0.20 c$ with a constant acceleration of $1500 {m s}^{- 2}$ . Calculate the time taken and the distance travelled during this acceleration. Use Newtonian mechanics.
2 marks
(b)
Outline why it is reasonable to use Newtonian mechanics in (a).
1 mark
(c)
The acceleration distance and time are negligible for the rest of the question. Calculate, in years, the time taken for the journey to the star in the reference frame of Earth.
1 mark
(d)
Calculate the time for the journey measured by a clock on the probe.
2 marks
(e)
Calculate the distance between Earth and the star in the reference frame of the probe.
1 mark
(f)
On arrival, the probe sends a radio signal to Earth. Determine, in the reference frame of Earth, the time between the probe passing Earth at its cruising speed and the signal arriving at Earth.
2 marks
(g)
Sketch a spacetime diagram in the reference frame of Earth, with $x$ and $c t$ axes, showing the world lines of Earth, the probe and the radio signal. Label the coordinates of the arrival at the star and the reception of the signal.
3 marks
(h)(i)
The probe's instruments are powered by a radioactive source of plutonium-238, which has a half-life of 88 years. Outline why the time measured on the probe, rather than the time measured on Earth, determines how much of the plutonium decays during the journey.
1 mark
(h)(ii)
The power output of the source is 300 W at the start of the cruise. Calculate the power output when the probe reaches the star. Assume the power is proportional to the activity of the source.
2 marks
(i)
The luminosity of the star is 1.5 times the luminosity of the Sun, which is $3.83 \times 10^{26}$ W. Mission scientists want to study a planet that receives the same intensity of starlight, $1.36 \times 10^{3} {W m}^{- 2}$ , as Earth receives from the Sun. Determine, in AU, the distance of this planet from the star.
3 marks
(j)
The surface temperature of the star is 6100 K. Determine the radius of the star.
2 marks

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Question 4 · PHY-00260Paper 1A · 1 mark

Star X has twice the luminosity of star Y and is four times as far from the Earth.

What is $\frac{apparent brightness of X}{apparent brightness of Y}$ ?

A
$\frac{1}{8}$
B
$\frac{1}{2}$
C
2
D
32

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Question 5 · PHY-01528Paper 1B · 12 marks

A group of students estimate the luminosity of the Sun. A flat aluminium disc of diameter 8.00 cm and mass 0.120 kg is painted matt black on its front face and mounted on an insulating stand. At midday on a clear day the front face is turned so that it faces the Sun directly. A digital thermometer, reading to 0.1 °C, is inserted into the disc and its temperature $θ$ is recorded every minute.

Specific heat capacity of aluminium = $900 {J kg}^{- 1} K^{- 1}$

The graph shows the variation of $θ$ with time $t$ .

Graph of disc temperature in degrees Celsius against time in minutes from 0 to 15. The temperature starts at 21.0 and rises by about 2.4 degrees in the first minute, then the curve bends over, reaching 42.1 at 15 minutes.

(a)
Explain why the students should use the gradient of the graph at $t = 0$ rather than the mean rate of rise of temperature over the 15 minutes.
2 marks
(b)
Determine, using the graph, the intensity of the sunlight absorbed by the front face of the disc.
3 marks
(c)
The uncertainty in the initial gradient is estimated to be 8 %. The mass is measured to $\pm 0.001$ kg and the diameter to $\pm 0.05$ cm. Calculate the percentage uncertainty in the intensity.
2 marks
(d)
The students assume that the intensity of sunlight above the atmosphere is equal to their value in (b). Calculate their value for the luminosity of the Sun.
2 marks
(e)
The accepted value of the luminosity of the Sun is $3.83 \times 10^{26} W$ . Discuss the students' result, with reference to your answer to (c).
3 marks

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Question 6 · PHY-00436Paper 2 · HL only · 20 marks

At the end of its life, a massive star collapses. Its core, of mass $2.8 \times 10^{30} kg$ , shrinks from a radius of $7.0 \times 10^{6} m$ to form a neutron star of radius $1.2 \times 10^{4} m$ . Before the collapse the core rotates once every $2.0$ hours. Assume that the core and the neutron star are both uniform spheres, for which $I = \frac{2}{5} M R^{2}$ , and that the core loses no mass and no angular momentum during the collapse.

(a)
State the name of the event in which the core of a red supergiant collapses and the outer layers of the star are ejected.
1 mark
(b)
Explain why the neutron star rotates much faster than the core did before the collapse.
2 marks
(c)
Show that the period of rotation of the neutron star is about $0.02 s$ .
2 marks
(d)(i)
Determine $\frac{rotational kinetic energy after the collapse}{rotational kinetic energy before the collapse}$ .
2 marks
(d)(ii)
Identify the source of the increase in rotational kinetic energy.
1 mark
(e)
Calculate the gravitational field strength at the surface of the neutron star.
2 marks
(f)
If the neutron star rotated too quickly, material at its equator would be thrown off. Determine the shortest possible period of rotation for the neutron star, and comment on your answer to (c).
3 marks
(g)
Observations of the neutron star show that its angular speed decreases by $0.18 {rad s}^{- 1}$ in one year ( $3.16 \times 10^{7} s$ ). Calculate the average torque acting on the neutron star.
2 marks
(h)
Determine the average rate at which the neutron star loses rotational kinetic energy.
2 marks
(i)
The surface temperature of the neutron star is $1.0 \times 10^{6} K$ . Calculate the luminosity of the neutron star due to its thermal radiation. Assume it radiates as a black body.
2 marks
(j)
Identify, with a calculation, the region of the electromagnetic spectrum in which the thermal radiation from the neutron star is most intense.
1 mark

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