Course: Separate Physics only | Tier: Foundation + Higher | Read the notes, then try the questions on paper.
Notes
Our solar system
Our solar system has one star (the Sun), eight planets, dwarf planets such as Pluto, and natural satellites (moons). Artificial satellites are made by people and put into orbit. The solar system is a small part of the Milky Way galaxy.
The Sun formed from a nebula – a cloud of dust and gas – pulled together by gravity.
The life cycle of a star
- Nebula: gravity pulls dust and gas together.
- Protostar: as it collapses, the temperature rises until hydrogen nuclei fuse to form helium.
- Main sequence star: stable for billions of years, because the outward pressure from fusion energy balances the inward pull of gravity.
- A star about the size of the Sun: red giant → white dwarf → black dwarf.
- A star much bigger than the Sun: red super giant → supernova → neutron star or black hole.
Fusion in stars makes all the naturally occurring elements. Elements heavier than iron are made in a supernova, and the explosion spreads the elements throughout the universe.
Orbital motion
Gravity provides the force that keeps planets and satellites in orbit. (HT) In a circular orbit the speed is constant but the direction keeps changing, so the velocity changes – the object is accelerating towards the centre. For a stable orbit, if the speed changes the radius must change: a faster object needs a smaller orbit.
Red-shift and the Big Bang
Light from distant galaxies is shifted towards the red end of the spectrum – its wavelength has increased. This red-shift shows the galaxies are moving away from us. The further away a galaxy is, the greater its red-shift, so the faster it is moving away. This shows the universe is expanding, which supports the Big Bang theory: the universe began from a very small region that was extremely hot and dense.
Since 1998, observations of supernovae suggest distant galaxies are moving away faster and faster. Scientists think this may involve dark mass and dark energy, which are not yet understood.
Questions
Q1 (F) Name the galaxy our solar system is part of.
Show answer
The Milky Way.
Q2 (F) What is the difference between a natural satellite and an artificial satellite?
Show answer
A natural satellite, like the Moon, is not made by people. An artificial satellite is made by people and launched into orbit, e.g. for communications or weather.
Q3 (F) Which force pulls a nebula together to form a star?
Show answer
Gravity.
Q4 (F) Put the life cycle of a star like the Sun in order, starting from the nebula.
Show answer
Nebula → protostar → main sequence star → red giant → white dwarf → black dwarf.
Q5 (F) Why does a main sequence star stay stable for billions of years?
Show answer
The outward pressure from the energy released by fusion balances the inward force of gravity.
Q6 (F/H) Describe what happens to a star much bigger than the Sun at the end of its life.
Show answer
It becomes a red super giant, then explodes as a supernova, leaving a neutron star or (for the most massive stars) a black hole.
Q7 (F/H) Where are elements heavier than iron made, and how do they spread through the universe?
Show answer
In a supernova. The explosion scatters these elements throughout the universe.
Q8 (F/H) What is red-shift, and what does it tell us about distant galaxies?
Show answer
An increase in the wavelength of light from a galaxy, shifting it towards the red end of the spectrum. It shows the galaxy is moving away from us – and more distant galaxies have a bigger red-shift, so they are moving away faster.
Q9 (H) A satellite moves in a circular orbit at constant speed. Explain why it is accelerating.
Show answer
Its direction is constantly changing, so its velocity is changing (velocity is a vector). A changing velocity means it is accelerating. Gravity provides the force towards the centre of the orbit.
Q10 (H) Explain how red-shift provides evidence for the Big Bang theory.
Show answer
Light from distant galaxies is red-shifted, so they are moving away from us. The further away a galaxy is, the faster it is moving away, so the universe is expanding. Running this backwards suggests the universe began from a very small, extremely hot and dense region – the Big Bang.
