GCSE · Chapter 21
Space physics: solar system, stars and the universe
By the end of this chapter you will be able to
- Name the planets in order and describe what keeps a planet in orbit
- Compare a low-orbit satellite with a geostationary one
- Put the stages of a star’s life cycle in order, for a Sun-like star and a massive star
- Explain red-shift and give evidence for the Big Bang
- [H] Use v = H0 d to find a distance or speed
You should already know
- Gravity and forces (Chapter 13)
- Waves, the electromagnetic spectrum and speed of light (Chapters 17 and 19)
Where this leads
- Energy and the Sun’s energy (Chapter 12, fusion in stars)
- Nuclear fusion (Chapter 12)
GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the solar system: the Sun, the eight planets, moons, comets and asteroids, and how gravity holds orbits in place, Describe artificial satellites in low orbit and in geostationary orbit, Describe the life cycle of a star, and how its mass decides its end (white dwarf, neutron star or black hole), Describe the universe: galaxies, the Big Bang, and the cosmic background radiation (idea), Explain red-shift of distant galaxies, [H] Use Hubble’s law, v = H0 d, to relate recession speed to distance.
Exercise P Do you remember?
1
FWhat is 70 multiplied by 100? Write the number.
7000
Method. Multiply.
- Multiply. 70 x 100 = 7000.7000
- Write the answer. 7000
Check. ✓
2
FWhat is the speed of light in a vacuum, in m/s, to one significant figure? Write as 3 x 10 to the power 8.
3 x 10^8
Method. Recall the value.
- Recall. The speed of light is about 3 x 108 m/s.3 x 10^8
- Write the answer. 3 x 10^8 m/s
Check. ✓
3
FWhich force keeps the Moon in orbit around the Earth? Write the word.
gravity
Method. Recall the force.
- Recall. Gravity pulls the Moon towards the Earth, keeping it in orbit.gravity
- Write the answer. gravity
Check. ✓
4
FWhich store holds energy when a ball is raised? Write the words.
gravitational potential
Method. Recall the store.
- Recall. A raised object has gravitational potential energy.gravitational potential
- Write the answer. gravitational potential
Check. ✓
5
FWhich type of EM radiation is given out by a warm body such as the Earth? Write the word.
infrared
Method. Recall the radiation.
- Recall. Warm bodies give out infrared radiation.infrared
- Write the answer. infrared
Check. ✓
21.1
The solar system
Video lesson: The solar systemComing soon
Context. On a clear night, Mia sees bright points of light. Some are stars; some are planets, which move against the background of stars. “What keeps the planets going round the Sun?” she asks. The answer is gravity.
The solar system is the Sun and everything that orbits it: the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), their moons, and smaller bodies such as comets and asteroids. The Sun has most of the mass of the solar system, so its gravity holds the planets in orbit. A planet moves in its orbit because its velocity is always changing direction, pulled towards the Sun by gravity.
The Sun and the eight planets on their orbits (not to scale)
Key ideas
- Eight planets orbit the Sun in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- Gravity between the Sun and a planet keeps the planet in orbit.
- Moons orbit planets; comets and asteroids also orbit the Sun.
- The Sun is a star; the planets shine by reflecting its light.
Worked example 21.1.1
Which planet is third from the Sun? Write the name.
Method. Use the order of the planets.
- Count. Mercury is first, Venus second and Earth third.Earth
- Decide. So the third planet from the Sun is Earth.Earth
- Write the answer. Earth
Check. ✓
Worked example 21.1.2
What keeps the Moon in orbit around the Earth? Write the word.
Method. Think about the force between the Earth and the Moon.
- Force. The Earth pulls the Moon with gravity.gravity
- Orbit. This pull changes the Moon’s direction, so it moves in a near-circle.changes direction
- Write the answer. gravity
Check. ✓
Worked example 21.1.3
A planet is pulled towards the Sun all the time. Which force does this? Write the word.
Method. Recall the force between masses.
- Look. Any two masses attract each other.attract
- Decide. The attraction between the planet and the Sun is gravity.gravity
- Write the answer. gravity
Check. ✓
Watch out
Watch out: a planet does not stay in orbit because there is no force on it. Gravity is the force that keeps the planet in orbit by constantly pulling it inwards.
Exam tip
Exam tip. When explaining an orbit, say gravity pulls the planet towards the Sun, and the planet’s direction keeps changing so it moves in an orbit.
Exercise 21A The solar system
1
FHow many planets are there in the solar system? Write the number.
8
Method. Count the planets.
- Count. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune: 8 planets.8
- Write the answer. 8
Check. ✓
2
FWhich planet is closest to the Sun? Write the name.
Mercury
Method. Recall the order.
- Recall. Mercury is the planet nearest the Sun.Mercury
- Write the answer. Mercury
Check. ✓
3
FWhich planet is fifth from the Sun? Write the name.
Jupiter
Method. Count from the Sun.
- Count. Mercury, Venus, Earth, Mars, Jupiter: Jupiter is fifth.Jupiter
- Write the answer. Jupiter
Check. ✓
4
FWhat is the name of the small icy bodies with long orbits that show a tail near the Sun? Write the name.
comet
Method. Recall the body.
- Recall. Comets have icy nuclei and tails when near the Sun.comet
- Write the answer. comet
Check. Accept asteroid for rocky bodies. ✓
5
FWhat is the name of the rocky bodies that orbit mainly between Mars and Jupiter? Write the name.
asteroids
Method. Recall the belt.
- Recall. The asteroid belt lies between Mars and Jupiter.asteroids
- Write the answer. asteroids
Check. ✓
6
FWhat force keeps the planets in orbit around the Sun? Write the word.
gravity
Method. Recall the force.
- Recall. Gravity from the Sun keeps the planets in orbit.gravity
- Write the answer. gravity
Check. ✓
7
FDo planets shine with their own light? Write yes or no.
no
Method. Planets reflect light from the Sun.
- Recall. Planets reflect light from the Sun; they do not make their own light.no
- Write the answer. no
Check. ✓
8
RExplain in one sentence why a planet moves in a curved path rather than in a straight line. Accept any sentence that says gravity from the Sun constantly pulls the planet towards it, changing its direction.
Accept any sentence that says gravity from the Sun constantly pulls the planet towards it, changing its direction.
Method. Think about the forces.
- Force. Gravity pulls the planet towards the Sun.towards the Sun
- Path. This changes the planet’s direction all the time, so it moves in an orbit.orbit
- Write the answer. Accept any sentence that says gravity from the Sun constantly pulls the planet towards it, changing its direction.
Check. ✓
9
RA pupil says “the planets stay in orbit because there is no force acting on them.” Explain why this is wrong. Accept any answer that says gravity is acting on them and pulling them towards the Sun.
Accept any answer that says gravity is acting on them and pulling them towards the Sun.
Method. Think about the forces.
- Force. Gravity from the Sun acts on each planet.gravity acts
- Orbit. Without gravity, planets would move in straight lines.straight lines
- Write the answer. Accept any answer that says gravity is acting on them and pulling them towards the Sun.
Check. ✓
10
FThe Sun is a star. Name the planet that is the largest in the solar system. Write the name.
Jupiter
Method. Recall the largest planet.
- Recall. Jupiter is the largest planet in the solar system.Jupiter
- Write the answer. Jupiter
Check. ✓
21.2
Artificial satellites and orbits
Video lesson: Artificial satellites and orbitsComing soon
Context. A weather satellite sends pictures of clouds, and a TV satellite dish always points at the same place in the sky. The two satellites are in different orbits.
An artificial satellite is a spacecraft that orbits the Earth, held in orbit by gravity. A low orbit satellite is close to the Earth, moves quickly, and passes over the surface in about 90 minutes or so; it is used for mapping and Earth observation. A geostationary satellite orbits high above the equator once every 24 hours, in the same direction as the Earth spins, so it stays above the same point. It is used for TV and communications.
Low-orbit and geostationary satellites above the Earth (not to scale)
Key ideas
- Low orbit: close to the Earth, fast, passes over the surface.
- Geostationary: high above the equator, 24 hour orbit, stays over one point.
- Both are held in orbit by gravity.
- Satellites use microwaves and radio for communication.
Worked example 21.2.1
A geostationary satellite has an orbit of 24 hours. How many orbits does it complete in 3 days? Write the number.
Method. Count the hours in 3 days.
- Hours. 3 days = 3 x 24 = 72 hours.72 h
- Divide. Orbits = 72 / 24 = 3.72 / 24 = 3
- Write the answer. 3
Check. ✓
Worked example 21.2.2
Which type of satellite stays above the same point on Earth? Write the name of the orbit.
Method. Recall which orbit keeps a satellite fixed in the sky.
- Recall. A geostationary orbit is above the equator and matches the Earth’s spin.geostationary
- Decide. So a geostationary satellite stays above the same point.geostationary
- Write the answer. geostationary
Check. ✓
Worked example 21.2.3
Why is a low-orbit satellite faster than a geostationary satellite? Write one reason in one sentence. Accept any sentence that says it is closer to the Earth, so gravity is stronger and its orbit is shorter.
Method. Think about the distance from the Earth.
- Close. A low-orbit satellite is closer to the Earth.closer
- Speed. The shorter orbit and stronger gravity mean it moves faster.faster
- Write the answer. Accept any sentence that says it is closer to the Earth, so gravity is stronger and its orbit is shorter.
Check. ✓
Watch out
Watch out: a geostationary satellite is above the equator. It is not above the same point at all latitudes.
Exam tip
Exam tip. For satellites, say low orbit for fast Earth observation and geostationary for communications that stay in one place.
Exercise 21B Satellites
1
FWhich type of orbit takes 24 hours? Write the name.
geostationary
Method. Recall the period.
- Recall. A geostationary satellite takes 24 hours to orbit the Earth.geostationary
- Write the answer. geostationary
Check. ✓
2
FWhich orbit is closer to the Earth: low orbit or geostationary? Write the name.
low orbit
Method. Compare the heights.
- Recall. Low orbit satellites are closer to the Earth.low orbit
- Write the answer. low orbit
Check. ✓
3
FWhat holds an artificial satellite in orbit? Write the word.
gravity
Method. Recall the force.
- Recall. Gravity from the Earth holds the satellite in orbit.gravity
- Write the answer. gravity
Check. ✓
4
FWhich orbit is used for satellite TV dishes that stay pointed at one spot? Write the name.
geostationary
Method. The dish stays fixed.
- Recall. A geostationary orbit lets the dish stay pointed at one spot.geostationary
- Write the answer. geostationary
Check. ✓
5
FHow many hours does a satellite in geostationary orbit take to go round the Earth once? Write the number.
24
Method. Recall the period.
- Recall. The orbit matches one full spin of the Earth, which is 24 hours.24
- Write the answer. 24 hours
Check. ✓
6
PSA low-orbit satellite goes round the Earth in about 90 minutes. How many orbits does it complete in 6 hours? Write the number.
4
Method. Convert 6 hours to minutes, then divide by 90.
- Convert. 6 hours = 360 minutes.360
- Divide. Orbits = 360 / 90 = 4.360 / 90 = 4
- Write the answer. 4 orbits
Check. ✓
7
FName one use of a low-orbit satellite. Write the use in a few words.
mapping
Method. Recall a use.
- Recall. Low-orbit satellites are used for mapping and Earth observation.mapping
- Write the answer. mapping
Check. Accept Earth observation or weather pictures. ✓
8
RExplain in one sentence why a geostationary satellite must be above the equator. Accept any sentence that says it must orbit in the same direction and at the same rate as the Earth spins.
Accept any sentence that says it must orbit in the same direction and at the same rate as the Earth spins.
Method. Think about the Earth’s spin.
- Spin. The Earth spins once every 24 hours.24 hours
- Match. The satellite must match this spin and direction to stay over one point.match spin
- Write the answer. Accept any sentence that says it must orbit in the same direction and at the same rate as the Earth spins.
Check. ✓
9
RA pupil says “a geostationary satellite is held up by air.” Explain why this is wrong. Accept any answer that says it is in space, held in orbit by gravity, with no air needed.
Accept any answer that says it is in space, held in orbit by gravity, with no air needed.
Method. Think about where it is.
- Space. It is in orbit far above the atmosphere.space
- Gravity. Gravity holds it in orbit; air is not needed.gravity
- Write the answer. Accept any answer that says it is in space, held in orbit by gravity, with no air needed.
Check. ✓
10
PSName the type of wave a communications satellite uses to send signals to Earth, from the list: radio, gamma or X-ray. Write the word.
radio
Method. Recall the waves used for communication.
- Recall. Communications satellites use radio or microwaves.radio
- Write the answer. radio
Check. Accept microwave. ✓
21.3
The life cycle of stars
Video lesson: The life cycle of starsComing soon
Context. A star is born from a cloud of gas and dust. It shines for billions of years, and then changes. The final stage depends on how massive the star is.
Stars form in a nebula, a cloud of gas and dust. Gravity pulls the gas together into a protostar, which heats up. When fusion starts, it becomes a main sequence star, like the Sun, which is stable for a long time. A Sun-like star then expands into a red giant, and then throws off its outer layers and leaves a hot, dense white dwarf. A much more massive star becomes a red supergiant, which can explode as a supernova. The remains are a neutron star or, if the star is massive enough, a black hole.
The life cycle of a Sun-like star and a massive star
Key ideas
- Nebula, protostar, main sequence (stable, fusion).
- Sun-like: red giant, then white dwarf.
- Massive: red supergiant, supernova, then neutron star or black hole.
- The mass of the star decides its final stage.
Worked example 21.3.1
What is the first stage of a star’s life cycle? Write the word.
Method. Think about where stars come from.
- Look. Think about where stars are born.birth
- Recall. A star forms from a cloud of gas and dust, called a nebula.nebula
- Write the answer. nebula
Check. ✓
Worked example 21.3.2
Put these stages in order for a Sun-like star: white dwarf, nebula, red giant, main sequence. Write the four words in order, separated by commas.
Method. Follow the life cycle.
- Look. Start at the birth of the star.birth
- Order. Nebula, then main sequence, then red giant, then white dwarf.nebula, main sequence, red giant, white dwarf
- Write the answer. nebula, main sequence, red giant, white dwarf
Check. ✓
Worked example 21.3.3
What does a massive star become after a supernova, if its core is very massive? Write the name.
Method. Recall the end states.
- Look. Think about the mass of the core.mass
- Recall. A very massive core can collapse into a black hole after a supernova.black hole
- Write the answer. black hole
Check. Accept neutron star for a less massive core. ✓
Watch out
Watch out: a red giant is a later stage of a Sun-like star, not the same as a red supergiant from a massive star. The mass decides which path the star takes.
Exam tip
Exam tip. When putting the life cycle in order, name the stages with the star’s mass in mind: Sun-like or massive.
Exercise 21C Stars and their life cycles
1
FWhat is a cloud of gas and dust called, in which stars form? Write the word.
nebula
Method. Recall the name.
- Recall. A nebula is a cloud of gas and dust where stars form.nebula
- Write the answer. nebula
Check. ✓
2
FWhat is the stable stage in which a star like the Sun spends most of its life? Write the phrase.
main sequence
Method. Recall the stage.
- Recall. The Sun is in the main sequence, and will be for billions of years.main sequence
- Write the answer. main sequence
Check. ✓
3
FWhat does a Sun-like star become after it expands into a red giant? Write the name.
white dwarf
Method. Recall the end stage.
- Recall. It throws off its outer layers and becomes a white dwarf.white dwarf
- Write the answer. white dwarf
Check. ✓
4
FWhat is the name of the explosion at the end of a very massive star? Write the word.
supernova
Method. Recall the event.
- Recall. A massive star can explode as a supernova.supernova
- Write the answer. supernova
Check. ✓
5
FWhat can a very massive star leave behind after a supernova, apart from a black hole? Write the name.
neutron star
Method. Recall the remnant.
- Recall. A supernova can leave a neutron star.neutron star
- Write the answer. neutron star
Check. ✓
6
FWhat decides the end stage of a star: its colour or its mass? Write the word.
mass
Method. The mass decides the path.
- Recall. The mass of the star decides which final stage it reaches.mass
- Write the answer. mass
Check. ✓
7
PSPut these in order for a massive star: black hole, nebula, red supergiant. Write the three words in order, separated by commas.
nebula, red supergiant, black hole
Method. Use the order of the life cycle.
- Order. Nebula, then red supergiant, then the black hole after a supernova.nebula, red supergiant, black hole
- Write the answer. nebula, red supergiant, black hole
Check. ✓
8
PSWhich stage follows the protostar for a star that is forming: main sequence or white dwarf? Write the name.
main sequence
Method. The main sequence comes after the protostar.
- Order. After the protostar, fusion starts and the star becomes a main sequence star.main sequence
- Write the answer. main sequence
Check. ✓
9
RExplain in one sentence why a Sun-like star becomes a white dwarf, rather than a black hole. Accept any sentence that says it is not massive enough to collapse into a black hole.
Accept any sentence that says it is not massive enough to collapse into a black hole.
Method. Think about the mass.
- Mass. A Sun-like star has too little mass for a supernova.too little mass
- Remnant. So it leaves a white dwarf, not a black hole.white dwarf
- Write the answer. Accept any sentence that says it is not massive enough to collapse into a black hole.
Check. ✓
10
RExplain why the main sequence is a stable stage for a star, in one sentence. Accept any sentence that says the outward pressure from fusion balances the inward pull of gravity.
Accept any sentence that says the outward pressure from fusion balances the inward pull of gravity.
Method. Think about the forces inside the star.
- Outward. Fusion in the core gives an outward pressure.outward
- Balance. This balances the inward pull of gravity, so the star is stable.balances
- Write the answer. Accept any sentence that says the outward pressure from fusion balances the inward pull of gravity.
Check. ✓
11
FWhich stage comes last for a Sun-like star: white dwarf or red giant? Write the name.
white dwarf
Method. The white dwarf is the final stage.
- Recall. The red giant is followed by the white dwarf.white dwarf
- Write the answer. white dwarf
Check. ✓
21.4
The universe: galaxies, the Big Bang and red-shift
Video lesson: The universe: galaxies, the Big Bang and red-shiftComing soon
Context. A bright smudge in the night sky turns out to be a galaxy: billions of stars held together by gravity. Astronomers find that most galaxies are moving away from us. What does this tell us about the universe?
The universe contains billions of galaxies. Our galaxy, the Milky Way, contains the Sun. Light from distant galaxies is red-shifted: its wavelength is stretched towards the red end of the spectrum, because the galaxy is moving away from us. The further away the galaxy, the greater the red-shift, so the universe is expanding.
The Big Bang theory says the universe began from a very hot, dense state about 13.8 billion years ago and has been expanding ever since. Evidence for it includes the red-shift of distant galaxies and the cosmic microwave background radiation, a faint radiation from all directions that is left over from the early universe (idea).
Red-shift: the lines from a distant galaxy are at longer wavelengths
| Evidence | What it shows |
|---|---|
| red-shift of distant galaxies | the universe is expanding; the more distant the galaxy, the faster it recedes |
| cosmic microwave background (CMB) | a faint radiation from all directions, left over from the hot early universe (idea) |
| Hubble’s law | speed is proportional to distance, which supports the expanding universe |
Evidence for an expanding universe and the Big Bang
Key ideas
- Galaxy: a huge group of stars held together by gravity.
- Red-shift: light from a receding galaxy is shifted to longer wavelengths.
- The universe is expanding; the Big Bang is the best-supported model.
- Cosmic microwave background: left over from the early universe (idea).
Worked example 21.4.1
What is the name of the theory that the universe began from a very hot dense state? Write the name.
Method. Recall the theory.
- Look. The question describes the start of the universe.start
- Recall. The Big Bang theory says the universe began from a hot, dense state.Big Bang
- Write the answer. Big Bang
Check. ✓
Worked example 21.4.2
Light from a distant galaxy is red-shifted. Does this mean its wavelength is longer or shorter? Write the word.
Method. Red-shift means the light is stretched.
- Look. Red-shift means the light moves towards the red end of the spectrum.red
- Decide. The red end has longer wavelengths, so the wavelength is longer.longer
- Write the answer. longer
Check. ✓
Worked example 21.4.3
Name the faint radiation from all directions in space, left over from the early universe. Write the name.
Method. Recall the evidence for the Big Bang.
- Look. This radiation is about 2.7 K, which is in the microwave range.microwave
- Recall. It is called the cosmic microwave background.cosmic microwave background
- Write the answer. cosmic microwave background
Check. Accept cosmic background radiation. ✓
Watch out
Watch out: red-shift does not mean the galaxy is red. It means its light is stretched to longer wavelengths, which is a sign that the galaxy is moving away.
Exam tip
Exam tip. Use the words “red-shift” and “longer wavelength” together when you explain why a galaxy is moving away.
Exercise 21D The universe and red-shift
1
FWhat is a huge group of stars held together by gravity called? Write the word.
galaxy
Method. Recall the word.
- Recall. A galaxy is a huge group of stars held together by gravity.galaxy
- Write the answer. galaxy
Check. ✓
2
FWhat is the name of the galaxy that contains our Sun? Write the name.
Milky Way
Method. Recall the galaxy.
- Recall. Our galaxy is the Milky Way.Milky Way
- Write the answer. Milky Way
Check. ✓
3
FIn red-shift, does the light move towards the red or the blue end? Write the word: red or blue.
red
Method. Red-shift means towards the red.
- Recall. Red-shift means the light is shifted to longer wavelengths, towards red.red
- Write the answer. red
Check. ✓
4
FIs the universe expanding or contracting, according to the evidence? Write the word.
expanding
Method. Recall the evidence.
- Recall. Red-shifts of distant galaxies show the universe is expanding.expanding
- Write the answer. expanding
Check. ✓
5
FWhat is the name of the faint radiation from all directions in space, left over from the early universe? Write the name.
cosmic microwave background
Method. Recall the radiation.
- Recall. The cosmic microwave background is left over from the early universe.cosmic microwave background
- Write the answer. cosmic microwave background
Check. Accept cosmic background radiation. ✓
6
FAbout how old is the universe, according to the Big Bang theory? Write the number of billion years.
13.8
Method. Recall the estimate.
- Recall. The universe is about 13.8 billion years old.13.8
- Write the answer. 13.8 billion years
Check. ✓
7
PSA galaxy has its light red-shifted. Is the galaxy moving towards us or away from us? Write the word: towards or away.
away
Method. Red-shift means the source is receding.
- Decide. Red-shift means the galaxy is moving away from us.away
- Write the answer. away
Check. ✓
8
RExplain in one sentence why the red-shift of distant galaxies is evidence for the Big Bang. Accept any sentence that says it shows the universe is expanding, so it was smaller and hotter in the past.
Accept any sentence that says it shows the universe is expanding, so it was smaller and hotter in the past.
Method. Think about what expansion means.
- Expand. Red-shift shows the galaxies are moving apart, so the universe is expanding.expanding
- Past. Running the expansion backwards, the universe was smaller and hotter in the past.smaller and hotter
- Write the answer. Accept any sentence that says it shows the universe is expanding, so it was smaller and hotter in the past.
Check. ✓
9
RA pupil says “red-shift means the galaxy is red in colour.” Explain why this is wrong. Accept any answer that says red-shift is a stretching of the wavelength of light, not the colour of the galaxy.
Accept any answer that says red-shift is a stretching of the wavelength of light, not the colour of the galaxy.
Method. Check the meaning.
- Meaning. Red-shift is a change in the wavelength of the light.wavelength
- Correction. It is not about the colour of the galaxy itself.not colour
- Write the answer. Accept any answer that says red-shift is a stretching of the wavelength of light, not the colour of the galaxy.
Check. ✓
10
RGive one piece of evidence that supports the Big Bang theory, in one sentence. Accept any answer that names red-shift or the cosmic microwave background, with a short explanation.
Accept any answer that names red-shift or the cosmic microwave background, with a short explanation.
Method. Name one piece of evidence.
- Red-shift. The red-shift of distant galaxies supports an expanding universe.red-shift
- Background. The cosmic microwave background is left over from the hot early universe.background
- Write the answer. Accept any answer that names red-shift or the cosmic microwave background, with a short explanation.
Check. ✓
21.5
[H] Hubble’s law
Video lesson: [H] Hubble’s lawComing soon
[H] Higher tier only. Edwin Hubble found that the further away a galaxy is, the faster it is moving away. This is Hubble’s law:
v = H0 x d
where v is the recession speed (km/s), d is the distance (megaparsecs, Mpc) and H0 is the Hubble constant. The value used here is an example: H0 = 70 km/s per Mpc.
Recession speed against distance: a straight line through the origin (example H0 = 70)
Key ideas
- [H] v = H0 x d. Speed in km/s, distance in Mpc, H0 in km/s per Mpc.
- [H] Speed is proportional to distance: double the distance, double the speed.
- [H] Rearrange to d = v / H0.
- [H] Red-shift is used to find the recession speed.
Worked example 21.5.1
[H] A galaxy is 100 Mpc away. Using H0 = 70 km/s per Mpc, find its recession speed. Write the number of km/s.
Method. Use v = H0 x d.
- Take the numbers. H0 = 70 km/s per Mpc and d = 100 Mpc.H0 = 70, d = 100
- Multiply. v = 70 x 100 = 7000 km/s.70 x 100 = 7000
- Write the answer. 7000 km/s
Check. ✓
Worked example 21.5.2
[H] A galaxy is moving away at 14 000 km/s. Using H0 = 70, find its distance in Mpc. Write the number of Mpc.
Method. Use d = v / H0.
- Take the numbers. v = 14000 km/s and H0 = 70.v = 14000, H0 = 70
- Divide. d = 14000 / 70 = 200 Mpc.14000 / 70 = 200
- Write the answer. 200 Mpc
Check. ✓
Worked example 21.5.3
[H] A galaxy is 30 Mpc away. Using H0 = 70 km/s per Mpc, find its recession speed. Write the number of km/s.
Method. Use v = H0 d.
- Take the numbers. H0 = 70 km/s per Mpc and d = 30 Mpc.H0 = 70, d = 30
- Multiply. v = 70 x 30 = 2100 km/s.70 x 30 = 2100
- Write the answer. 2100 km/s
Check. ✓
Watch out
Watch out: [H] check the units. Distance is in megaparsecs (Mpc) and H0 is in km/s per Mpc, so the speed comes out in km/s.
Exam tip
Exam tip. [H] Write v = H0 d, put the numbers in with units, and rearrange if you need d.
Exercise 21E [H] Hubble’s law calculations
1
F[H] Using H0 = 70, what is the speed of a galaxy 50 Mpc away? Write the number of km/s.
3500
Method. v = H0 d.
- Multiply. v = 70 x 50 = 3500 km/s.70 x 50 = 3500
- Write the answer. 3500 km/s
Check. ✓
2
F[H] Using H0 = 70, what distance is a galaxy moving at 7000 km/s? Write the number of Mpc.
100
Method. d = v / H0.
- Divide. d = 7000 / 70 = 100 Mpc.7000 / 70 = 100
- Write the answer. 100 Mpc
Check. ✓
3
PS[H] If a galaxy is twice as far away, how many times faster is it moving? Write the number.
2
Method. Speed is proportional to distance.
- Recall. v = H0 d, so doubling d doubles v.2
- Write the answer. 2 times
Check. ✓
4
PS[H] A galaxy is 150 Mpc away with H0 = 70. What is its recession speed? Write the number of km/s.
10500
Method. v = H0 d.
- Multiply. v = 70 x 150 = 10 500 km/s.70 x 150 = 10500
- Write the answer. 10500 km/s
Check. ✓
5
PS[H] A galaxy has a recession speed of 3500 km/s. What is its distance? Write the number of Mpc.
50
Method. d = v / H0.
- Divide. d = 3500 / 70 = 50 Mpc.3500 / 70 = 50
- Write the answer. 50 Mpc
Check. ✓
6
R[H] Explain in one sentence what the straight line through the origin on a speed-distance graph shows. Accept any sentence that says speed is directly proportional to distance, as Hubble’s law says.
Accept any sentence that says speed is directly proportional to distance, as Hubble’s law says.
Method. Think about the line.
- Line. A straight line through the origin means the two are directly proportional.proportional
- Law. So speed is proportional to distance, as Hubble’s law says.Hubble
- Write the answer. Accept any sentence that says speed is directly proportional to distance, as Hubble’s law says.
Check. ✓
7
PS[H] Using H0 = 70, a galaxy moves away at 1400 km/s. Find its distance in Mpc. Write the number of Mpc.
20
Method. d = v / H0.
- Divide. d = 1400 / 70 = 20 Mpc.1400 / 70 = 20
- Write the answer. 20 Mpc
Check. ✓
8
PS[H] Which galaxy moves faster: one 50 Mpc away or one 80 Mpc away? Write 50 or 80.
80
Method. Speed is proportional to distance.
- Compare. The galaxy at 80 Mpc has the larger distance, so it moves faster.80
- Write the answer. 80
Check. ✓
9
PS[H] Using H0 = 70, find the speed of a galaxy 10 Mpc away. Write the number of km/s.
700
Method. v = H0 d.
- Multiply. v = 70 x 10 = 700 km/s.70 x 10 = 700
- Write the answer. 700 km/s
Check. ✓
10
R[H] Explain in one sentence why Hubble’s law supports an expanding universe. Accept any sentence that says galaxies further away move faster, so the universe is expanding.
Accept any sentence that says galaxies further away move faster, so the universe is expanding.
Method. Think about the pattern.
- Pattern. The further a galaxy is, the faster it moves away.faster
- Link. So space itself is stretching, which means the universe is expanding.expanding
- Write the answer. Accept any sentence that says galaxies further away move faster, so the universe is expanding.
Check. ✓
Chapter summary
| Skill | Method |
|---|---|
| Solar system | Sun, eight planets, moons, comets and asteroids; gravity keeps orbits in place |
| Satellites | Low orbit: fast, close; geostationary: 24 hours, above the equator, stays over one point |
| Life cycle | Sun-like: nebula, protostar, main sequence, red giant, white dwarf; massive: ends in a supernova |
| Universe | Galaxies; red-shift; expanding universe; Big Bang; cosmic microwave background |
| [H] Hubble’s law | v = H0 d; speed proportional to distance |
Can you…?
- I can name the planets and say what keeps them in orbit
- I can compare low-orbit and geostationary satellites
- I can put the stages of a star’s life cycle in order
- I can explain red-shift and the Big Bang evidence
- [H] I can use Hubble’s law
Key terms
- solar system
- the Sun and everything that orbits it
- galaxy
- a huge group of stars held together by gravity
- nebula
- a cloud of gas and dust in which stars form
- geostationary orbit
- an orbit above the equator that takes 24 hours, so the satellite stays over one point
- red-shift
- the stretching of light to longer wavelengths from a receding source
- Big Bang
- the theory that the universe began from a hot, dense state and has been expanding ever since
- [H] Hubble constant (H0)
- the constant in v = H0 d, relating recession speed to distance
Exercise 21Z Check-up
1
Name the planet that is closest to the Sun. Write the name.
Mercury
Method. Recall.
- Recall. Mercury is the closest planet to the Sun.Mercury
- Write the answer. Mercury
Check. ✓
2
Which force keeps the planets in orbit around the Sun? Write the word.
gravity
Method. Recall.
- Recall. Gravity keeps the planets in orbit.gravity
- Write the answer. gravity
Check. ✓
3
Which satellite orbit takes 24 hours and stays over one point? Write the name.
geostationary
Method. Recall.
- Recall. A geostationary orbit takes 24 hours.geostationary
- Write the answer. geostationary
Check. ✓
4
What is the name of the cloud of gas and dust where stars form? Write the word.
nebula
Method. Recall.
- Recall. A nebula is where stars form.nebula
- Write the answer. nebula
Check. ✓
5
What does a Sun-like star become after its red giant stage? Write the name.
white dwarf
Method. Recall.
- Recall. A white dwarf is left after the red giant stage.white dwarf
- Write the answer. white dwarf
Check. ✓
6
Name the theory that the universe began from a hot, dense state. Write the name.
Big Bang
Method. Recall.
- Recall. The Big Bang theory describes the start of the universe.Big Bang
- Write the answer. Big Bang
Check. ✓
7
Is the red-shift of distant galaxies evidence for a moving or a still universe? Write the word: expanding or still.
expanding
Method. Red-shift shows galaxies moving apart.
- Recall. Red-shift shows the universe is expanding.expanding
- Write the answer. expanding
Check. ✓
8
What is the name of the star that ends as a supernova, leaving a neutron star or black hole? Write the name of the explosion.
supernova
Method. Recall.
- Recall. A massive star ends in a supernova.supernova
- Write the answer. supernova
Check. ✓
9
[H] Using H0 = 70, a galaxy at 200 Mpc moves at what speed? Write the number of km/s.
14000
Method. v = H0 d.
- Multiply. v = 70 x 200 = 14 000 km/s.70 x 200 = 14000
- Write the answer. 14000 km/s
Check. ✓
10
Explain in one sentence why a planet moves in an orbit. Accept any sentence that says gravity pulls it towards the Sun, changing its direction all the time.
Accept any sentence that says gravity pulls it towards the Sun, changing its direction all the time.
Method. Think about the force.
- Gravity. Gravity pulls the planet towards the Sun.towards the Sun
- Orbit. So the planet keeps changing direction, which keeps it in orbit.orbit
- Write the answer. Accept any sentence that says gravity pulls it towards the Sun, changing its direction all the time.
Check. ✓
Total 29 marks. Allow about 29 minutes.
Exercise 21X Exam-style questions
1
FPut these stages in order for a Sun-like star: red giant, nebula, white dwarf, main sequence. Write the four words in order, separated by commas.[2 marks]
nebula, main sequence, red giant, white dwarf
Method. Follow the life cycle.
- Order. Nebula, main sequence, red giant, white dwarf.nebula, main sequence, red giant, white dwarf
- Write the answer. nebula, main sequence, red giant, white dwarf
Check. ✓
2
RDescribe what keeps a satellite in a geostationary orbit, and say why it appears to stay still in the sky. Accept any answer that says gravity holds it in orbit, and it orbits once every 24 hours in the same direction as the Earth spins.[3 marks]
Accept any answer that says gravity holds it in orbit, and it orbits once every 24 hours in the same direction as the Earth spins.
Method. Describe the orbit.
- Force. Gravity holds the satellite in orbit.gravity
- Period. It takes 24 hours for one orbit, matching the Earth’s spin.24 hours
- Direction. It orbits in the same direction, so it appears to stay still.same direction
- Write the answer. Accept any answer that says gravity holds it in orbit, and it orbits once every 24 hours in the same direction as the Earth spins.
Check. ✓
3
RDescribe the life cycle of a massive star, from its birth to its end. Accept any answer that names the nebula, protostar, main sequence, red supergiant, supernova, and a neutron star or black hole.[4 marks]
Accept any answer that names the nebula, protostar, main sequence, red supergiant, supernova, and a neutron star or black hole.
Method. Give the stages in order.
- Birth. It forms from a nebula, then a protostar.nebula, protostar
- Stable. It then spends time on the main sequence.main sequence
- Expand. It becomes a red supergiant.red supergiant
- End. It explodes as a supernova, leaving a neutron star or black hole.supernova
- Write the answer. Accept any answer that names the nebula, protostar, main sequence, red supergiant, supernova, and a neutron star or black hole.
Check. ✓
4
RExplain how red-shift provides evidence that the universe is expanding. Accept any answer that says light from distant galaxies is stretched to longer wavelengths because they are moving away, so the further galaxies move away faster.[3 marks]
Accept any answer that says light from distant galaxies is stretched to longer wavelengths because they are moving away, so the further galaxies move away faster.
Method. Link red-shift to expansion.
- Red-shift. Light from distant galaxies is stretched to longer wavelengths.longer wavelength
- Moving. This means the galaxies are moving away from us.moving away
- Expansion. The further away a galaxy, the faster it moves away, so the universe is expanding.expanding
- Write the answer. Accept any answer that says light from distant galaxies is stretched to longer wavelengths because they are moving away, so the further galaxies move away faster.
Check. ✓
5
PS[H] A galaxy is 120 Mpc away. Using H0 = 70 km/s per Mpc, calculate its recession speed. Write the number of km/s.[2 marks]
8400
Method. v = H0 d.
- Multiply. v = 70 x 120 = 8400 km/s.70 x 120 = 8400
- Write the answer. 8400 km/s
Check. ✓
6
RExplain why a low-orbit satellite is used for mapping, and why a geostationary satellite is used for TV. Accept any answer that says low-orbit satellites pass close to the surface for detailed pictures, while geostationary satellites stay over one point for a fixed dish.[4 marks]
Accept any answer that says low-orbit satellites pass close to the surface for detailed pictures, while geostationary satellites stay over one point for a fixed dish.
Method. Compare the two uses.
- Low. A low-orbit satellite is close to the surface, giving detailed pictures.close, detailed
- Geo. A geostationary satellite stays over one point.one point
- Dish. So a fixed dish on the ground can always receive its signal for TV.fixed dish
- Write the answer. Accept any answer that says low-orbit satellites pass close to the surface for detailed pictures, while geostationary satellites stay over one point for a fixed dish.
Check. ✓
7
RDescribe the evidence that the universe began in a Big Bang, giving two pieces of evidence. Accept any two of: red-shift of distant galaxies, the cosmic microwave background, and the relative abundance of light elements (idea).[4 marks]
Accept any two of: red-shift of distant galaxies, the cosmic microwave background, and the relative abundance of light elements (idea).
Method. Name two pieces of evidence and explain.
- First. The red-shift of distant galaxies shows the universe is expanding.red-shift
- Second. The cosmic microwave background is left over from the hot early universe.microwave background
- Conclusion. Both support a hot, dense beginning, which is the Big Bang.Big Bang
- Write the answer. Accept any two of: red-shift of distant galaxies, the cosmic microwave background, and the relative abundance of light elements (idea).
Check. ✓
8
RA pupil says “the planets are held in orbit by the Sun because the Sun pushes them.” Explain why this is wrong, in terms of the force. Accept any answer that says the force is gravitational attraction towards the Sun, not a push.[3 marks]
Accept any answer that says the force is gravitational attraction towards the Sun, not a push.
Method. Correct the idea.
- Force. The force is gravity, which is an attraction towards the Sun.attraction
- Orbit. It is not a push; the planets move sideways and gravity changes their direction.not a push
- Write the answer. Accept any answer that says the force is gravitational attraction towards the Sun, not a push.
Check. ✓
9
PS[H] Using H0 = 70 km/s per Mpc, calculate the recession speed of a galaxy 40 Mpc away. Write the number of km/s.[2 marks]
2800
Method. Use v = H0 d.
- Multiply. v = 70 x 40 = 2800 km/s.70 x 40 = 2800
- Write the answer. 2800 km/s
Check. ✓
10
RName two uses of geostationary satellites. Accept any two of: television broadcasting, satellite communications, and weather forecasting.[2 marks]
Accept any two of: television broadcasting, satellite communications, and weather forecasting.
Method. Recall the uses.
- First. Geostationary satellites are used for television broadcasting.television
- Second. They are also used for communications and weather forecasting.communications
- Write the answer. Accept any two of: television broadcasting, satellite communications, and weather forecasting.
Check. ✓