2.1.1Space
The Universe — SQA National 5 Physics Revision Notes & Topical Questions
5 min read 5 parts Lesson 1 of 12
Part 1 of 5Objects in Space
The universe is everything that existsall matter, energy, space and time.
Star
- A star is a huge ball of hot gas that releases light and heat energy from nuclear fusion reactions.
Planet
- A planet is a large object orbiting a star that does not produce its own light and is big enough to clear its path.
Moon
- A moon is a natural satellite which orbits a planet or dwarf planet.
Galaxy
- A galaxy is a vast collection of billions of stars, together with gas and dust, held together by gravity.
Part 2 of 5Scale and Light-Years
Scale and Light-Years
Solar system
- A star together with the planets, moons and other objects that orbit it; ours is centred on the Sun.
Milky Way
- Our galaxy contains hundreds of billions of stars, and the Sun is just one of them.
Light-year
- A light-year is the distance travelled by light in one year, about 9.5 × 10¹⁵ m.
Speed of light
- Light travels at 3.0 × 10⁸ m/s in a vacuum, which is why long distances are expressed using light time.
| Object | Approximate size order |
|---|---|
| Planet | 10⁷ m across |
| Star | 10⁹ m across |
| Galaxy | 10²¹ m across |
| Universe | 10²⁶ m across |
Part 3 of 5Ideas about the Universe
1
Observation shows that light from distant galaxies is shifted towards the red end of the spectrum, indicating the galaxies are moving apart.
2
This suggests that the universe is expanding.
3
Tracing the expansion backwards suggests that everything began from a very small, hot, dense state, called the Big Bang.
4
The age of the universe is estimated at about 13.8 billion years.
5
New observations and technology continue to refine the theories.
Part 4 of 5Worked Example
Question
- Sunlight takes 500 s to reach the Earth.
- The speed of light is 3.0 × 10⁸ m/s.
- Find the distance from the Sun to the Earth.
- How long is a light-year in metres, using 365 days?
- d = v t = 3.0 × 10⁸ × 500 = 1.5 × 10¹¹ m.
- One year = 365 × 24 × 3600 = 31 536 000 s.
- 1 light-year = 3.0 × 10⁸ × 31 536 000 = 9.46 × 10¹⁵ m (about 9.5 × 10¹⁵ m).
Answer 1.5 × 10¹¹ m; 9.5 × 10¹⁵ m.
Part 5 of 5Exam Checklist & Tips
2.1.1 – Key Points for the Exam
- Light-year The distance light travels in one year.
- Galaxy A very large group of stars bound by gravity.
- Solar system A star and all the objects that orbit it.
- Big Bang The theory that the universe expanded from a hot, dense state.
Common Mistakes
- Thinking a light-year is a unit of time; it is a unit of distance.
- Saying that planets give out their own light; they reflect light from a star.
- Mixing up a galaxy with a solar system.
- Treating the Moon as a planet.
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