The Universe & Big Bang - Year 10 Science

What you will learn

describe the hierarchical structure of the universe (moons, planets, stars, solar systems, galaxies, clusters),
interpret redshift as evidence for an expanding universe (Hubble’s law),
explain the Big Bang model and its three main lines of evidence,
describe the life cycle of a star,
appreciate the scale of astronomical distances and times.

Worked example 0 Real-world example: why the night sky is dark (Olbers' paradox)

If the universe were infinite, static and full of stars, every line of sight should eventually hit a star, making the sky as bright as the Sun’s surface. It isn’t. Why?

The universe is not infinitely old — it is $\sim 13.8$ billion years old.
Light from galaxies beyond a certain distance has not yet reached us.
The universe is expanding, so light from distant galaxies is red-shifted toward lower energy and longer wavelengths — much of it shifted out of the visible range.
Both effects keep the night sky dark.

Key idea: even something as simple as the darkness of the night sky is evidence for a finite, expanding universe. Olbers saw this paradox in the 1820s — it was not fully resolved until the 20th century.

1. Structure of the universe

From smallest to largest:

Moons orbit planets. Earth has one; Jupiter has 95 confirmed.
Planets orbit stars. Our solar system has eight.
Stars: self-luminous balls of plasma powered by nuclear fusion. Range in mass from $\sim 0.08$ to $\sim 100$ solar masses.
Solar systems: a star plus orbiting planets, moons, asteroids and comets.
Galaxies: $10^8$ to $10^{12}$ stars bound by gravity. The Milky Way (ours) contains $\sim 200$ billion stars.
Galaxy clusters: groups of galaxies. Our Local Group has $\sim 50$ galaxies including Andromeda.
Observable universe: $\sim 93$ billion light-years across, containing $\sim 2$ trillion galaxies.

Black holes are regions where gravity is so strong that nothing — not even light — can escape. They form when massive stars collapse. Supermassive black holes of millions to billions of solar masses sit at the centre of most galaxies.

Worked example 1 The scale of a light-year

A light-year is the distance light travels in one year. Calculate it, given $c = 3.0 \times 10^8$ m/s.

Seconds in one year: $365.25 \times 24 \times 60 \times 60 \approx 3.156 \times 10^7$ s.
Distance: $3.0 \times 10^8 \times 3.156 \times 10^7 \approx 9.47 \times 10^{15}$ m.
That is roughly $9.5$ trillion km.

The nearest star to the Sun, Proxima Centauri, is $\sim 4.24$ light-years away — so we see it as it was in $2020$ .

2. Evidence for the Big Bang

1. Redshift and Hubble’s law. Light from a receding object is stretched to longer wavelengths (redshifted), just as a siren is pitch-shifted as it moves away (Doppler effect). Hubble showed in 1929 that recession velocity is proportional to distance:

v = H_0 d

where $H_0 \approx 70$ km/s per megaparsec is the Hubble constant. This is exactly what an expanding universe predicts.

2. Cosmic microwave background (CMB). Predicted as the “afterglow” of the hot early universe, the CMB was discovered in 1965. It fills the entire sky at a temperature of $2.73$ K, with tiny temperature fluctuations that match theoretical predictions with extraordinary precision.

3. Abundance of light elements. Big Bang nucleosynthesis predicts $\sim 75\%$ hydrogen and $\sim 25\%$ helium by mass, plus tiny traces of lithium and deuterium. Observations match.

Worked example 2 Using Hubble's law

A galaxy is measured to be receding at $7\,000$ km/s. Estimate its distance using $H_0 = 70$ km/s per megaparsec.

$d = \dfrac{v}{H_0} = \dfrac{7\,000}{70} = 100$ megaparsecs.
One megaparsec $\approx 3.26$ million light-years, so $d \approx 326$ million light-years.

Key idea: we are seeing that galaxy as it was $326$ million years ago — light takes that long to reach us.

3. Timeline of the universe

Major events in the history of the universe.

$t = 0$ : Big Bang — tiny, hot, dense.
$10^{-32}$ s: cosmic inflation — rapid exponential expansion.
$3$ minutes: Big Bang nucleosynthesis — hydrogen and helium nuclei form.
$380\,000$ years: recombination — universe cools enough for neutral atoms to form; photons (now the CMB) travel freely.
$\sim 200$ million years: first stars ignite.
$13.8$ billion years: today.

4. Life cycle of a star

Worked example 3 The Sun's future

Describe the stages the Sun will go through from now to the end of its life.

Main sequence (current, $\sim 4.6$ Gyr old, $\sim 5$ Gyr remaining): fusing hydrogen to helium in its core.
Red giant ( $\sim 5$ Gyr from now): core hydrogen exhausted; outer layers expand beyond Earth’s orbit.
Planetary nebula: outer layers puff off as a beautiful shell of gas.
White dwarf: the exposed core, about Earth-sized but half the Sun’s mass, slowly cooling over billions of years.

More massive stars end more violently: they collapse, trigger a supernova explosion, and leave behind a neutron star or (for the most massive) a black hole. Elements heavier than iron are made in supernovae and merging neutron stars.

Worked example 4 Where did your calcium come from?

A calcium atom in your bones was made billions of years ago in a different star. Describe the chain of events.

Calcium (atomic number 20) is made in massive stars by fusion of lighter elements during the red-giant phase.
The star eventually goes supernova, scattering the heavy elements into space.
A gas cloud enriched in these heavy elements forms a new solar system — ours, $\sim 4.6$ billion years ago.
Earth accreted from that enriched material; calcium became part of rocks, oceans, plants, and your skeleton.

Key idea: every atom heavier than lithium in your body was forged in a star. “We are all star-stuff.” — Carl Sagan.

Practice: Year 10

Fluency

Structure and scale

Arrange in order of size: moon, galaxy, star, solar system, planet, galaxy cluster.
How many stars are in the Milky Way, approximately?
Name the closest galaxy to our own that is comparable in size.
Define a light-year in your own words.
What is a black hole?

Fluency

Big Bang evidence

Name three lines of evidence for the Big Bang.
State Hubble’s law in words and as a formula.
What is the cosmic microwave background, and why is it important?
What percentage of the universe by mass is hydrogen? Helium?
How old is the universe, approximately?

Fluency

Stars

What is the main energy source of a star?
What is a main-sequence star?
Name the final state(s) of a star like our Sun.
Name the possible final states of a star much more massive than the Sun.
Where are elements heavier than iron produced?

Reasoning

Explain and interpret

Explain the Doppler effect with an everyday example (e.g. a passing ambulance) and link it to astronomical redshift.
Using Hubble’s law, explain why the universe must have had a beginning.
Why does the CMB have a nearly uniform temperature across the sky?
Why is it said that “looking far away is looking back in time”?
Why are massive stars shorter-lived than low-mass stars, even though they contain more fuel?

Problem solving

Apply

A galaxy is measured to have a redshift corresponding to $v = 21\,000$ km/s. Using $H_0 = 70$ km/s/Mpc, find its distance in Mpc and in millions of light-years ( $1$ Mpc $\approx 3.26$ million ly).
The Andromeda galaxy is $\sim 2.5$ million light-years away. How long has the light been travelling? What does this tell you about what you see when you look at it?
The Sun will reach red-giant phase in $\sim 5$ Gyr. Express this in seconds using scientific notation.
A supernova releases $\sim 10^{44}$ J of energy. Compare this with the Sun’s total lifetime output of $\sim 10^{44}$ J. What does this tell you about supernovae?

Challenge

Reasoning

Harder reasoning

The cosmic microwave background has a temperature of $2.73$ K today. In the early universe, photons were in equilibrium with plasma at $\sim 3\,000$ K. Explain, using expansion and wavelength stretching, why the observed CMB temperature is so much lower.
An astronomer measures redshift $z = 1.0$ for a distant galaxy, meaning its observed wavelength is twice the rest wavelength. Explain why this implies the universe was half its current size when the light was emitted.
The Hubble constant has been measured by two independent methods (CMB + early universe, and nearby supernovae) giving slightly different values ( $67.4$ vs $73$ km/s/Mpc). Why is this “Hubble tension” potentially a big deal? What might it imply about cosmology?
Explain why life’s building blocks (C, N, O, heavy elements) require a previous generation of stars to have existed. Why is it unlikely we are the first life in the universe based on this argument?

Answers

Answer key

Attempt the practice first. When you're ready to check, expand the answers below.

Show the full answer key

Year 10 answers

Fluency

Structure and scale

Moon $<$ planet $<$ star $<$ solar system $<$ galaxy $<$ galaxy cluster.
About $200$ billion.
Andromeda (M31).
The distance that light travels in one year, about $9.5$ trillion km.
A region of space where gravity is so strong that nothing, not even light, can escape.

Fluency

Big Bang evidence

Redshift (Hubble’s law); cosmic microwave background; abundance of light elements (H and He).
The recession speed of a galaxy is proportional to its distance: $v = H_0 d$ .
The faint thermal radiation from when the universe first became transparent ( $\sim 380\,000$ years after the Big Bang). It is direct evidence of the hot, dense early universe.
About $75\%$ hydrogen, $25\%$ helium by mass (of ordinary matter).
About $13.8$ billion years.

Fluency

Stars

Nuclear fusion of hydrogen into helium (and later heavier elements).
A star that is steadily fusing hydrogen to helium in its core — the longest, most stable life stage.
Red giant, planetary nebula, white dwarf.
Supernova followed by a neutron star or black hole.
In supernova explosions and neutron-star mergers.

Reasoning

Explain and interpret

A passing ambulance’s siren is higher pitched as it approaches and lower as it recedes. Similarly, light from receding galaxies is stretched to longer (redder) wavelengths. Galaxies all showing redshift indicates the universe is expanding.
If all galaxies are moving apart, running the film backwards shows they were all together at one point in the past. Extrapolating gives a starting time about $13.8$ Gyr ago.
Before recombination, the universe was so dense it behaved like a uniform plasma; all regions had the same temperature. Inflation ironed out any variations, so the CMB is almost perfectly uniform except for tiny fluctuations.
Light takes time to travel. The further away we look, the longer the light has been in transit, so we see objects as they were in the past.
Massive stars burn their fuel much faster to counteract their stronger gravity; higher temperatures and pressures consume hydrogen in $\sim 10$ Myr vs the Sun’s $\sim 10$ Gyr.

Problem solving

Apply

$d = \dfrac{21\,000}{70} = 300$ Mpc $\approx 978$ million ly.
$\sim 2.5$ million years. You are seeing Andromeda as it was $2.5$ Myr ago.
$5 \times 10^9 \text{ yr} \times 3.156 \times 10^7 \text{ s/yr} \approx 1.58 \times 10^{17}$ s.
A supernova releases in seconds roughly the same energy the Sun emits over its entire $10^{10}$ -year lifetime. Supernovae are extraordinarily energetic, visible across the universe.

Reasoning

Challenge

As the universe expands, photon wavelengths are stretched by the same factor. Going from $\sim 3\,000$ K (emission) to $2.73$ K (today) corresponds to a $\sim 1\,100$ -fold stretch — the universe is now $1\,100$ times larger than at the epoch of recombination.
$z = 1.0$ means observed wavelength is twice emitted. Wavelength has been stretched by a factor $1 + z = 2$ , meaning space itself has doubled in scale since the light was emitted. So the universe was half its current size.
The two methods probe different epochs (early universe vs nearby universe). If both are right, something in between (new physics, dark energy behaviour) may be missing from the standard model of cosmology.
Life requires carbon, oxygen, nitrogen, calcium, iron — none of which existed in the pristine Big Bang hydrogen/helium mix. Stars had to form, die, and seed the gas clouds with heavy elements before planets and life were possible. This pushes first-life possibilities back only once enough massive stars had exploded — likely $\sim 1$ - $2$ Gyr after the Big Bang.

Prefer paper? Print the answer key as a separate booklet: open print view ->