☀️ How the Sun Works

Guided Notes — Unit 1: Discovering New Worlds

Lesson 1 review · what the Sun is made of · how it releases energy · how that energy reaches us

How to Use This

You have: the packet Guided Notes: How the Sun Works

On screen: the same content, Part for Part

Anything shown like this goes in a blank on your sheet.

The numbered questions in the packet are not answered on these slides. We work those together.

PART A

Light, Wavelength, and the Visible Spectrum

Wavelength and Color

PACKET PART A

A wavelength is the distance between one wave peak and the next.

For light, wavelength determines the color we see.

Wavelengths are measured in nanometers (nm) — one billionth of a meter.

The visible spectrum runs from about 400 nm to about 700 nm.

  • Shortest (≈400 nm) → violet
  • Longest (≈700 nm) → red
long λ λ short λ crest trough longer wavelength — red, ~700 nm shorter wavelength — violet, ~400 nm

The Tools

PACKET PART A

A prism separates light by wavelength into a band of colors — a spectrum (plural: spectra).

A spectroscope (or spectrometer) does this and lets us measure exactly which wavelengths are present.

We cannot take a star apart. Everything we know about what a star is made of comes from analyzing its light.

PART B

Three Kinds of Spectra

Three Kinds of Spectra

PACKET PART B
1. Continuous
2. Absorption
light through a cooler gas
3. Emission
a hot thin gas glowing
400450500550600650700 nm
  • Continuous — every wavelength, no gaps. From a hot, glowing solid, liquid or dense gas
  • Absorption — dark lines where wavelengths are missing
  • Emission — bright lines on a dark background

The Rule That Ties Them Together

PACKET PART B

A gas absorbs and emits light at the same wavelengths.

Look at bars 2 and 3 on the previous slide. The dark lines and the bright lines sit in the same places — about 410, 434, 486 and 656 nm.

That is why an emission spectrum measured in a laboratory on Earth can be matched against an absorption spectrum measured from a star.

PART C

Every Element Has a Fingerprint

Repeatable + Unique = Fingerprint

PACKET PART C

Atoms of each element absorb and emit a characteristic set of wavelengths — belonging to that element and no other.

1. Repeatable
Every sample of the same gas gives the same pattern, every time.

2. Unique
Different elements give different patterns.

Repeatable + unique = a fingerprint we can use to identify an element — even in tiny amounts, and across enormous distances.

Hydrogen
Helium
Oxygen

Why This Counts as Evidence

PACKET PART C

Pattern — a repeated, consistent relationship in data

Empirical evidence — evidence from observation or measurement

"The lines look like they match" is not evidence.

Citing the wavelengths is — hydrogen at 410, 434, 486 and 656 nm in every sample.

PART D

What Is the Sun Made Of?

First: Is the Sun's Spectrum Consistent?

PACKET PART D
Observation 1
Observation 2
Observation 3
400450500550600650700 nm

All three are identical — dark lines above the same wavelengths every time. The Sun's gases don't change, so the absorbed wavelengths don't either.

Match the Sun Against the Lab

PACKET PART D
☀️ The Sun
Hydrogen
Helium
Oxygen

Every Sun line matches a hydrogen or helium line, with nothing left over. Oxygen's pattern is not there.

Conclusion: The Sun's Composition

PACKET PART D

The Sun is composed mostly of hydrogen and helium.

By mass: about 73% hydrogen, about 25% helium, and roughly 2% heavier elements.

The same method works on any star. Studying stars' light spectra and brightness tells us their composition, their motion, and their distance from Earth.

PART E

How the Sun Releases Energy

Two Candidate Processes

PACKET PART E
🔥 Chemical: combustion ☢️ Nuclear: fusion
What changes Atoms rearrange into new molecules. Nuclei untouched Nuclei combine into a heavier nucleus. A new element forms
Conserved Atoms are conserved Atoms are not — but protons + neutrons are
Requires Oxygen. Makes water No oxygen. Enormous temperature and pressure
In words hydrogen + oxygen → water + energy hydrogen + hydrogen → helium + energy

The Evidence That Decides It

PACKET PART E
Process Energy from ONE reaction How long the Sun could last
Chemical (combustion) 0.000000593 MeV 50,000 years
Nuclear (fusion) 17.6 MeV 10 billion years

One fusion reaction releases about 30 million times more energy than one combustion reaction.

The Scale of Time

PACKET PART E
Time span Years
Combustion could power the Sun 50,000
Humans have existed 2,000,000
Liquid water on Earth (at least) 4,000,000,000
Age of the Sun and solar system 4,600,000,000
Fusion could power the Sun 10,000,000,000

Marine sedimentary rock shows oceans for nearly 4 billion years.

Conclusion: Scale Is the Reasoning

PACKET PART E

The Sun releases energy by nuclear fusion, not by burning.

Combustion fails on scale — it would exhaust the Sun in 50,000 years, about 80,000 times too short for 4 billion years of liquid water.

The significance of a process depends on the scale at which it occurs.

A Closer Look

What nuclear fusion actually is

Inside One Fusion Reaction

Deep in the Sun's core, four hydrogen nuclei (four protons) end up as one helium nucleus.

The nuclei themselves combine. That is what makes it nuclear rather than chemical — a brand new element comes out.

+ + + + 4 protons (hydrogen nuclei) + + n n helium-4 nucleus + ENERGY

Where the Energy Comes From

GOES BEYOND THE PACKET

Weigh the ingredients against the product and something is missing:

4 hydrogen nuclei 4.0291 u 1 helium nucleus 4.0015 u the missing 0.7% of the mass becomes the energy

is the speed of light, so is about . A tiny amount of mass becomes an enormous amount of energy.

Every second the Sun fuses about 600 million tonnes of hydrogen into about 596 million tonnes of helium.

The missing ~4 million tonnes leaves as energy.

Why Fusion Needs a Star

GOES BEYOND THE PACKET

Protons are all positively charged — so they repel each other. The closer they get, the harder they push apart.

To fuse, they must be driven close enough for the strong nuclear force to take over.

That takes crushing pressure and extreme temperature — conditions only a star's core can supply.

ordinary conditions — they bounce apart + + 15 million K and crushing pressure + + close enough to fuse

This is why fusion happens only in the core — and why gravity is what makes a star work. Gravity supplies the squeeze.

PART F

From the Core to Earth

The Path Energy Takes

PACKET PART F

Fusion happens only in the core.

☢️ Core — fusion releases energy
↓
Radiative zone — energy travels as light, absorbed and re-emitted
↓
Convective zone — hot gas rises, cooler sinks
↓
Surface — light escapes, radiating in all directions
↓
🌍 Earth — arrives about 8 minutes later

Energy leaves the Sun as electromagnetic radiation — light.

Radiation — energy transfer by electromagnetic waves; the only mechanism that crosses the vacuum of space

This is the light you analyzed. The spectrum that told us hydrogen and helium is the Sun's energy arriving.

PART G

The Sun's Life Span

What Happens as the Sun Ages

PACKET PART G

This reaction is irreversible — helium does not break back into hydrogen.

  • Hydrogen is the reactant → amount decreases
  • Helium is the product → amount increases

ESS1.A: The Sun is changing and will burn out over a life span of about 10 billion years.

It is currently about 4.6 billion years old — roughly halfway.

"Burn out" is figurative. No combustion is involved — the Sun runs low on fusion fuel.

PART H

Vocabulary

Vocabulary (1 of 2)

Term Definition
Wavelength Distance between wave peaks; determines color. Measured in nm
Spectrum (pl. spectra) The band of wavelengths light separates into
Continuous spectrum All wavelengths present, no gaps
Absorption spectrum Continuous spectrum with dark lines where a cooler gas absorbed specific wavelengths
Emission spectrum Bright lines on a dark background from a hot, thin glowing gas — at the same wavelengths it absorbs
Spectroscope / spectrometer Instrument that separates light so wavelengths can be measured
Empirical evidence Evidence obtained from observation or measurement

Vocabulary (2 of 2)

Term Definition
Pattern A repeated, consistent relationship in data. Requires empirical evidence
Nuclear fusion Light nuclei combining into a heavier nucleus, releasing energy and forming a new element
Combustion A chemical reaction of fuel with oxygen. Atoms rearrange; nuclei unchanged
Radiation Energy transfer by electromagnetic waves; the only mechanism that crosses a vacuum
Scale The size, amount, or time span at which something occurs
Habitable Suitable for life
Exoplanet A planet orbiting a star other than our Sun

The Four Statements to Know Cold

  1. Atoms of each element absorb and emit characteristic wavelengths — so we can identify an element in tiny quantities and at great distance.

  2. The study of stars' light spectra and brightness tells us what stars are made of, how they move, and how far away they are.

  3. Nuclear fusion in the Sun's core releases the energy that ultimately reaches Earth as radiation.

  4. The Sun is changing and will burn out over a life span of about 10 billion years.

WHAT THIS DECK IS: the projection companion to the guided-notes packet "Guided Notes: How the Sun Works" (ess-u1-guided-notes.pdf). Slides follow the packet Part for Part. Students fill in their blanks as each slide comes up. NOT the teaching deck. The 5E instruction lives in U1_Discovering_New_Worlds_L0-L1_Slides. This is end-of-unit consolidation: content knowledge only. EVERY BLANK ANSWER on these slides is shown in gold. Advance, let students write, then discuss. STANDARDS: ESS1.A(1) Sun's ~10-billion-year life span · ESS1.A(2) spectra and brightness identify composition · PS3.D(1) fusion in the core releases the energy that reaches Earth as radiation · PS4.B(4) atoms absorb and emit characteristic frequencies. TIMING: about one class period at a brisk pace, or two if you stop to work the questions.

TEACHER MOVE: Set the expectation that the gold text is what they copy. The constructed-response questions are deliberately not on the slides so students have to reason rather than transcribe.

PACKET: Part A, first four blanks. THE DIAGRAM: one wave whose wavelength shrinks left to right. Crest-to-crest is ONE wavelength — the gold arrows mark three of them so students can see the distance getting smaller while the wave itself never stops being a wave. COLOR IS NOT PAINTED ON. The stroke runs red to violet because that IS what changing wavelength does to visible light. Long (~700 nm) reads red; short (~400 nm) reads violet. COMMON SLIP: students write "colour" for the first blank but then say wavelength IS colour. Wavelength DETERMINES colour; they are not the same thing. ASK: "Between which two points do I measure one wavelength?" Accept crest-to-crest or trough-to-trough — any two matching points one cycle apart. WATCH FOR: students reading the shrinking wave as the wave "slowing down" or "losing energy." In a vacuum every wavelength of light travels at the same speed; shorter wavelength means HIGHER frequency and higher energy per photon. (Beyond the assessment boundary here — offer only if asked.)

PACKET: Part A, closing Key idea box. KEY POINT: This is the premise the whole lesson rests on. A telescope fitted with a spectroscope is the only instrument that reaches the Sun.

PACKET: Part B. One blank ("missing"). THE SOURCE RULE matters and students skip it: continuous comes from a hot glowing SOLID, LIQUID or DENSE GAS — a light bulb filament, or the Sun's interior.

PACKET: Part B, "The rule that ties them together." WHY IT MATTERS: without this, the whole lab-reference method is unjustified. Students need to see that the lab and the star are showing the same physics from opposite directions.

PACKET: Part C, three blanks. THIS IS PS4.B(4): "Atoms of each element emit and absorb characteristic frequencies of light. These characteristics allow identification of the presence of an element, even in microscopic quantities."

PACKET: Part C, "Why this counts as evidence." CCC #1 HS element: empirical evidence is needed to identify patterns. This is the single most-tested habit in the unit — make students say numbers out loud.

PACKET: Part D opening, questions 2 and 3. WHY THIS STEP: a consistent solar spectrum is what makes the lab comparison meaningful. If the Sun's spectrum changed night to night, matching it against a reference would prove nothing. PUSH FOR NUMBERS: "They look the same" is an impression. Naming the wavelengths makes it empirical evidence.

PACKET: Part D, the comparison figure. A MATCH MUST WORK BOTH WAYS. Two failure modes: (1) matching one or two lines and stopping — ask "is there any Sun line you have NOT accounted for?"; (2) seeing one coincidental overlap and declaring a match — ask "does that element have lines the Sun does NOT have?"

PACKET: Part D, Conclusion box. Two blanks. ESS1.A(2) verbatim: "The study of stars' light spectra and brightness is used to identify compositional elements of stars, their movements, and their distances from Earth." BRIDGE TO L2: that last sentence is what the Star Life Cycles lesson is built on.

PACKET: Part E, first table. BOTH ARE REAL PROCESSES that genuinely release energy. The question is never "is combustion real" — it is whether it works at the required SCALE.

PACKET: Part E, data table (two blanks) and question 6. MAKE THEM DO THE DIVISION. The ratio is the quantitative half of the argument; the time span is the other half.

PACKET: Part E, "Now the time evidence." THE ROCK IS THE POINT. The 4-billion-year figure is physical evidence from Earth, not an assumption about the Sun. Liquid water needs steady energy, so the Sun has run at roughly this rate that whole time.

PACKET: Part E, Conclusion box. CCC #3 — Scale, Proportion and Quantity, stated as the curriculum states it. THE "IT'S SO BIG" OBJECTION (packet question 7): the 50,000-year figure ALREADY assumes the Sun burns all of its hydrogen. Size is inside the calculation. SPECTRUM EVIDENCE TOO (packet question 8): helium is present — the product of fusion. No oxygen and no water — which combustion requires and produces.

PLACEMENT: deliberately AFTER the scale argument. Students first establish THAT fusion powers the Sun from evidence; only then do we open up what fusion is. That order matches how the class built the claim and keeps the evidence doing the work. ASSESSMENT BOUNDARY — SAY THIS TO STUDENTS: HS-ESS1-1 states "Assessment does not include details of the atomic and sub-atomic processes involved with the sun's nuclear fusion." These three slides are enrichment. They answer the question students always ask, and they make the Part E conclusion feel earned rather than asserted — but nothing here is on a blank in the packet and nothing here is required. SKIP IF SHORT ON TIME. Nothing later in the deck depends on these three slides.

WHY TWO OF THE PROTONS BECOME NEUTRONS: in the chain, two protons each convert to a neutron, emitting a positron and a neutrino. Students do not need this — but they will notice the n's appear from nowhere, so have the one-sentence answer ready. CONNECT BACK: this is exactly why helium shows up in the Sun's spectrum. Helium is the PRODUCT. That was one of the spectrum arguments against combustion in Part E. NOT ASSESSED. Enrichment only.

THIS IS THE ANSWER to "but where does the energy actually COME from?" — the question the Part E scale argument never answers. THE NUMBERS CHECK OUT: luminosity 3.83x10^26 W, so m = E/c^2 = 3.83x10^26 / 9x10^16 = 4.3x10^9 kg/s, about 4 million tonnes per second. At 0.7% conversion that needs ~600 million tonnes of hydrogen per second. MASS UNITS: u = atomic mass unit. If students have not met it, say "a unit for weighing single atoms" and move on — the POINT is that the bars are almost but not quite the same length. CONNECT TO L2: this 0.7% is why a star's fuel is finite, and why mass controls lifespan. NOT ASSESSED. Enrichment only.

THE CAUSAL LOOP WORTH NAMING: gravity creates the conditions for fusion, and fusion pushes back against gravity. That balance is EQUILIBRIUM — the central idea of the whole Star Life Cycles lesson. These slides set it up. SUN'S CORE: about 15 million K, and roughly 150 g/cm3 — denser than lead, but still a gas because it is fully ionised plasma. STRONG NUCLEAR FORCE: enormously strong but only over a tiny range. Protons have to be nearly touching before it beats the electrical repulsion. One sentence is plenty. DO NOT go into quantum tunnelling, the Coulomb barrier by name, or the p-p chain steps. Well past the boundary. NOT ASSESSED. Enrichment only.

PACKET: Part F, four blanks. HS-ESS1-1 emphasis: "the energy transfer mechanisms that allow energy from nuclear fusion in the sun's core to reach Earth." PS3.D(1) says the same. CLOSE THE LOOP OUT LOUD. Students spent two days on spectra without realising the light IS the energy transfer. WHY NOT CONDUCTION OR CONVECTION (packet question 9): both need matter. Space is a vacuum.

PACKET: Part G, four blanks. THE WORDING NOTE MATTERS. Students have just spent a whole Part ruling out burning; the DCI's own phrase "burn out" undoes that if you leave it unexamined. PACKET QUESTIONS 10 AND 11 ask why stability over time matters, and what happens if output changes. Work those together — they are the bridge to the anchor phenomenon.

PACKET: Part H closing box. THESE FOUR ARE THE DCIs for this lesson, in student language: PS4.B(4), ESS1.A(2), PS3.D(1), ESS1.A(1). If a student remembers nothing else, these four carry the assessable content. UP NEXT: Star Life Cycles — does the exoplanet have a star like our Sun?