Coaching Centre
The Universe review
A summary of the whole Universe chapter, key words, and a mixed test covering the scale of the universe, parallax and light years, star colour and brightness, the H–R diagram, the life cycles of stars, galaxies and Hubble's law, the Big Bang and its evidence, gravity and dark matter, telescopes and space exploration, and exoplanets.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 6A Structure and scale | Earth → Solar System → Milky Way (a barred spiral, about 100 000 ly across, 100–400 billion stars) → Local Group → Laniakea supercluster → observable universe (about 93 billion ly across). 1 ly = 9.46 × 10¹² km. Looking far away is looking back in time. Use scientific notation for huge numbers. |
| 6B Measuring distances | Parallax: measure a star's shift six months apart; d (pc) = 1 ÷ p (arcseconds); 1 pc = 3.26 ly. Works to thousands of light years with Gaia. Apparent vs absolute magnitude (smaller = brighter; 5 magnitudes = 100×). Brightness falls with distance². Further away: standard candles (Cepheids, type Ia supernovae), then Hubble's law. |
| 6C Colour and brightness | Stars are hot glowing objects: hotter stars peak at shorter wavelengths and look blue-white; cooler stars look red. Spectral classes O B A F G K M (hot to cool); the Sun is G2, 5800 K. Luminosity depends on radius² and temperature⁴. Spectra reveal composition (mostly hydrogen and helium; Cecilia Payne). |
| 6D H–R diagram | Luminosity against temperature (hot on the left). Main sequence (90% of stars, fusing hydrogen; mass decides position and lifetime), red giants and supergiants (top right), white dwarfs (bottom left). Massive stars burn out far faster. Cluster turn-offs give ages. |
| 6E Life cycles of stars | Nebula → protostar → main sequence (gravity balanced by pressure from fusion). Low mass: red giant → planetary nebula → white dwarf. High mass (above about 8 Suns): red supergiant → iron core → supernova → neutron star or black hole. Stars and supernovae made most elements heavier than helium. |
| 6F Galaxies and expansion | Elliptical, spiral, barred spiral, lenticular, irregular galaxies. Redshift = galaxy moving away. Hubble–Lemaître law: v = H₀ d, H₀ ≈ 70 km/s/Mpc; space itself is expanding. Age ≈ 1/H₀ ≈ 14 billion years. The Hubble tension: different methods give 67 or 73. |
| 6G The Big Bang | Universe began 13.8 billion years ago, hot and dense, expanding and cooling; not an explosion from a point. Evidence: expansion, CMB (released at 380 000 years, now 2.725 K, with ripples), 25% helium, galaxies changing over time. Steady State rejected after the CMB (1965). E = mc². Dark energy (1998) speeds up expansion. |
| 6H Gravity | F = Gm₁m₂/d² (inverse square). Gravity grew CMB ripples into stars, galaxies and the cosmic web; spinning collapsing clouds form discs. Flat rotation curves, cluster motions and lensing show dark matter (about 5× ordinary matter). Gravitational waves detected by LIGO in 2015. |
| 6I Technology | Aperture: light collected ∝ D², resolution ≈ 1.22λ/D. Segmented mirrors, adaptive optics, CCDs, multi-fibre spectrographs. The atmosphere blocks most wavelengths, so many telescopes must be in space; JWST (2021) observes infrared from L2. Radio arrays, FRBs, the EHT black hole images (2019, 2022). Spacecraft and Artemis II (2026). |
| 6J Are we alone? | More than 6000 exoplanets. Transits give size (depth = (Rp/Rs)²) and period; radial velocity gives mass; microlensing finds distant planets. Habitable zone: liquid water possible, closer to cooler stars. Searching Mars, icy moons, exoplanet atmospheres (biosignatures) and SETI; no confirmed life yet. |
Putting it together
The whole chapter is about reading the light. Almost everything we know about the universe comes from radiation that has travelled to us across space, sometimes for billions of years. Its direction and tiny yearly shifts give distances to nearby stars (6B). Its brightness, compared with how bright the source really is, gives distances further out. Its colour gives temperature (6C), and together with brightness places a star on the H–R diagram, which reveals the star's mass, stage of life and future (6D, 6E). The dark lines in its spectrum give what it is made of, and their shift gives how fast it is moving: that is how Hubble found the expansion of the universe (6F), which leads back to the Big Bang (6G). Light from the beginning itself still reaches us as the cosmic microwave background. Where the light doesn't add up, as with galaxies spinning too fast, we infer things we can't see, such as dark matter (6H). Each new instrument that collects more light, finer detail or new wavelengths has changed the picture (6I), and the same tools are now reading the atmospheres of other worlds (6J). At every step, ideas were tested, argued over and revised by the scientific community, and the universe has turned out to be far bigger, older and stranger than anyone expected.
Key words
absolute magnitude · adaptive optics · aperture · apparent magnitude · astronomical unit · Big Bang theory · biosignature · black hole · blueshift · cosmic microwave background · cosmic web · dark energy · dark matter · elliptical galaxy · exoplanet · fast radio burst · fusion · galaxy · gravitational lensing · gravitational waves · habitable zone · Hertzsprung–Russell diagram · Hubble constant · Hubble–Lemaître law · interferometry · inverse square law · irregular galaxy · light year · Local Group · luminosity · main sequence · nebula · neutron star · nucleosynthesis · parallax · parsec · planetary nebula · protostar · pulsar · radial velocity · red giant · redshift · resolution · spectral class · spectrum · spiral galaxy · standard candle · Steady State theory · supergiant · supernova · transit · white dwarf
Test yourself
A star has a parallax of 0.25 arcseconds. Its distance is
d = 1 ÷ 0.25 = 4 pc, about 13 light years.
Star X is blue-white and Star Y is orange-red. Which statement is correct?
Colour gives temperature. A cool star can still be very luminous if it is very large.
On an H–R diagram, a star in the bottom left is
White dwarfs are in this region.
A star of 15 solar masses will end its life as
Above about 8 solar masses the core collapses; up to roughly 20–25, a neutron star remains.
A galaxy is 200 Mpc away. With H₀ = 70 km/s/Mpc, its recession speed is about
v = 70 × 200 = 14 000 km/s.
Which observation is best explained by dark matter?
The speeding up of expansion is attributed to dark energy.
Which is NOT a reason for putting a telescope in space?
A few hundred thousand kilometres is nothing compared with the distances to stars.
Label the spectra and absorption lines.
What does each observation provide evidence for?
Order these from smallest to largest.
Stars shine because of nuclear in their cores. The distance light travels in one year is a . A star's colour shows its surface . Light from distant galaxies shows a , which shows the universe is . The faint glow left over from 380 000 years after the Big Bang is the cosmic background. A planet that passes in front of its star produces a .
A massive star (20 times the Sun's mass) forms in a nebula in the Milky Way. Describe the life of this star from its formation to its end, and explain how stars like it are connected to the existence of planets and life.
The star begins in a cold, dense part of a nebula of gas and dust. Gravity makes a clump collapse; as it shrinks it heats up and spins faster, forming a protostar surrounded by a disc. When the core reaches about 10 million K, hydrogen nuclei fuse into helium and the star joins the main sequence. Here the outward pressure from fusion energy balances the inward pull of gravity. Because it has 20 times the Sun's mass, the star is very hot, blue-white and tens of thousands of times more luminous than the Sun, at the top left of the main sequence on the H–R diagram. It uses its fuel so quickly that it lasts only about 10 million years, compared with 10 billion for the Sun.
When the hydrogen in its core runs out, the core contracts and heats, and the outer layers swell and cool, so the star becomes a red supergiant. The core fuses helium into carbon and oxygen, then heavier elements such as neon, magnesium and silicon, forming layers like an onion, until the centre is iron. Fusing iron takes in energy rather than giving it out, so the core can no longer support itself. In less than a second it collapses, and the rebound and flood of neutrinos blow the outer layers apart in a supernova. The core is left as a neutron star, or if enough mass falls back, a black hole.
This matters for planets and life because almost all elements heavier than helium were made inside stars. Supernovae scatter carbon, oxygen, silicon, iron and other elements into space (and neutron-star mergers make heavy elements like gold), enriching the nebulae where new stars form. The Solar System formed from such a cloud about 4.6 billion years ago, so the rocky Earth and the carbon, nitrogen, oxygen, phosphorus and sulfur in living things all came from earlier generations of stars like this one.