Space physics flashcards

Key points

• The Solar System is the Sun and everything orbiting it: eight planets, dwarf planets such as Pluto, asteroids and comets.
• Moons orbit planets. Planets, dwarf planets, asteroids and comets orbit the Sun.
• The Sun is one of billions of stars in our galaxy, the Milky Way.
• The universe holds billions of galaxies.

Smallest to largest

Key points

• A star begins as a nebula: a huge cloud of dust and gas, mostly hydrogen.
• Gravity pulls the cloud together. As it collapses, the particles speed up and the core heats: a protostar.
• When the core is hot and dense enough, hydrogen nuclei fuse into helium, releasing energy.
• The star is born and joins the main sequence.

Key points

• A main sequence star is in equilibrium: two effects balance.
• Gravity pulls inwards, and the outward pressure from the energy released by fusion pushes outwards.
• So the star stays a steady size for billions of years. The Sun is a main sequence star now.
• When the hydrogen in the core runs low, the balance changes and the star moves on.

Key points

• When its hydrogen runs low, the star swells into a red giant.
• It sheds its outer layers, leaving its hot, dense core: a white dwarf.
• No fusion happens in a white dwarf, so it slowly cools and fades, in theory into a black dwarf.

The Sun's life cycle

Key points

• It swells into a red super giant.
• Its core collapses and it explodes as a supernova, which can briefly outshine a whole galaxy.
• What is left depends on the mass: an incredibly dense neutron star, or for the biggest stars a black hole.
• A black hole's gravity is so strong that not even light escapes, so it cannot be seen directly.

A massive star's life cycle

Key points

• Fusion joins light nuclei into heavier ones, releasing energy: hydrogen into helium first.
• As a star ages it fuses heavier elements, up to iron.
• Elements heavier than iron, such as gold and uranium, are only made in a supernova.
• The supernova scatters the elements through space, into clouds that form new stars and planets.

Key points

• Gravity: it pulls the Moon towards the Earth, the planets towards the Sun and satellites towards their planet.
• The Moon is a natural satellite. Artificial satellites are the ones people launch.
• In a circular orbit, gravity acts towards the centre, at right angles to the motion.
• So it changes the direction of motion, not the speed. Without it, the object would fly off in a straight line.

Key points

• In a circle the direction keeps changing, so the velocity changes even at a constant speed.
• A changing velocity is an acceleration: here it is towards the centre, caused by gravity.
• For a stable orbit, a smaller radius needs a higher speed.
• Closer in, gravity pulls harder, so the object must move faster or it would fall inwards.

Closer in, faster round

Key points

• Light from distant galaxies arrives with a longer wavelength than expected, shifted towards the red end of the spectrum.
• It happens because the galaxies are moving away from us.
• The further away a galaxy is, the bigger its red-shift, so the faster it is moving away.
• So the universe is expanding: from any galaxy, the others would all seem to be moving away.

Key points

• The universe began from a tiny, extremely hot and dense region about 13.8 billion years ago, and has expanded ever since.
• Red-shift is the key evidence: run the expansion backwards and everything was once squeezed together.
• The cosmic microwave background, faint radiation left from the early universe, is further evidence.
• Distant supernovae suggest the expansion is speeding up. Dark energy and dark matter are not yet understood.