What does Fusion and Stars cover in IB Physics?

This topic covers the processes of fusion in stars, stellar stability, evolution, and methods for determining stellar properties. Stellar stability relies on an equilibrium between outward thermal or radiation pressure and inward gravitational forces. Fusion is the energy source in stars, requiring high density and temperature.

Is Fusion and Stars SL or HL?

Both. SL and HL students study Fusion and Stars to the same depth.

How does FourtyFive help me practise Fusion and Stars?

FourtyFive has 31 Fusion and Stars questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for Fusion and Stars practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite Fusion and Stars answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

Topic E.5 · SL and HL

Fusion and Stars: notes and practice questions

Q: How do I revise Fusion and Stars for IB Physics?

Start from the core idea: this topic covers the processes of fusion in stars, stellar stability, evolution, and methods for determining stellar properties. In the exam: both papers, both levels. May 2025 Paper 2 TZ1 ran E.5 as the tail of the long greenhouse question at both levels: calculate in MeV the energy released by the proton-proton chain from a given helium mass (2), outline the role of fusion in maintaining a stable stellar radius (2), outline how helium in the Sun is confirmed empirically (2), then from an HR diagram state a star type (1) and discuss how fusion in a supergiant differs from fusion in the Sun (3). Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

12 min read Jump to 20 practice questions

Summary

This topic covers the processes of fusion in stars, stellar stability, evolution, and methods for determining stellar properties.
Stellar stability relies on an equilibrium between outward thermal or radiation pressure and inward gravitational forces.
Fusion is the energy source in stars, requiring high density and temperature.
Stellar mass affects a star's evolution.
The Hertzsprung-Russell (HR) diagram describes stellar properties (luminosity vs. temperature).
Stellar distance $d$ can be found using parallax: $d(\text{parsec}) = \frac{1}{p(\text{arc-second})}$
Stellar radii are determined using luminosity and surface temperature.
Energy release calculations and conversions between AU, ly, and pc are required.

How it is examined

Both papers, both levels. May 2025 Paper 2 TZ1 ran E.5 as the tail of the long greenhouse question at both levels: calculate in MeV the energy released by the proton-proton chain from a given helium mass (2), outline the role of fusion in maintaining a stable stellar radius (2), outline how helium in the Sun is confirmed empirically (2), then from an HR diagram state a star type (1) and discuss how fusion in a supergiant differs from fusion in the Sun (3). Note how often outline appears at 2 marks: two distinct points, briefly.

Given in the booklet

The parallax relation, the AU, light year and parsec conversions, the solar luminosity, mass and radius. Stellar radius comes from combining $L = \sigma A T^4$ (B.1) with $A = 4\pi R^2$ , which is not printed as a single equation. The HR diagram itself is not in the booklet, so its axes, orientation and regions are recall. Cepheid variables were on the retired 2016 option and are explicitly excluded now.

Note on the November 2024 update

The first understanding used to read "outward radiation pressure". The Nov 2024 update inserted "thermal or", so it now reads "outward thermal or radiation pressure", to account for low and medium mass stars where gas pressure dominates against high mass stars where radiation pressure does. Any source predating Nov 2024 has the older wording.

Key ideas

that the stability of stars relies on an equilibrium between outward thermal or radiation pressure and inward pressure due to gravitational forces
that fusion is a source of energy in stars
the conditions leading to fusion in stars in terms of density and temperature
the effect of stellar mass on the evolution of a star

Not assessed

Cepheid variables are not required.

Guiding questions

How are elements created?
What physical processes lead to the evolution of stars?
Can observations of the present state of the universe predict the future outcome of the universe?

Linking questions

How is fusion like and unlike fission?
How can the understanding of black-body radiation help determine the properties of stars?
How do emission spectra provide information about observations of the cosmos?
HR diagrams have been helpful in the classification of stars by finding patterns in their properties. Which other areas of physics use classification to help our understanding? (NOS)
In which ways has technology helped to collect data from observations of distant stars? (NOS)
How can gas laws be used to model stars? (NOS)

Practice questions

31 questions · 10 easy · 19 medium · 2 hard

Showing 20 of 20

Question 1

EasyPaper 1A · calculator1 mark

Which graph best shows the variation of binding energy per nucleon with nucleon number?

Graph showing binding energy per nucleon on the y-axis and nucleon number on the x-axis. The curve is a straight line with a positive slope starting from the origin.

Graph showing binding energy per nucleon on the y-axis and nucleon number on the x-axis. The curve is a straight line with a negative slope starting from a positive y-intercept.

Graph showing binding energy per nucleon on the y-axis and nucleon number on the x-axis. The curve starts at the origin, rises sharply to a peak around nucleon number 50-60, and then slowly decreases for higher nucleon numbers.

Graph showing binding energy per nucleon on the y-axis and nucleon number on the x-axis. The curve is a horizontal line, indicating a constant value.

Question 2

MediumPaper 2 · calculator3 marks

A newly discovered exoplanet system orbits a star named Celestia. Observations indicate that Celestia is located at a distance of $5.0 \times 10^{17}$ m from Earth.

(a) Calculate the parallax angle of star Celestia as observed from Earth.

Question 3

HardPaper 2 · calculator13 marks

(a)

An astronomical observatory is studying a distant exoplanet system. They collect the following data for the host star, named "Exo-Star", and compare it with known data for the Sun.

Data about the Exo-Star:

Luminosity = $8.0 \times 10^5 \times$ luminosity of the Sun

Observed brightness on Earth = $6.5 \times 10^{-9}$ W m $^{-2}$

Peak wavelength of its emitted radiation = $480$ nm

Data about the Sun:

Observed brightness on Earth (solar constant) = $1.36 \times 10^3$ W m $^{-2}$

Average distance of the Sun from the Earth = $1.0$ AU

(a) Calculate the luminosity of the Sun.

[3]

(b)

(b) Calculate the distance of the Exo-Star from Earth.

[4]

(c)

[2]

(d)

(d) Calculate the radius of the Exo-Star.

[4]

Question 4

EasyPaper 1A · calculator1 mark

Astronomers are studying a newly discovered exoplanet orbiting a distant star. They measure the star's parallax angle to be $0.025$ arc-seconds.

What is the distance from Earth to this star?

A. $25 \text{ pc}$

B. $40 \text{ pc}$

C. $50 \text{ pc}$

D. $20 \text{ pc}$

Question 5

MediumPaper 2 · calculator4 marks

(a)

An astronomer observes a distant main-sequence star. The star is stable and maintains a constant radius.

(a) Explain, in terms of forces, why the star does not collapse under its own gravity.

[3]

(b)

(b) The star has a mass similar to the Sun and is powered by the fusion of hydrogen into helium. State the name of the primary reaction sequence for this process in such a star.

[1]

Question 6

HardPaper 2 · calculator20 marks

(a)

An exoplanet named Xylos orbits a star named Aethel. Scientists are studying its atmospheric conditions and the properties of its host star.

(a) State what is meant by the stellar constant for Xylos.

[1]

(b)(i)

(b) The following data are given for exoplanet Xylos:

Average albedo of Xylos $= 0.40$

Average orbital distance from Aethel $= 2.0 \times 10^{11} \text{ m}$

Average global surface temperature of Xylos $= 250 \text{ K}$

Luminosity of star Aethel $= 2.0 \times 10^{26} \text{ W}$

(i) Outline the physical mechanism by which some of the infrared radiation emitted by the surface of Xylos is absorbed by its atmospheric gases and re-radiated back towards the surface.

[2]

(b)(ii)

(ii) Show that the average global intensity of radiation absorbed by the surface of Xylos is about $60 \text{ W m}^{-2}$ .

[2]

(b)(iii)

(iii) Determine the average intensity re-radiated by Xylos's atmosphere towards its surface. Assume that the emissivity of the surface is $0.95$ .

[3]

(c)

[2]

(d)(i)

(d) A possible fusion reaction occurring in stars like Aethel is the deuterium-tritium (D-T) fusion reaction:

$^2_1\text{H} + ^3_1\text{H} \rightarrow ^4_2\text{He} + ^1_0\text{n}$

Relevant atomic masses are:

Mass of deuterium ( $^2_1\text{H}$ ) $= 2.013553 \text{ u}$

Mass of tritium ( $^3_1\text{H}$ ) $= 3.015501 \text{ u}$

Mass of helium-4 ( $^4_2\text{He}$ ) $= 4.001506 \text{ u}$

Mass of neutron ( $^1_0\text{n}$ ) $= 1.008665 \text{ u}$

(i) Calculate, in $\text{MeV}$ , the energy released in the reaction.

[2]

(d)(ii)

(ii) Outline the role of fusion reactions in maintaining a stable radius for a star like Aethel.

[2]

(d)(iii)

(iii) Outline how the presence of hydrogen in Aethel can be confirmed empirically.

[2]

(e)(i)

(e) Aethel has a surface temperature of $5200 \text{ K}$ and a luminosity $0.5$ times that of the Sun.

(i) State the star type of Aethel.

[1]

(e)(ii)

(ii) Discuss how nuclear fusion processes in a red dwarf star differ from those in the Sun.

[3]

Question 7

EasyPaper 1A · calculator1 mark

The Hertzsprung–Russell (HR) diagram shows four stars P, Q, R and S.

Hertzsprung–Russell diagram with logarithmic scales for luminosity (y-axis) and temperature (x-axis, decreasing to the right). The main sequence band runs from top-left to bottom-right. P is on the main sequence, similar to the Sun. Q is in the top-right (red giant region). R is in the bottom-left (white dwarf region). S is in the top-left of the main sequence (blue giant region).

Which of the following sequences represents a possible evolutionary path for a star with a mass similar to the Sun?

A. S → Q → R

B. P → Q → R

C. P → S

D. Q → P → R

Question 8

MediumPaper 2 · calculator4 marks

A meteorite is found to contain various elements. Spectroscopic analysis reveals the presence of silicon-28 ( $^{28}\text{Si}$ ), nickel-58 ( $^{58}\text{Ni}$ ), and gold-197 ( $^{197}\text{Au}$ ). The early universe consisted almost entirely of hydrogen and helium.

(a) Outline the stellar processes that are responsible for the creation of these three elements found in the meteorite.

Question 9

EasyPaper 1A · calculator1 mark

The stellar parallax method is used to determine the distance to nearby stars. What is the primary reason this method is not suitable for determining the distance to very distant stars?

A. The parallax angle becomes too large to measure accurately.

B. The parallax angle becomes too small to measure accurately.

C. The light from distant stars is redshifted, which affects the measurement.

D. Interstellar dust absorbs too much light from distant stars.

Question 10

MediumPaper 2 · calculator11 marks

(a)

The Hertzsprung-Russell (HR) diagram below shows four regions, labelled A, B, C, and D, where stars are typically found.

HR diagram with axes for Luminosity (increasing upwards) and Surface Temperature (decreasing to the right). Region A is the diagonal band of the main sequence. Region B is the area above the main sequence to the right (red giants). Region C is the area at the very top (supergiants). Region D is the area at the bottom left (white dwarfs).

(a) Identify the group of stars found in each region.

[4]

(b)(i)

An astronomer collects data for three stars, X, Y, and Z, in a newly discovered star cluster. The data are shown in the table. The luminosity is given in terms of the Sun's luminosity, $L_{\odot}$ , and the surface temperature is in Kelvin. The Sun's surface temperature is $5800$ K.

Star	Luminosity ( $L/L_{\odot}$ )	Temperature ( $T$ /K)
X	5	7000
Y	850	3500
Z	0.01	15000

(b) (i) Using the data, show that the radius of Star Y is approximately 80 times the radius of the Sun ( $R_{\odot}$ ).

[3]

(b)(ii)

(ii) Star X is on the main sequence. State the principal process of energy production within Star X.

[1]

(b)(iii)

(iii) Star Z is a white dwarf. Outline the stages of evolution for a low-mass star, similar to the one that became Star Z, after it leaves the main sequence. Your answer should refer to the regions on the HR diagram from part (a).

[3]

Question 11

EasyPaper 1A · calculator1 mark

A Hertzsprung–Russell (HR) diagram shows the position of the Sun and four other stars W, X, Y and Z.

An HR diagram with Luminosity on the y-axis (increasing upwards) and Surface Temperature on the x-axis (increasing to the left). The Sun is located on the main sequence. Star W is in the top right (high luminosity, low temperature). Star X is in the top left (high luminosity, high temperature). Star Y is in the bottom left (low luminosity, high temperature). Star Z is in the bottom right (low luminosity, low temperature).

Which star is a red supergiant?

A. W

B. X

C. Y

D. Z

Question 12

MediumPaper 2 · calculator8 marks

(a)

A protostar is a very young star that is still gathering mass from its parent molecular cloud.

(a) State the nuclear process that will eventually power the star once it reaches the main sequence.

[1]

(b)(i)

For the process identified in (a) to begin in the core of the protostar, two key conditions must be met.

(i) Outline why a very high temperature is a necessary condition.

[2]

(b)(ii)

(ii) Outline why a very high density is a necessary condition.

[2]

(c)

(c) Explain how the core of the protostar reaches the extreme temperature and density required for nuclear fusion to begin.

[3]

Question 13

EasyPaper 1A · calculator1 mark

A Hertzsprung-Russell (HR) diagram is shown with four stars labelled P, Q, R, and S. The Sun is also shown for reference on the main sequence. Which star is a red supergiant?

Hertzsprung-Russell diagram with luminosity relative to the Sun on the y-axis (logarithmic scale, increasing upwards) and surface temperature in Kelvin on the x-axis (logarithmic scale, decreasing to the right). The main sequence is a diagonal band from top-left to bottom-right. Star P is in the top-right quadrant (high luminosity, low temperature). Star Q is in the bottom-left quadrant (low luminosity, high temperature). Star R is on the main sequence, below and to the right of the Sun. Star S is on the main sequence, above and to the left of the Sun.

A. P

B. Q

C. R

D. S

Question 14

MediumPaper 1A · calculator1 mark

Star Alpha and Star Beta are main sequence stars. Star Alpha has a radius $R$ and a surface temperature $T$ . It is located at a distance $d$ from Earth. Star Beta has the same radius $R$ but its surface temperature is $2T$ . It is located at a distance $4d$ from Earth.

What is the ratio of the apparent brightness of Star Beta to Star Alpha, $\frac{b_{\text{Beta}}}{b_{\text{Alpha}}}$ ?

A. $\frac{1}{16}$

B. $\frac{1}{4}$

C. $1$

D. $4$

Question 15

EasyPaper 1A · calculator1 mark

The black-body radiation spectrum for a star with a surface temperature of $5000 \text{ K}$ is shown. The intensity units are arbitrary.

Graph showing the black-body radiation spectrum for a star at 5000 K, with intensity on the y-axis and wavelength on the x-axis. The curve has a single peak.

Later in its life cycle, the star's surface temperature increases to $7500 \text{ K}$ . Which graph shows the new radiation spectrum? The original spectrum is shown as a dashed line.

Graph showing the new curve with its peak at a shorter wavelength and a higher intensity than the original dashed curve.

Graph showing the new curve with its peak at a longer wavelength and a higher intensity than the original dashed curve.

Graph showing the new curve with its peak at a shorter wavelength and a lower intensity than the original dashed curve.

Graph showing the new curve with its peak at a longer wavelength and a lower intensity than the original dashed curve.

Question 16

MediumPaper 1A · calculator1 mark

Star Alpha has a surface temperature $T_A$ , luminosity $L_A$ and radius $R_A$ .

Star Beta has a luminosity $L_B = 16L_A$ and a surface temperature $T_B = 2T_A$ .

What is the radius of Star Beta?

A. $\frac{R_A}{4}$

B. $\frac{R_A}{2}$

C. $R_A$

D. $2R_A$

Question 17

EasyPaper 1A · calculator1 mark

A star on the main sequence has a mass of approximately $15$ solar masses. What is the expected evolutionary sequence for this star after it leaves the main sequence?

A. Red giant → Planetary nebula → White dwarf

B. Red supergiant → Supernova → Neutron star

C. Red giant → Supernova → Black hole

D. Red supergiant → Planetary nebula → White dwarf

Question 18

MediumPaper 1A · calculator1 mark

The black-body radiation curves for two stars, P and Q, are shown. Star P has a peak wavelength of $400$ nm, and star Q has a peak wavelength of $600$ nm.

Graph showing black-body radiation curves for two stars P and Q, with intensity on the y-axis and wavelength/nm on the x-axis. Curve P peaks at a shorter wavelength (400 nm) and higher intensity than curve Q, which peaks at a longer wavelength (600 nm) and lower intensity.

What is $\frac{\text{temperature of P}}{\text{temperature of Q}}$ ?

A. $\frac{1}{2}$

B. $\frac{2}{3}$

C. $\frac{3}{2}$

D. $2$

Question 19

EasyPaper 1A · calculator1 mark

Which of the following correctly describes the sequence of the main evolutionary stages for a star of approximately one solar mass, after it leaves the main sequence?

A. Red giant → Supernova → Neutron star

B. Red supergiant → Planetary nebula → White dwarf

C. Red giant → Planetary nebula → White dwarf

D. Red supergiant → Supernova → Black hole

Question 20

MediumPaper 1A · calculator1 mark

A proposed fusion power plant uses the reaction of one deuterium nucleus ( $^2\text{H}$ ) and one tritium nucleus ( $^3\text{H}$ ). This reaction converts a mass of $0.019\text{u}$ into energy. In a conventional fission power plant, the fission of one uranium-235 nucleus converts a mass of $0.28\text{u}$ into energy.

What is the ratio of $\frac{\text{energy released per unit mass of hydrogen fuel}}{\text{energy released per unit mass of uranium fuel}}$ ?

A. $0.068$

B. $0.31$

C. $3.2$

D. $7.9$

11 more Fusion and Stars questions in the app

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Stopping one step short of the conclusion. Two numbers and no sentence is two marks out of three.
Answering a procedure question with a platitude.
Losing precision in Paper 1B. Uniquely to this paper, quoting the right number badly loses marks.

Fusion and Stars: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

11 more Fusion and Stars questions in the app

Where marks are lost

Practise this topic as a session

Questions,
answered.

Start with the IB question
bank built for you.

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

11 more Fusion and Stars questions in the app

Where marks are lost

Questions,answered.

Start with the IB questionbank built for you.

Questions,
answered.

Start with the IB question
bank built for you.