Stars
Section: Space Physics | Syllabus: Cambridge IGCSE Physics (0625)
The Sun as a Star
The Sun is a star of medium size, one of many billions of stars that make up the Milky Way galaxy.
- The Sun consists mostly of hydrogen (~75%) and helium (~24%)
- It radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum
- The Sun has a surface temperature of about 5800 K and a core temperature of about 15 000 000 K
Nuclear Fusion in Stars
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- Stars are powered by nuclear reactions that release energy
- In stable stars like the Sun, these reactions involve the fusion of hydrogen into helium
- Hydrogen nuclei combine at extremely high temperatures and pressures to form helium, releasing large amounts of energy - this is what makes stars shine
Galaxies
- A galaxy is a massive system made up of many billions of stars held together by gravity
- The Sun is a star in a galaxy called the Milky Way
- The Milky Way is a spiral galaxy with a diameter of about 100 000 light-years
- Other stars that make up the Milky Way are much further away from the Earth than the Sun is - the Sun is by far the closest star to us
- The nearest star after the Sun is Proxima Centauri, about 4.2 light-years away
The Milky Way - our galaxy - contains billions of stars, of which the Sun is just one
Light-Years
The distances between stars and galaxies are so enormous that ordinary units like kilometres become impractical. Astronomers instead measure large distances in light-years.
- A light-year is the distance that light travels through a vacuum in one year - it is a unit of distance, not time
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The Value of a Light-Year
- 1 light-year = 9.5 × 10^15 m
Worked Example: Calculating One Light-Year
Museli checks this value using d = v × t, with v = 3 × 10^8 m/s and t = 365.25 days.
- Convert t to seconds: t = 365.25 × 24 × 3600 = 3.156 × 10^7 s
- d = v × t = (3 × 10^8) × (3.156 × 10^7) ≈ 9.5 × 10^15 m
| Object / Scale | Distance from Earth | Light Travel Time |
|---|---|---|
| The Sun | 1.5 × 10^11 m (150 million km) | ~8 minutes |
| Proxima Centauri (nearest star) | 4.2 light-years | 4.2 years |
| Across the Milky Way | ~100 000 light-years | 100 000 years |
- When you observe a star 4.2 light-years away, you are seeing light that left it 4.2 years ago - looking at distant objects in space means looking back in time
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