Space Physics: Orbits, Stars and the Expanding Universe
Cosmic Navigator: From the Moon to the Big Bang 🚀
Introduction
1. Introduction
Yo! Quick-fire space physics. Why do we get day and night, why does the Moon never fall on us, why are summer and winter a thing, and how do we know the Universe started with a bang? Same toolkit every time: things go round in orbits, gravity does the pulling, and light carries the evidence across billions of years.
This is your fast refresh, not the full textbook. One key formula per concept, one quick worked example, and the exact sentence the marker wants. Skim it the night before, lock in `v = 2πr/T` and `v = H₀d`, walk in calm. Let's launch! 🚀
This is your fast refresh, not the full textbook. One key formula per concept, one quick worked example, and the exact sentence the marker wants. Skim it the night before, lock in `v = 2πr/T` and `v = H₀d`, walk in calm. Let's launch! 🚀
2. The Earth, the Moon and Orbital Speed
The Earth spins on its tilted axis once in about 24 hours (giving day and night) and orbits the Sun once in about 365 days. The tilt of the Earth's axis, not any tilt of the Sun, is what gives us seasons. The Moon orbits the Earth roughly once a month, and the changing slice of its sunlit half that we see is the cycle of phases. Anything in a circular orbit moves at a steady speed set by how big the orbit is and how long one lap takes.
Key idea🔑 Key formula: orbital speed : circumference of the orbit () divided by the time for one lap (). Keep in metres and in seconds for m/s. 🎯
Worked example
Worked Example: How Fast Is the Moon Moving?
Worked Example: 1 km/s in Space 🛰️
The Moon orbits the Earth at a radius of m, taking days for one lap. Find its orbital speed.
- 1Convert the period to seconds: s. Always work in seconds before using the formula.
- 2Apply :
The Moon looks like it's drifting lazily across the sky, but it's actually screaming around us at about a kilometre every second. It doesn't fall on us because that sideways motion keeps "missing" the Earth as gravity bends its path into a circle.
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Answer
3. The Solar System and Orbits
Our Solar System is one star (the Sun) plus eight planets, their moons, plus asteroids and comets, all held in orbit by the Sun's gravity. The four inner planets are small and rocky; the four outer ones are large and gassy. Orbits are elliptical (squashed circles), with the Sun not at the centre unless the orbit is nearly circular. A planet (or a comet) speeds up when it is closer to the Sun and slows down when it is farther away, by conservation of energy: moving in, gravitational potential energy turns into kinetic energy; moving out, the reverse. The Sun's gravitational field gets weaker with distance, and the planets' orbital speeds fall with distance too. You should also be able to read a table of planetary data (distance from the Sun, orbital period, density, surface temperature, surface ) and spot the trends: the period gets longer with distance, the inner planets are much denser, temperature mostly falls with distance (Venus is the exception), and surface follows the planet's mass, not its distance.
Key idea🔑 Key idea: orbits are ellipses; a body moves fastest at its closest point to the Sun (most kinetic energy, least gravitational potential energy) and slowest at its farthest. Inner planets orbit faster than outer ones.
Worked example
Worked Example: Mars vs Jupiter — Which Orbits Faster?
Worked Example: Closer = Faster 🪐
Mars orbits closer to the Sun than Jupiter. Without any numbers, state which planet has the greater orbital speed and why.
- 1Mars is closer, so the Sun's gravitational pull on it is stronger, and a stronger pull is needed to bend a faster body into orbit, so Mars orbits faster than Jupiter. The rule to remember: closer to the Sun always means a higher orbital speed.
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4. The Sun, Stars and the Life Cycle
The Sun is a medium-sized star, made mostly of hydrogen and helium, powered by nuclear fusion of hydrogen into helium in its core. It radiates most of its energy as infrared, visible light and ultraviolet. Stars are grouped into galaxies of many billions of stars (ours is the Milky Way, about light-years across), and distances are so vast we measure them in light-years: the distance light travels in space in one year, light-year m.
A star's life: an interstellar cloud of gas and dust (containing hydrogen) collapses under its own gravity and heats up, forming a protostar. It becomes a stable star when the inward force of gravity is balanced by an outward force due to the high temperature in its centre. When the hydrogen in the centre runs out, most stars (including the Sun) swell into a red giant, which forms a planetary nebula with a white dwarf at its centre. A much more massive star swells into a red supergiant, explodes as a supernova, and leaves a nebula containing hydrogen and new heavier elements, with a neutron star or black hole at its centre. That nebula may form new stars with orbiting planets.
A star's life: an interstellar cloud of gas and dust (containing hydrogen) collapses under its own gravity and heats up, forming a protostar. It becomes a stable star when the inward force of gravity is balanced by an outward force due to the high temperature in its centre. When the hydrogen in the centre runs out, most stars (including the Sun) swell into a red giant, which forms a planetary nebula with a white dwarf at its centre. A much more massive star swells into a red supergiant, explodes as a supernova, and leaves a nebula containing hydrogen and new heavier elements, with a neutron star or black hole at its centre. That nebula may form new stars with orbiting planets.
Key idea🔑 Key idea: the Sun fuses hydrogen → helium. Every star: cloud → protostar → stable star. Then it depends on mass: most stars → red giant → planetary nebula + white dwarf; massive → red supergiant → supernova → nebula + neutron star or black hole, and the nebula can form new stars and planets.
Worked example
Worked Example: Order a Star's Life Cycle
Worked Example: Supergiant to Supernova 💥
Put these stages of a massive star (much heavier than the Sun) in order: supernova, interstellar cloud, red supergiant, stable star, protostar, black hole.
- 1Every star starts the same way: interstellar cloud → protostar → stable star. The cloud collapses under gravity and heats up to make the protostar, and the star is stable once gravity inwards is balanced by the outward force from its hot centre.
- 2Now read the word "massive" and take the massive-star branch: stable star → red supergiant → supernova → black hole, with a nebula of hydrogen and heavier elements thrown out by the supernova. A star like the Sun would instead become a red giant, then a planetary nebula with a white dwarf, so the mass tells you which branch to take.
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5. The Expanding Universe and Hubble's Law
Light from distant galaxies is stretched to longer (redder) wavelengths: redshift. The further away a galaxy is, the bigger its redshift, which means the faster it is moving away from us. That is Hubble's law, and it is evidence that the whole Universe is expanding from an initial hot, dense state: the Big Bang. A galaxy's speed comes from the change in wavelength of its light, and the distance of a far galaxy comes from the brightness of a supernova in it (the dimmer it looks, the further away). The faint microwave radiation of a specific frequency reaching us from all directions (cosmic microwave background radiation, CMBR) was produced shortly after the Universe formed and has been stretched into the microwave region as the Universe expanded; it is the second main piece of evidence. The Hubble constant is , about per second, and running the expansion backwards estimates the age of the Universe as .
Key idea🔑 Key formula: Hubble's law : recession speed equals the Hubble constant times distance ( s). Redshift and the CMBR are the two main pieces of evidence for the Big Bang; age of Universe .
Worked example
Worked Example: How Fast Is That Galaxy Receding?
Worked Example: Plug Into v = H₀d 🧮
A galaxy is m away. Taking the Hubble constant as s, find how fast it is moving away from us.
- 1State Hubble's law and substitute. Units already match (metres and per-second), so no conversion is needed:
- 2Sanity-check the size: that's about of the speed of light ( m/s), which is huge but believable for a galaxy this far away. The further out you look, the faster things recede, which is exactly why a bigger distance means a bigger redshift.
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