P30: Space Physics
The solar system, planetary orbits, artificial and geostationary satellites, red-shift, the Big Bang theory, cosmic microwave background radiation, and the life cycle of stars.
The Solar System
The solar system consists of the Sun and everything that orbits it, including eight planets, dwarf planets, asteroids, comets and moons. The Sun contains over 99% of the mass of the solar system.
The eight planets in order from the Sun:
- Mercury — smallest, closest to the Sun
- Venus — similar size to Earth, very thick atmosphere
- Earth — the only planet known to support life
- Mars — the red planet, thin atmosphere
- Jupiter — largest planet, gas giant
- Saturn — famous for its ring system, gas giant
- Uranus — ice giant, rotates on its side
- Neptune — furthest planet, ice giant
The inner planets (Mercury, Venus, Earth, Mars) are rocky with solid surfaces. The outer planets (Jupiter, Saturn, Uranus, Neptune) are gas or ice giants with no solid surface.
Orbits
Planets orbit the Sun due to the gravitational attraction between the Sun and the planet. Gravity provides the centripetal force needed to keep the planet in its orbit. The orbit is an ellipse, but for most planets it is nearly circular.
- For a stable orbit, the gravitational force must equal the centripetal force required
- The closer a planet is to the Sun, the stronger the gravitational force and the faster it must travel to stay in orbit
- Planets closer to the Sun have shorter orbital periods (they orbit faster)
- Mercury orbits in 88 days; Neptune takes about 165 years
The radius of an orbit can change if the speed of the object changes. If a satellite slows down, it moves closer to the Earth. If it speeds up, it moves further away.
Artificial Satellites
Artificial satellites are human-made objects that orbit the Earth. They are used for communication, weather monitoring, navigation, and scientific research.
| Feature | Low Earth Orbit (LEO) | Geostationary Orbit (GEO) |
|---|---|---|
| Altitude | 160 – 2000 km | 35,786 km |
| Orbital period | About 90 minutes | 24 hours (matches Earth's rotation) |
| Position relative to Earth | Moves across the sky | Appears stationary above one point on the equator |
| Uses | Weather imaging, spying, ISS, mapping | Communication, satellite TV, GPS |
| Speed | Fast (about 7.5 km/s) | Slower (about 3 km/S) |
| Coverage | Small area at a time, whole Earth over 24h | Fixed large area of Earth's surface |
Geostationary satellites must orbit directly above the equator, travel in the same direction as the Earth's rotation (west to east), and have an orbital period of exactly 24 hours. This makes them appear stationary from Earth.
Red-Shift
Red-shift is the observed increase in wavelength (and decrease in frequency) of light from distant galaxies. It is evidence that the universe is expanding.
- When a source of light is moving away from an observer, the wavelength of the light appears to increase (shift towards the red end of the spectrum)
- The further away a galaxy is, the greater its red-shift, meaning it is moving away faster
- This relationship between distance and speed of recession is called Hubble's law
- Red-shift provides evidence that the universe is expanding
- More distant galaxies have greater red-shifts, showing they are receding faster
Red-shift is analogous to the Doppler effect. When a source moves away, the waves are stretched, increasing their wavelength. When a source moves towards an observer, the waves are compressed (blue-shift).
The Big Bang Theory
The Big Bang theory states that the universe began from a very small, very hot and very dense point (a singularity) approximately 13.8 billion years ago. The universe has been expanding and cooling ever since.
Evidence for the Big Bang:
- Red-shift of galaxies: distant galaxies are moving away from us in all directions, suggesting everything started from a single point
- Cosmic Microwave Background Radiation (CMBR): the universe is filled with microwave radiation that is the remnant of the initial hot Big Bang. As the universe expanded, the high-energy radiation was stretched to longer wavelengths and is now detected as microwaves
CMBR is the afterglow of the Big Bang. It was discovered accidentally by Penzias and Wilson in 1965. It has a temperature of about 2.7 K (-270.5 C) and comes from all directions in space.
The Expanding Universe
The universe is expanding. The rate of expansion is determined by the balance between the kinetic energy of the expanding galaxies and the gravitational attraction trying to pull them together.
Three possible futures for the universe:
- Open universe: expansion continues forever if there is not enough mass to stop it
- Closed universe: gravity eventually stops the expansion and the universe contracts back (the Big Crunch)
- Flat universe: expansion slows but never quite stops
Current observations suggest the expansion of the universe is actually accelerating, driven by a mysterious force called dark energy. This points towards an open universe.
Life Cycle of Stars
Stars form from clouds of gas and dust called nebulae. The life cycle of a star depends on its mass. Low-mass stars (like the Sun) follow a different path from high-mass stars.
Stage 1: Nebula
Stars begin as a nebula, a large cloud of gas (mainly hydrogen) and dust in space. Gravity causes the gas and dust to collapse and clump together. As the material collapses, it heats up.
Stage 2: Protostar
As the gas collapses under gravity, it forms a protostar. The temperature and pressure increase as more material is pulled in. The protostar is not yet hot enough for nuclear fusion.
Stage 3: Main Sequence Star
When the core temperature reaches about 15 million degrees, hydrogen nuclei begin to fuse into helium (nuclear fusion). This releases enormous amounts of energy. The star becomes stable because the outward radiation pressure balances the inward gravitational force. The Sun is currently a main sequence star and has been for about 5 billion years.
Life Cycle of a Low-Mass Star (like the Sun)
- Nebula — cloud of gas and dust collapses under gravity
- Protostar — material heats up as it collapses further
- Main sequence star — hydrogen fuses to helium; stable for billions of years
- Red giant — when hydrogen runs low, the core contracts and the outer layers expand and cool; the star becomes a red giant
- White dwarf — the outer layers are ejected (planetary nebula) and the hot core remains as a white dwarf
- Black dwarf — the white dwarf cools down over billions of years to become a cold, dark black dwarf (theoretical; none yet exist as the universe is not old enough)
Life Cycle of a High-Mass Star
- Nebula — cloud of gas and dust collapses under gravity
- Protostar — material heats up as it collapses further
- Main sequence star — hydrogen fuses to helium; much hotter and brighter than low-mass stars; shorter life (millions rather than billions of years)
- Red supergiant — when hydrogen runs low, the star expands enormously; it then fuses heavier elements up to iron
- Supernova — when the core collapses, the outer layers are blown off in a massive explosion called a supernova. This distributes heavier elements into space
- Neutron star or black hole — the remnant core becomes either a neutron star (incredibly dense, composed mainly of neutrons) or, if massive enough, a black hole (where gravity is so strong that not even light can escape)
Comparison of Star Life Cycles
| Stage | Low-Mass Star (like the Sun) | High-Mass Star (8+ solar masses) |
|---|---|---|
| Birth | Nebula → Protostar | Nebula → Protostar |
| Stable phase | Main sequence (billions of years) | Main sequence (millions of years) |
| Expansion | Red giant | Red supergiant |
| Death | Planetary nebula | Supernova |
| Remnant | White dwarf → Black dwarf | Neutron star or Black hole |
| Elements produced | Helium (up to carbon in some cases) | All elements up to iron; heavier elements in supernova |
Nuclear Fusion in Stars
Stars are powered by nuclear fusion. In the core of a main sequence star, hydrogen nuclei fuse to form helium nuclei. This process releases enormous amounts of energy as radiation.
- Fusion requires extremely high temperatures and pressures (about 15 million K in the core)
- The energy released by fusion creates an outward radiation pressure that balances the inward gravitational force
- This balance is called hydrostatic equilibrium and keeps the star stable
- When fusion fuel runs low, the balance is disrupted and the star evolves
- Heavy elements (up to iron) are formed by fusion in the cores of massive stars
- Elements heavier than iron are formed during supernova explosions
All the elements heavier than hydrogen and helium were formed inside stars and distributed into space by supernovae. The iron in your blood was made in a star that exploded billions of years ago.
Practice Questions
1. Describe the life cycle of a star similar in mass to the Sun. [5 marks]
Starts as a nebula (cloud of gas and dust). Gravity causes collapse into a protostar. Core reaches fusion temperature and becomes a main sequence star (hydrogen fuses to helium). When hydrogen runs low, the core contracts and outer layers expand into a red giant. The outer layers are ejected as a planetary nebula, leaving a white dwarf. The white dwarf cools to become a black dwarf.
2. Explain what red-shift tells us about the universe. [3 marks]
Red-shift is the increase in wavelength of light from distant galaxies. It shows that galaxies are moving away from us in all directions. The further away a galaxy is, the greater its red-shift, meaning it is moving away faster. This is evidence that the universe is expanding.
3. State two pieces of evidence for the Big Bang theory. [2 marks]
The red-shift of distant galaxies (showing the universe is expanding) and cosmic microwave background radiation (the afterglow of the hot early universe).
4. Compare a geostationary satellite with a low Earth orbit satellite. [4 marks]
Geostationary satellites orbit at 35,786 km with a 24-hour period and appear stationary above one point on the equator. They are used for communications and TV. Low Earth orbit satellites orbit at 160-2000 km with about a 90-minute period and move across the sky. They are used for weather imaging, mapping and the ISS.
5. Explain why a supernova is important for the formation of planets and life. [3 marks]
A supernova explosion creates elements heavier than iron and distributes all the heavy elements into space. These elements form the dust and gas that later collapse to form new stars, planets and eventually life. Without supernovae, there would be no heavy elements like carbon, oxygen and iron available to form rocky planets and living organisms.
Maths Skills
Orbital Speed
Orbital Speed Equation
v = 2πr / T
v = orbital speed (m/s)
r = orbital radius (m)
T = orbital period (s)
2πr = circumference of the orbit (m)
Worked Example 1: Earth's Orbital Speed
The Earth orbits the Sun at an average distance of 1.5 × 10¹¹ m with a period of 1 year (3.15 × 10⁷ s). Calculate the orbital speed.
v = 2πr / T = 2 × 3.14 × 1.5 × 10¹¹ / 3.15 × 10⁷
v = 9.42 × 10¹¹ / 3.15 × 10⁷ = 2.99 × 10⁴ m/s ≈ 30 km/s
Worked Example 2: Geostationary Satellite
A geostationary satellite orbits at a radius of 4.22 × 10⁷ m with a period of 24 hours (86,400 s). Calculate its orbital speed.
v = 2πr / T = 2 × 3.14 × 4.22 × 10⁷ / 86,400
v = 2.65 × 10⁷ / 86,400 = 3.07 × 10³ m/s ≈ 3.1 km/s
Gravitational Force Relationships
- Gravitational force is inversely proportional to the square of the distance: F ∝ 1/r²
- If the distance doubles, the gravitational force becomes 1/4 of its original value
- If the distance triples, the force becomes 1/9 of its original value
- Closer orbits require higher speeds (stronger gravity provides greater centripetal force)
Red-Shift Calculations
Hubble's Law
v = H₀ × d
v = recession velocity of the galaxy (km/s)
H₀ = Hubble constant ≈ 70 km/s/Mpc
d = distance to the galaxy (Mpc, megaparsecs)
Worked Example 3: Galaxy Recession Velocity
A galaxy is 100 Mpc away from Earth. Calculate its recession velocity using H₀ = 70 km/s/Mpc.
v = H₀ × d = 70 × 100 = 7000 km/s
The galaxy is moving away from us at about 7000 km/s.
Worked Example 4: Distance from Red-Shift
A galaxy's red-shift indicates a recession velocity of 21,000 km/s. Calculate its distance from Earth.
v = H₀ × d, so d = v / H₀ = 21,000 / 70 = 300 Mpc
Common Misconceptions
"The Big Bang was an explosion in space." The Big Bang was not an explosion at a point in space. It was the expansion of space itself. Before the Big Bang, all the matter and energy in the universe was compressed into an extremely hot, dense singularity. Space itself began to expand, carrying matter with it. There is no centre of the expansion — every point in the universe is moving away from every other point, like dots on the surface of an inflating balloon. The galaxies are not moving through space away from a central point; rather, the space between them is stretching.
"Red-shift means stars are turning red in colour." Red-shift means the wavelengths of light from distant galaxies are stretched towards the red end of the spectrum. The light is shifted to longer wavelengths but does not necessarily appear red to the eye. A blue star that is red-shifted might appear greenish or white, depending on the amount of shift. The term refers to the direction of the shift in the spectrum (towards longer wavelengths), not the observed colour of the object.
"Satellites stay in orbit because there is no gravity in space." There is gravity in space — it is what keeps satellites in orbit. Without gravity, a satellite would fly off in a straight line. Gravity provides the centripetal force that constantly pulls the satellite towards the Earth, bending its path into a circle. A satellite is actually falling towards the Earth all the time, but it moves forward fast enough that the Earth's surface curves away beneath it at the same rate.
6-Mark Extended Question
Describe the life cycle of a star like our Sun. Compare it with the life cycle of a star much more massive than the Sun. [6 marks]
Both stars begin in the same way: as a nebula, a large cloud of gas (mainly hydrogen) and dust. Gravity causes the material to collapse and clump together, forming a protostar. As the core temperature and pressure increase, eventually reaching about 15 million K, hydrogen nuclei begin to fuse into helium. This releases enormous energy and the star becomes a main sequence star, where the outward radiation pressure from fusion balances the inward gravitational force. A star like the Sun remains in the main sequence for billions of years. A much more massive star is hotter and burns through its hydrogen fuel much faster, so it remains on the main sequence for only millions of years. When hydrogen runs low, both stars expand: the Sun-like star becomes a red giant, while the massive star becomes a red supergiant. The red giant eventually ejects its outer layers as a planetary nebula, leaving a hot, dense core called a white dwarf, which cools over billions of years to become a black dwarf. The red supergiant, however, undergoes further fusion creating elements up to iron in its core. When the core collapses, the outer layers are blown off in a supernova explosion, which distributes heavy elements into space and can create elements heavier than iron. The remnant core becomes either a neutron star (incredibly dense, composed mainly of neutrons) or, if the original star was massive enough, a black hole, where gravity is so strong that not even light can escape.
AO3: Analyse and Evaluate
astronomers measured the recession velocities and distances of five galaxies. Their data is shown below:
| Galaxy | Distance (Mpc) | Recession velocity (km/s) |
|---|---|---|
| A | 10 | 650 |
| B | 30 | 2150 |
| C | 50 | 3600 |
| D | 70 | 4800 |
| E | 100 | 7100 |
(a) Plot a graph of recession velocity (y-axis) against distance (x-axis) and draw a line of best fit.
(b) Use the gradient of your line of best fit to determine the Hubble constant H₀ in km/s/Mpc.
(c) Galaxy F has a recession velocity of 14,000 km/s. Use your value of H₀ to estimate its distance.
(d) Galaxy G is 200 Mpc away but has a recession velocity of only 12,000 km/s. Suggest a reason why this galaxy does not follow Hubble's law as closely as the others.
(a) The data points should lie approximately on a straight line through the origin. The graph shows a linear relationship between recession velocity and distance, which is Hubble's law.
(b) The gradient can be calculated using any two points on the line of best fit. Using Galaxy A (10, 650) and Galaxy E (100, 7100):
Gradient = (7100 – 650) / (100 – 10) = 6450 / 90 = 71.7 km/s/Mpc
The Hubble constant H₀ ≈ 72 km/s/Mpc (accept 65–75 km/s/Mpc from the data).
(c) Using H₀ = 72 km/s/Mpc: d = v / H₀ = 14,000 / 72 = 194 Mpc.
(d) Hubble's law applies to galaxies that are moving away due to the expansion of space alone. Galaxy G's lower-than-expected velocity could be because it is in a gravitationally bound cluster where local gravitational attraction counteracts some of the expansion. Nearby galaxies in our Local Group (like Andromeda) are actually moving towards us because gravitational attraction dominates over cosmic expansion at short distances. Peculiar motion from local gravitational effects can cause individual galaxies to deviate from the Hubble flow.
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