Physics · 6 Space physics

6.2.1 The Sun as a star

Before you start

The order of the electromagnetic spectrum, and that all electromagnetic waves travel at $3.0 \times 10^{8}$ m/s in a vacuum (3.3).
The wave equation $v = f λ$ (3.1).
Speed = distance ÷ time, rearranged to find a time (1.2).
The nuclear model of the atom and nuclide notation $_{Z}^{A} X$ , where $A$ is the mass number and $Z$ the proton number (5.1).
That the Sun is at the centre of the Solar System, with the planets orbiting it (6.1).

Syllabus objectives covered (2)

Know that the Sun is a star of medium size, consisting mostly of hydrogen and helium, and that it radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum
Know that stars are powered by nuclear reactions that release energy and that in stable stars the nuclear reactions involve the fusion of hydrogen into helium

Part 1 of 5

What makes the Sun a star

A star is a huge ball of extremely hot gas that gives out its own light. The Sun is a star, and it is the nearest star to Earth.
That is what separates the Sun from everything else in the Solar System. The Moon and the planets do not produce their own visible light. You see them because they reflect sunlight.
The stars you see at night are also suns. They look like tiny points only because they are enormously further away than the Sun.
Stars come in a very wide range of sizes. Some are many times wider than the Sun; others are much smaller.
Against that range, the Sun is a star of medium size. That is the syllabus phrase, and it is the one to write.
The trap: the Sun looks far bigger and brighter than any other star, so it is tempting to call it a large star. It looks that way only because it is so close to us.

Part 2 of 5

What the Sun is made of

The Sun consists mostly of two elements: hydrogen and helium.
Hydrogen is the more plentiful. Roughly three-quarters of the Sun's mass is hydrogen and most of the rest is helium; all the other elements together make up only a small fraction. You do not need these fractions. You need the two names.
Write the names of the elements. 'Gas' or 'hot gas' describes the state the material is in, not what it is made of, so it does not answer a question about composition.
Notice which two elements they are. Hydrogen is the fuel for the reactions that power the Sun, and helium is what those reactions produce. The last two sections show how.

Part 3 of 5

The radiation the Sun gives out

The Sun transfers energy into space as electromagnetic radiation. Radiation is how that energy crosses space to Earth: the space in between is a vacuum, conduction and convection need a medium, and electromagnetic waves do not.
Recall the order of the electromagnetic spectrum, from longest wavelength to shortest: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
The Sun radiates most of its energy in three neighbouring regions: infrared, visible light and ultraviolet. It gives out small amounts of other kinds of electromagnetic radiation too, but the bulk of the energy is in these three.
graph diagram — not drawn yet
A graph of the relative intensity of the Sun's radiation (vertical axis, no numbers) against wavelength (horizontal axis, increasing to the right), drawn as one smooth curve that rises steeply through a band labelled 'ultraviolet', peaks inside a narrow band labelled 'visible', and falls away gradually through a wide band labelled 'infrared', with almost nothing beyond either end.
Almost all the area under the curve, which represents the energy, lies inside the three labelled regions.
You meet all three every day. Infrared is the warmth you feel on your skin in sunshine. Visible light is what your eyes detect. Ultraviolet is what causes sunburn.
The trap is to name only the one you can see. 'Light' on its own, or 'light and heat', leaves out ultraviolet, and 'heat' is not a region of the spectrum. Write all three names.
Keep the order of the three straight, because a question can ask you to compare them. Infrared has the longest wavelength and the lowest frequency. Ultraviolet has the shortest wavelength and the highest frequency. Visible light lies between them.
All three are electromagnetic waves, so in the vacuum of space they all travel at the same speed, $3.0 \times 10^{8}$ m/s. Radiation that leaves the Sun's surface at one moment arrives at Earth together, whatever its region.

Part 4 of 5

What powers a star: nuclear fusion

Stars are powered by nuclear reactions that release energy. These are reactions between atomic nuclei, not between atoms or molecules.
So the Sun is not burning. Burning is a chemical reaction with oxygen: atoms rearrange, but their nuclei stay the same and no new element appears. Burning hydrogen would make water, not helium.
The reaction in stars is nuclear fusion: two light nuclei join to form a single heavier nucleus, and energy is released.
Do not confuse it with nuclear fission, the splitting of a heavy nucleus such as uranium into smaller nuclei. Nuclear power stations use fission. Stars are powered by fusion.
Where does the energy come from? When hydrogen nuclei fuse into helium, the nuclei produced have slightly less total mass than the nuclei that went in. That difference in mass is released as energy. You need this idea only qualitatively, with no values.
This point is background, not something the syllabus asks you to recall, but it explains where fusion happens. Fusion is hard to make happen. Every nucleus is positively charged, so two nuclei repel each other strongly as they approach. They fuse only if they collide fast enough to get extremely close despite that repulsion.
That is why fusion happens in the core of the Sun. The temperature there is millions of degrees, so the nuclei move very fast. The huge pressure also packs them close together, so collisions happen all the time.
The energy released in the core travels out to the surface. From the surface it is radiated into space as the infrared, visible light and ultraviolet of the previous section.

Part 5 of 5

Stable stars fuse hydrogen into helium

A stable star is one that stays the same size for a long time, because the inward pull of its own gravity is balanced by the outward push from its hot centre. The Sun is a stable star.
In a stable star, the nuclear reactions are the fusion of hydrogen into helium. Hydrogen nuclei join in a series of steps, and the end product is helium nuclei.
Overall, four hydrogen nuclei end up as one helium nucleus. The syllabus does not ask you to recall the individual steps. What you can do with a step that is given to you is check that it balances, exactly as for any nuclide equation.
Two rules decide every nuclide equation. The mass numbers (top) add up to the same total on each side. The proton numbers (bottom) add up to the same total on each side. Worked example 3 applies them to one of the Sun's reactions.
The proton number then tells you the element: $Z = 1$ is always hydrogen and $Z = 2$ is always helium, whatever the mass number.
This links back to what the Sun is made of. Hydrogen, the main ingredient, is the fuel, and helium is the product. As time passes, the hydrogen in the core is used up and the helium there builds up.
Because the core's hydrogen is being converted to helium, the supply is limited, so a star cannot fuse hydrogen for ever. What happens when it runs low belongs to the life cycle of stars in 6.2.2 Stars.

Worked examples

Example 1. Radiation leaving the Sun travels $1.5 \times 10^{11}$ m to reach the Earth. Calculate the time it takes, in seconds and in minutes.

Show the working

The radiation is electromagnetic, so its speed in the vacuum of space is $3.0 \times 10^{8}$ m/s. You must recall this value: Paper 4 does not give it to you.
Speed = distance ÷ time, so rearrange to make time the subject.
$t = \frac{d}{v}$
Substitute, keeping each number with its power of ten.
$t = \frac{1.5 \times 10^{11}}{3.0 \times 10^{8}} = 500 s$
Divide by 60 to convert to minutes.
$500 \div 60 = 8.3 min$
The infrared, visible light and ultraviolet all take this same time, because every electromagnetic wave travels at the same speed in a vacuum.

Answer: $500$ s, which is about $8.3$ minutes.

Example 2. Describe the Sun as a star. (4 marks)

Show the working

Four marks means four separate facts. Plan them before you write, one per phrase of the objective, then add one more.
Size: the Sun is a star of medium size. 'Big' or 'huge' does not do; the description compares the Sun with other stars, and medium is the correct comparison.
Composition: it consists mostly of hydrogen and helium. Name both elements; 'gas' alone describes the state, not the substance.
Radiation: it radiates most of its energy as infrared, visible light and ultraviolet. Name all three regions.
A fourth fact: it is the nearest star to Earth and gives out its own light. The fusion of hydrogen into helium in its core is also a correct fact.
Avoid 'it gives out light and heat'. 'Heat' is not a region of the spectrum, and the phrase leaves out ultraviolet.

Answer: The Sun is a star of medium size; it consists mostly of hydrogen and helium; it radiates most of its energy as infrared, visible light and ultraviolet; it is the nearest star to Earth and gives out its own light.

Example 3. One of the reactions in the Sun's core is $_{1}^{2} H +_{1}^{1} H \to_{Z}^{A} X + energy$ . Determine $A$ and $Z$ , identify X, and explain why this is a fusion reaction.

Show the working

Mass numbers balance: the top numbers on the left must add up to $A$ .
$A = 2 + 1 = 3$
Proton numbers balance: the bottom numbers on the left must add up to $Z$ .
$Z = 1 + 1 = 2$
A proton number of 2 means the nucleus is helium. So X is helium-3, $_{2}^{3} He$ , with 2 protons and $3 - 2 = 1$ neutron.
It is fusion because two light nuclei have joined to form one heavier nucleus, releasing energy. Two hydrogen nuclei have become a helium nucleus, which is the direction a stable star works in: hydrogen into helium.

Answer: $A = 3$ , $Z = 2$ ; X is helium ( $_{2}^{3} He$ ). Two light nuclei join to make a heavier one and energy is released, so it is fusion.

Example 4. A student writes: 'The Sun gives out energy because it is a huge ball of burning hydrogen.' Comment on this statement.

Show the working

'Comment' asks for an informed opinion, so say what is right, say what is wrong, then give the correct version.
What is right: hydrogen is the Sun's fuel, and the Sun does release energy from it.
What is wrong: the process is not burning. Burning is a chemical reaction with oxygen; it leaves the nuclei unchanged and, for hydrogen, makes water.
The correct version: the energy comes from nuclear fusion. In the core, hydrogen nuclei fuse to form helium nuclei, and energy is released.
A short supporting reason makes the comment informed: the fusion needs the very high temperature of the core, where fast-moving nuclei can get close enough to join.

Answer: The statement is partly right: hydrogen is the fuel. But the Sun is not burning, which is a chemical reaction with oxygen. Its energy comes from nuclear fusion, in which hydrogen nuclei in the very hot core join to form helium nuclei, releasing energy.

Where people go wrong

Mistake: Saying the Sun is burning, like a fire.

Instead: The Sun releases energy by nuclear fusion of hydrogen nuclei into helium nuclei.

Burning is a chemical reaction with oxygen. It rearranges atoms but never changes their nuclei, and burning hydrogen makes water, not helium.

Mistake: Saying stars are powered by nuclear fission, or writing 'fission and fusion' to be safe.

Instead: Stars are powered by fusion: light nuclei joining to form a heavier nucleus.

Both are nuclear reactions that release energy, which is why they get mixed up, but fission splits heavy nuclei such as uranium. Writing both is a contradiction, and a contradiction cannot score.

Mistake: Answering 'light and heat' when asked how the Sun radiates its energy.

Instead: Name the three regions: infrared, visible light and ultraviolet.

'Heat' is not a region of the electromagnetic spectrum, and the answer leaves ultraviolet out completely.

Mistake: Calling the Sun a large or giant star because it looks so much bigger than other stars.

Instead: The Sun is a star of medium size.

It looks bigger and brighter only because it is so much closer to us than any other star.

Mistake: Thinking fusion is hydrogen atoms or molecules combining.

Instead: Fusion is the joining of nuclei.

Atoms or molecules joining is chemistry, and chemistry never changes one element into another. Turning hydrogen into helium, a different element, needs the nuclei themselves to join.

Check yourself

State the two elements that make up most of the Sun. Which is more plentiful?
Answer
Hydrogen and helium. Hydrogen is more plentiful.
State the three regions of the electromagnetic spectrum in which the Sun radiates most of its energy.
Answer
Infrared, visible light and ultraviolet.
Put those three regions in order of increasing frequency.
Answer
Infrared, visible light, ultraviolet.
The Sun looks much bigger and brighter than every other star. Does this mean it is a large star?
Answer
No. The Sun is a star of medium size; it looks bigger and brighter because it is much closer to us than any other star.
Infrared radiation from the Sun has a wavelength of $1.0 \times 10^{- 6}$ m. Calculate its frequency.
Answer
$f = v / λ = (3.0 \times 10^{8}) / (1.0 \times 10^{- 6}) = 3.0 \times 10^{14}$ Hz.
What is nuclear fusion?
Answer
The joining of two light nuclei to form a heavier nucleus, with the release of energy.
In a stable star, what is fused into what? Where in the star does it happen?
Answer
Hydrogen is fused into helium, in the core.
Why does fusion need a very high temperature?
Answer
Nuclei are positively charged and repel each other. Only at very high temperatures do they move fast enough to get close enough to fuse.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. The Sun is the star at the centre of the Solar System. A space probe is $4.5 \times 10^{10}$ m from the Sun.

(a) State the two elements that make up most of the Sun. 2 marks No calculator

Mark scheme

B1 hydrogen
B1 helium; 'gas' alone is insufficient

(b) State the size of the Sun compared with other stars. 1 mark No calculator

Mark scheme

B1 medium (size) / average size, owtte

(c) The Sun radiates most of its energy in three regions of the electromagnetic spectrum. State the three regions. 2 marks No calculator

Mark scheme

B2 infrared, visible light (accept 'light'), ultraviolet; all three correct for 2 marks, any two correct for 1 mark; 'heat' is not a region and does not score; any wrong region counts as one of the three answers

(d) Calculate the time taken for radiation from the Sun to reach the probe. 3 marks

Mark scheme

C1 speed of electromagnetic waves taken as 3.0 × 10⁸ m/s
C1 t = d / v, or (4.5 × 10¹⁰) / (3.0 × 10⁸)
A1 150 s (accept 2.5 min); unit required

(e) The probe detects infrared and ultraviolet radiation that left the Sun's surface at the same moment. Deduce which arrives at the probe first, and explain your answer. 2 marks No calculator

Mark scheme

B1 they arrive at the same time
B1 all electromagnetic waves travel at the same speed in a vacuum / in space

Question 2. Stars such as the Sun are powered by nuclear reactions in their cores.

(a) Describe the nuclear reactions that take place in a stable star. 2 marks No calculator

Mark scheme

B1 (nuclear) fusion / nuclei join together, releasing energy; 'fission' given as well is CON
B1 hydrogen (nuclei) form helium (nuclei)

(b) One of the reactions is $_{2}^{3} He +_{2}^{3} He \to_{2}^{4} He + 2_{Z}^{A} X$ . Determine the values of $A$ and $Z$ , and identify X. 3 marks No calculator

Mark scheme

B1 A = 1
B1 Z = 1
B1 X is hydrogen (nucleus) / a proton; ecf from their value of Z

(c) Explain, in terms of mass, how energy is released when hydrogen nuclei fuse to form helium. 2 marks No calculator

Mark scheme

B1 total mass of the products / helium nucleus is less than total mass of the hydrogen nuclei that fused, owtte
B1 the (lost) mass is converted to / released as energy

(d) A student says: 'The Sun can keep fusing hydrogen for ever.' Comment on this statement. 2 marks No calculator

Mark scheme

B1 hydrogen (in the core) is converted into helium / is used up by the fusion
B1 the amount of hydrogen is limited, so the statement is wrong / hydrogen fusion must eventually stop, owtte

How this is examined

What powers a stable star: hydrogen nuclei fusing into helium: a one-mark State naming the process, or a two-mark Describe of the reactions in a stable star such as the Sun 'Nuclear fusion', or 'nuclei join together', for one mark; 'hydrogen (nuclei) form helium (nuclei)' for the other. Writing 'fission' as well as 'fusion' is a contradiction and cancels the mark. 'Burning' earns nothing.
Where the energy released by fusion comes from: a two-mark Explain, in terms of mass, of how fusion releases energy Two linked points, with no values: the nuclei produced have less total mass than the nuclei that fused; that difference in mass is released as energy. 'Fusion gives out energy' on its own explains nothing.
The three regions the Sun radiates most of its energy in: a two-mark State, or an Identify where the regions are labelled on a spectrum All three names: infrared, visible light, ultraviolet. 'Heat' is not a region. Under the list rule, a wrong extra region such as X-rays counts as one of your three answers.
What the Sun is made of, and how big it is compared with other stars: one- or two-mark State parts at the start of a longer question 'Hydrogen' and 'helium' as two separate names; 'medium' for the size. 'Gas' and 'large' do not earn the marks.
Balancing a fusion reaction written in nuclide notation: a Determine with one nucleus or one number missing from a given equation Mass numbers adding to the same total on both sides; proton numbers adding to the same total on both sides; then the proton number used to name the element (1 is hydrogen, 2 is helium).
The time the Sun's radiation takes to cross space: a three-mark Calculate giving a distance from the Sun, possibly followed by a Deduce about which radiation arrives first $3.0 \times 10^{8}$ m/s recalled; $t = d / v$ written or substituted; the answer in seconds with the unit. For the Deduce: 'they arrive at the same time because all electromagnetic waves travel at the same speed in a vacuum'.

Words to know

star: A huge ball of extremely hot gas that gives out its own light, powered by nuclear reactions inside it.
nuclear fusion: The joining of two light nuclei to form a heavier nucleus, with the release of energy.
nuclear fission: The splitting of a heavy nucleus into smaller nuclei, with the release of energy. It is not the process that powers stars.
stable star: A star that stays the same size because the inward pull of gravity is balanced by the outward push from its hot centre. Its energy comes from fusing hydrogen into helium.
core: The central region of a star, where the temperature and pressure are high enough for fusion.
infrared: Electromagnetic radiation with a longer wavelength and lower frequency than visible light; felt as warmth.
ultraviolet: Electromagnetic radiation with a shorter wavelength and higher frequency than visible light; causes sunburn.
electromagnetic radiation: Energy transferred by electromagnetic waves, which can travel through a vacuum at $3.0 \times 10^{8}$ m/s.

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