Space physics

1. Describe the solar system and its place in the Milky Way

   Our solar system is one star - the Sun - orbited by eight planets, dwarf planets like Pluto, moons orbiting the planets, and smaller bodies such as asteroids and comets. The whole system sits in the Milky Way, a galaxy of billions of stars, and the Milky Way is just one of billions of galaxies in the universe. Keep the hierarchy in order: moons orbit planets, planets orbit stars, and stars gather in galaxies.

2. Explain how a star forms from a nebula

   A star begins as a nebula - a vast cloud of dust and gas, mostly hydrogen. Gravity slowly pulls the cloud together, and as it collapses the particles speed up and the core heats, forming a protostar. When the core becomes hot and dense enough, hydrogen nuclei fuse into helium, and a star is born. It is now a main sequence star, where it will spend most of its life.

3. Explain how a main sequence star stays stable

   A main sequence star is in equilibrium: gravity pulls everything inward, while the enormous outward pressure from fusion energy pushes back. The two balance, so the star stays a steady size for billions of years - the Sun is in this stage now. When the hydrogen fuel runs low, the balance shifts, and the star moves into the next stage of its life cycle.

4. Describe the life cycle of a star about the size of the Sun

   When a Sun-sized star runs low on hydrogen, fusion in the core slows, gravity briefly wins, and then the outer layers swell - the star becomes a red giant. It sheds its outer layers, leaving the hot, dense core behind as a white dwarf, which fuses nothing and simply cools and fades over time. Small and medium stars end this quiet way.

5. Describe the life cycle of a star much more massive than the Sun

   A massive star swells into a red super giant, then ends spectacularly: the core collapses and the star explodes as a supernova, briefly outshining a whole galaxy. What remains depends on the leftover mass - a neutron star, which is incredibly dense, or for the very biggest stars a black hole, whose gravity is so strong that not even light escapes. The route splits by mass: bigger star, more dramatic ending.

6. Explain how fusion in stars creates the chemical elements

   Fusion joins small nuclei into bigger ones, releasing energy - hydrogen into helium at first, then helium into heavier elements as the star ages. Elements up to iron are made this way inside stars; anything heavier than iron needs the extreme energy of a supernova. The explosion then scatters all these elements through space - the atoms in your body were made inside stars.

7. Explain orbital motion as the result of gravity

   Gravity is the force that keeps every orbit going: it pulls the Moon towards the Earth, the planets towards the Sun, and satellites towards the planet they circle. For a body in a circular orbit, gravity acts towards the centre of the circle, constantly changing the direction of motion without changing the speed. Without gravity, an orbiting body would fly off in a straight line.

8. Explain why an object in a circular orbit has changing velocity but constant speedHigher only

   Velocity is speed with a direction, and in a circular orbit the direction changes at every moment even though the speed stays the same - so the velocity is always changing. A changing velocity means the object is accelerating, and that acceleration is caused by gravity pulling towards the centre. For a stable orbit, the radius and speed must match: move a satellite closer in, and it must travel faster to stay in orbit.

9. Describe red-shift and what it tells us about distant galaxies

   Light from distant galaxies is stretched to longer wavelengths, shifting it towards the red end of the spectrum - this is red-shift. The further away a galaxy is, the bigger its red-shift, which means the most distant galaxies are moving away from us fastest. That single observation tells us the whole universe is expanding.

10. Explain how red-shift supports the Big Bang model

    If galaxies are all moving apart now, running the film backwards means everything was once squeezed into a single, incredibly small and dense point. The Big Bang model says the universe began there about 13.8 billion years ago and has been expanding ever since - and red-shift is the key evidence. Scientists are still exploring open questions, such as why the expansion seems to be speeding up, which ideas like dark energy try to explain.

The Solar System

The Sun and everything orbiting it: eight planets, dwarf planets, moons orbiting planets, and smaller objects such as asteroids and comets. It is a tiny part of the Milky Way galaxy.

Main sequence star

The long, stable phase of a star's life, in which the inward pull of gravity is balanced by the outward pressure from fusion in its core. The Sun is a main sequence star.

Supernova

The enormous explosion at the end of a massive star's life, which flings the elements the star made out into space.

Orbit

The path of one body around another, held by gravity: planets orbit the Sun, moons and artificial satellites orbit planets.

Red-shift

The observed increase in the wavelength of light from distant galaxies. The further away a galaxy is, the bigger its red-shift, showing the more distant galaxies are moving away faster.

The Big Bang theory

The theory that the universe began from an extremely small, hot and dense region around 13.8 billion years ago and has been expanding ever since. Red-shift is key evidence for it.

More Physics topics

• Energy
• Electricity
• Particle model of matter
• Atomic structure & radioactivity
• Forces & motion
• Waves

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