Stars are huge balls of gas that shine by nuclear fusion. Their mass decides how bright they are, how long they live and how they die.
📏 Distance and brightness
Stars are so far away that we use the light-year, the distance light travels in one year (about 9.5 trillion km). For nearby stars astronomers measure parallax, the small apparent shift of a star as Earth orbits the Sun: distance in parsecs = 1 ÷ parallax angle in arcseconds, and 1 parsec = 3.26 light-years. Apparent magnitude is how bright a star looks; luminosity is how much energy it really emits.
parallax 0.25 arcsec → d = 1/0.25 = 4 parsecs
4 parsecs = 4 · 3.26 = 13.04 light-years
star 4 light-years away: light takes 4 years to arrive
we see it as it was 4 years ago
🖼️ See it
A star like our Sun skips the supernova: it swells into a red giant and then shrinks to a white dwarf.
💥 Life and death of stars
A star is born in a nebula that collapses under gravity. When the core is hot enough, hydrogen fuses into helium and a main-sequence star is born. Stars like the Sun later become red giants, then shed their outer layers as a planetary nebula and leave a white dwarf. Stars of about 8 times the Sun’s mass or more explode as a supernova, leaving a neutron star or a black hole. More massive stars burn faster and live shorter lives.
Sun: total lifetime about 10 billion years
age now 4.6 billion → remaining ≈ 10 − 4.6 = 5.4 billion years
Sun-like: nebula → main sequence → red giant → white dwarf
massive: nebula → main sequence → supergiant → supernova → neutron star or black hole
We are made of stardust. Elements heavier than hydrogen and helium (carbon, oxygen, iron) were made in stars and spread by supernovae.
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Fusion
Main-sequence stars shine by fusing:
Lightest fuel.
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Sun’s fate
The Sun will end its life as a:
Small star.
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Massive star life
Compared with the Sun, a very massive star lives:
Fast burn.
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Light-year
A light-year is a unit of:
A long way.
✍️ Type your answer · 5 of 7
A star has a parallax angle of 0.25 arcseconds.
What is its distance, in parsecs? (1/0.25)
Enter an exact answer, without rounding.
parsecs
distance = 1 ÷ parallax.
✍️ Type your answer · 6 of 7
A star is 4 parsecs away. 1 parsec = 3.26 light-years.
How many light-years is that?
Enter an exact answer, without rounding.
ly
Multiply.
✍️ Type your answer · 7 of 7
The Sun lasts about 10 billion years and is 4.6 billion years old.
How many billion years remain?
Enter an exact answer, without rounding.
billion years
Subtract.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Parallax is used to measure the distance to:
Question 2 of 5
A supernova leaves behind a:
Question 3 of 5
A star is born in a:
Question 4 of 5
Heavy elements like iron mostly come from:
Question 5 of 5
Luminosity is a star’s:
Lesson 2 · Cosmology
🌌 Galaxies & the Expanding Universe
Stars gather in galaxies, and the galaxies are moving apart. The universe began in a hot, dense state about 13.8 billion years ago.
🌀 Galaxies
A galaxy is a huge system of stars, gas and dust held together by gravity, often with a supermassive black hole at the centre. Our home, the Milky Way, is a spiral galaxy about 100 000 light-years across with a few hundred billion stars. The nearest large galaxy is Andromeda, about 2.5 million light-years away. Galaxies are grouped into clusters.
Milky Way diameter ≈ 100 000 light-years
light crosses it in 100 000 years
Andromeda: 2.5 million ly away
we see it as it looked 2.5 million years ago
🖼️ See it
The Hubble constant H₀ is about 70 km/s per megaparsec. The straight line shows the whole universe expanding, not galaxies flying through space.
🎈 Expansion and the Big Bang
Light from distant galaxies is red-shifted (stretched to longer wavelengths), showing they are moving away. In 1929 Edwin Hubble found that the farther a galaxy, the faster it recedes: v = H₀ × d, with H₀ ≈ 70 km/s per megaparsec. Running the expansion backwards gives a start, the Big Bang, about 13.8 billion years ago. Other evidence: the cosmic microwave background and the amounts of hydrogen and helium.
v = 70 · d (d in Mpc)
galaxy at 100 Mpc: v = 70 · 100 = 7000 km/s
redshift z = (observed − emitted) / emitted
(550 − 500)/500 = 50/500 = 0.1
The universe is not expanding into anything. Space itself stretches. Imagine dots on an inflating balloon.
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Milky Way
The Milky Way is a:
Galaxy.
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Redshift
A red-shifted galaxy is:
Receding.
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Big Bang evidence
Evidence for the Big Bang includes the:
Afterglow.
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Hubble
Hubble’s law says that more distant galaxies:
Farther, faster.
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Hubble’s law: v = 70 × d, where d is in megaparsecs and v is in km/s. A galaxy is 100 Mpc away.
What is its speed, in km/s?
Enter an exact answer, without rounding.
km/s
Multiply.
✍️ Type your answer · 6 of 7
Light of 500 nm is observed at 550 nm. z = (550 − 500)/500.
What is the redshift z?
Enter an exact answer, without rounding.
Divide.
✍️ Type your answer · 7 of 7
The Milky Way is 100 000 light-years across.
How many years does light take to cross it?
Enter an exact answer, without rounding.
years
Same number.
📝 Lesson quiz · 5 questions
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Question 1 of 5
The universe began about:
Question 2 of 5
The Big Bang was:
Question 3 of 5
Galaxies are held together by:
Question 4 of 5
A light-year measures:
Question 5 of 5
Dark matter is known from:
Lesson 3 · Climate
🌡️ Climate Change
Earth’s climate has changed naturally for billions of years. The current rapid warming is caused mainly by human activity.
📈 The evidence
Global average temperature has risen about 1.2 °C since the late 1800s. Carbon dioxide has risen from about 280 ppm before industry to about 420 ppm today. Other evidence: shrinking glaciers and sea ice, rising sea level (about 3–4 mm a year), warming oceans and more extreme heat. Ice cores give a record of past CO₂ and temperature for 800 000 years.
CO₂: 420 − 280 = 140 ppm increase
140/280 · 100 = 50% rise
sea level 3 mm/year · 50 years = 150 mm = 15 cm
🖼️ See it
Approximate yearly averages. The rise of almost 100 ppm in sixty years comes mainly from burning fossil fuels.
🧯 Causes, impacts and responses
Burning fossil fuels, deforestation, farming (methane) and industry add greenhouse gases. Impacts include heatwaves, drought, flooding, melting ice, ocean acidification and ecosystem change. Mitigation reduces emissions (renewables, efficiency, forests); adaptation prepares for change (sea walls, drought-resistant crops).
emissions 40 units/year → cut 25% → 40 · 0.25 = 10 saved
new emissions = 40 − 10 = 30
glacier loses 2 m a year → 30 years: 60 m
Natural vs human: volcanoes and the Sun alone cannot explain the modern warming. Isotopes in the added CO₂ match fossil fuels.
✏️ Quick check · 1 of 7
Greenhouse source
The largest human source of CO₂ is:
Coal, oil and gas.
✏️ Quick check · 2 of 7
Mitigation
Which is a mitigation action?
Cut the cause.
✏️ Quick check · 3 of 7
Adaptation
A sea wall is an example of:
Adjust.
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Ice cores
Ice cores tell scientists about:
Old air.
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CO₂ rose from 280 ppm to 420 ppm.
What is the increase, in ppm?
Enter an exact answer, without rounding.
ppm
Subtract.
✍️ Type your answer · 6 of 7
A rise from 280 to 420 ppm is an increase of 140 ppm.
What percent increase is this? (140/280)
Enter an exact answer, without rounding.
%
Divide, ×100.
✍️ Type your answer · 7 of 7
Sea level is rising 3 mm a year.
How many mm in 50 years?
Enter an exact answer, without rounding.
mm
Multiply.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Deforestation contributes to climate change because trees:
Question 2 of 5
Ocean acidification is caused by oceans absorbing:
Question 3 of 5
Sea level rises partly because water:
Question 4 of 5
The Paris Agreement aims to limit warming to well below:
Question 5 of 5
A key sign that warming is global is that:
Lesson 4 · Weather
🌀 Severe Weather
Thunderstorms, tornadoes and hurricanes are powered by heat and moisture. Understanding them saves lives.
⚡ Thunderstorms and tornadoes
Thunderstorms form when warm, moist air rises quickly into a tall cumulonimbus cloud. Ice and water collide, building up electric charge that discharges as lightning; the sudden heating makes the air expand and produce thunder. Light arrives almost instantly but sound travels at about 340 m/s. A tornado is a rotating column of air from a storm cloud, rated on the Enhanced Fujita scale (EF0–EF5).
thunder 5 s after the flash
distance = 340 · 5 = 1700 m = 1.7 km
rule of thumb: 3 s per km
🖼️ See it
Hurricanes weaken quickly over land or cold water because they lose their supply of heat and moisture.
🌀 Hurricanes
Hurricanes (typhoons, cyclones) form over warm ocean water at least about 26.5 °C. Warm, moist air rises, spinning in toward low pressure. Winds of at least 119 km/h define a hurricane; categories 1–5 describe wind strength. The most deadly hazard is usually the storm surge, a rise of sea level pushed ashore, plus heavy rain and flooding. Hurricanes weaken over land or cold water because they lose their energy source.
hurricane 600 km away moving 20 km/h
time = 600/20 = 30 h
evacuate: 2400 people, buses hold 60
2400/60 = 40 buses
Water, not wind, causes most hurricane deaths. Follow evacuation orders.
✏️ Quick check · 1 of 7
Thunder
Thunder is caused by:
Heated air.
✏️ Quick check · 2 of 7
Hurricane water
Hurricanes form over:
Warm sea.
✏️ Quick check · 3 of 7
Storm surge
The storm surge is:
Sea rises.
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Tornado
A tornado is:
Spinning column.
✍️ Type your answer · 5 of 7
You count 5 seconds between lightning and thunder. Sound travels 340 m/s.
How far away is the lightning, in m?
Enter an exact answer, without rounding.
m
distance = speed · time.
✍️ Type your answer · 6 of 7
A hurricane is 600 km from the coast and moves at 20 km/h.
How many hours until it arrives?
Enter an exact answer, without rounding.
h
time = distance ÷ speed.
✍️ Type your answer · 7 of 7
2400 people must be evacuated. Each bus holds 60 people.
How many buses are needed?
Enter an exact answer, without rounding.
buses
Divide.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Hurricanes weaken over land because they:
Question 2 of 5
Lightning is:
Question 3 of 5
A hurricane watch means conditions are:
Question 4 of 5
The centre of a hurricane, which is calm, is the:
Question 5 of 5
Which is safest during a thunderstorm?
Lesson 5 · Society
⚠️ Natural Hazards & Risk
A hazard becomes a disaster when it meets people who are unprepared. Scientists, planners and communities work to reduce risk.
🎲 Hazard, exposure and vulnerability
A hazard is a natural event that could cause harm (earthquake, flood, landslide, tsunami, drought). Risk depends on the hazard’s probability, how many people and assets are exposed, and how vulnerable they are. A return period is the average time between events of a given size: a “100-year flood” has a 1% chance in any year. It does not mean one flood every 100 years.
return period 100 years → probability 1/100 = 1% per year
return period 50 years → 1/50 = 2% per year
expected loss = probability · damage
0.02 · 5 000 000 = 100 000
🖼️ See it
Risk grows with the hazard and with people’s vulnerability, and shrinks with their ability to cope.
🛡️ Reducing risk
Prediction and monitoring (seismographs, tide gauges, satellite data), warnings and evacuation plans, land-use planning (no building on floodplains or steep unstable slopes), building codes (earthquake-resistant design) and education all cut losses. Rich and poor countries are not equally vulnerable: the same earthquake can kill a few people in one place and thousands in another.
tsunami wave speed 200 m/s, coast 120 km = 120 000 m away
time = 120 000/200 = 600 s = 10 min
warnings can give minutes to hours to act
Preparedness works. Early warnings and drills cut deaths far more than they cost.
✏️ Quick check · 1 of 7
Risk
Risk is greatest where a hazard strikes a place that is:
Exposure and vulnerability.
✏️ Quick check · 2 of 7
Return period
A “100-year flood” means a flood with:
One in a hundred.
✏️ Quick check · 3 of 7
Land use
Which is good land-use planning for floods?
Avoid.
✏️ Quick check · 4 of 7
Building codes
Earthquake damage is reduced by:
Design.
✍️ Type your answer · 5 of 7
A flood has a return period of 50 years.
What is the chance in any one year, as a percent?
Enter an exact answer, without rounding.
%
Divide 100 by 50.
✍️ Type your answer · 6 of 7
The chance of a damaging flood is 0.02 per year and the damage would be 5 000 000.
What is the expected loss per year?
Enter an exact answer, without rounding.
probability · damage.
✍️ Type your answer · 7 of 7
A tsunami wave moves at 200 m/s and the coast is 120 000 m away.
How many seconds until it arrives?
Enter an exact answer, without rounding.
s
time = distance ÷ speed.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which is a natural hazard?
Question 2 of 5
A disaster occurs when a hazard:
Question 3 of 5
Early warning systems reduce:
Question 4 of 5
Which factor makes a community more vulnerable?
Question 5 of 5
A levee is a:
Lesson 6 · Resources
⛏️ Mineral Resources & Mining
Metals, building stone and fertiliser all come from the ground. Mining supplies them, but it has costs for land, water and people.
💎 Ores and extraction
An ore is a rock that contains enough of a valuable mineral to be mined at a profit. The grade is the percentage of useful metal in the ore. Near-surface deposits are removed by open-pit or strip mining; deeper ones by underground mining. The ore is crushed and the mineral is separated, leaving waste rock called tailings.
grade 2% copper, 500 t of ore:
0.02 · 500 = 10 t of copper
tailings = 500 − 10 = 490 t of waste
🖼️ See it
The depth and shape of the ore body decide the method; both need the land restored once mining ends.
♻️ Impacts and sustainability
Mining can destroy habitats, produce acid mine drainage that pollutes rivers, and use large amounts of water and energy. Companies must plan reclamation (restoring land after mining). Recycling saves ore and energy: making aluminium from scrap uses about 5% of the energy needed from ore. Good design, repair and reuse cut demand.
new aluminium: 100 units of energy
recycled aluminium: 5% → 0.05 · 100 = 5 units
saving = 100 − 5 = 95 units
gold grade 5 g/t, mining 200 t → 5 · 200 = 1000 g
Mining your own phone: old electronics hold more gold per tonne than many ores. Recycle them.
✏️ Quick check · 1 of 7
Ore
An ore is a rock that:
Worth mining.
✏️ Quick check · 2 of 7
Tailings
Tailings are:
Leftover.
✏️ Quick check · 3 of 7
Acid drainage
Acid mine drainage is a problem because it:
Pollution.
✏️ Quick check · 4 of 7
Recycling
Recycling aluminium saves mainly:
Large energy saving.
✍️ Type your answer · 5 of 7
A copper ore has a grade of 2% and 500 t of ore is mined.
How many tonnes of copper does it contain?
Enter an exact answer, without rounding.
t
Multiply by the grade.
✍️ Type your answer · 6 of 7
Of 500 t of ore, 10 t is copper.
How many tonnes of waste (tailings)?
Enter an exact answer, without rounding.
t
Subtract.
✍️ Type your answer · 7 of 7
Making aluminium from ore uses 100 units of energy. Recycling uses 5% of that.
How many units of energy does recycling use?
Enter an exact answer, without rounding.
Multiply.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Open-pit mining is used for deposits that are:
Question 2 of 5
Reclamation means:
Question 3 of 5
Which is a non-renewable resource?
Question 4 of 5
Ore grade is the:
Question 5 of 5
A way to reduce demand for new metals is to:
Final Challenge
🏆 12-Question Challenge Quiz
Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.
0 / 12
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Stars & Their Life Cycles
Which stage comes after the main sequence for a Sun-like star?
Q2 · Stars & Their Life Cycles
Luminosity is a star’s:
Q3 · Galaxies & the Expanding Universe
Galaxies show redshift because:
Q4 · Galaxies & the Expanding Universe
Dark matter is known from:
Q5 · Climate Change
The enhanced greenhouse effect is caused by:
Q6 · Climate Change
A key sign that warming is global is that:
Q7 · Severe Weather
The scale used to rate tornadoes is the:
Q8 · Severe Weather
Which is safest during a thunderstorm?
Q9 · Natural Hazards & Risk
Risk is best described as a combination of:
Q10 · Natural Hazards & Risk
A levee is a:
Q11 · Mineral Resources & Mining
Tailings must be managed carefully because they can: