6.2.2 Stars – Core Content
Learning objectives (syllabus wording)
By the end of this lesson you must be able to state that:
- Syllabus point 1: Galaxies are each made up of many billions of stars.
- Syllabus point 2: The Sun is a star in the galaxy known as the Milky Way.
- Syllabus point 3: All other stars that make up the Milky Way are much farther away from the Earth than the Sun is.
- Syllabus point 4: Astronomical distances can be measured in light‑years, where one light‑year is the distance travelled in vacuum by light in one year (a distance, not a unit of time).
Recap of 6.1 – The Solar System (Core)
- Earth’s rotation → day/night; Earth’s orbit around the Sun → year, seasons.
- Solar‑System hierarchy (core): Sun → eight planets (Mercury → Neptune) → dwarf planets, asteroids, comets.
- All planets lie well inside the Sun’s orbit. The farthest planet, Neptune, is ≈30 AU ≈ 0.0005 ly from the Sun.
- Therefore every star other than the Sun lies far beyond the whole Solar System.
Key concepts (Core)
1. Galaxies and the Milky Way
- A galaxy is a massive system of stars, gas, dust and dark matter held together by gravity.
- Typical galaxies contain many billions (10⁹ – 10¹²) of stars – exactly what the syllabus requires.
- The Milky Way is the spiral galaxy that contains our Solar System.
2. The Sun’s place in the Milky Way
- The Sun is one of the many billions of stars in the Milky Way.
- It orbits the galactic centre at a distance of about 8 kpc ≈ 26 000 ly (order of magnitude ≈ 10⁴ ly). This figure is useful for extended study but not required for the core.
3. Relative distances of stars
4. Light‑year – definition (Core)
A light‑year (ly) is “the distance travelled in vacuum by light in one year”. It is a unit of distance only, not a unit of time.
For extended study you may use:
- Speed of light, \(c = 3.00\times10^{8}\ \text{m s}^{-1}\).
- One year = 365.25 days = \(3.156\times10^{7}\) s.
- Hence \(1\ \text{ly}=c\times1\ \text{year}\approx9.46\times10^{15}\ \text{m}\).
Comparison table (Core)
| Object | Distance from Earth | Distance in light‑years |
|---|
| Sun | 1 AU = 1.496 × 10¹¹ m | 1.58 × 10⁻⁵ ly |
| Alpha Centauri (nearest star) | 4.37 ly | 4.37 ly |
| Sirius (bright naked‑eye star) | ≈8.6 ly | ≈8.6 ly |
| Centre of the Milky Way | ≈8 kpc ≈ 26 000 ly | ≈26 000 ly |
Suggested diagram (Core)
A simple schematic of the Milky Way should show:
- The Sun’s position about 8 kpc from the galactic centre.
- Nearby stars such as Alpha Centauri (≈4 ly) and Sirius (≈8 ly).
- Scale bars for 1 AU, 1 ly and 10 ly to illustrate the huge difference between Solar‑System and inter‑stellar distances.
Summary (Core)
Galaxies contain many billions of stars. Our Sun is one of those stars in the Milky Way and lies only 1 AU (≈1.6 × 10⁻⁵ ly) from Earth. Every other star in the Milky Way is much farther away – at least a few light‑years, i.e. hundreds of thousands of times the Sun‑Earth distance. Because such distances are enormous, astronomers use the light‑year as a convenient unit of distance.
Supplementary material (optional for extended candidates)
Additional details on galaxies
- Types: spiral, barred‑spiral, elliptical, irregular.
- Typical star counts: 10⁹ – 10¹² per galaxy.
- Andromeda Galaxy distance: ≈2.54 × 10⁶ ly (extragalactic – not required for core).
Measuring stellar distances
- Parallax method – baseline = Earth’s orbital diameter; angle measured in arc‑seconds.
- Parsec definition (useful for AO2): 1 pc = distance at which 1 AU subtends 1″; 1 pc = 3.26 ly.
Cross‑topic link (Physics 3.1 – Wave speed)
Since \(c = f\lambda\), the speed of light can be related to its wavelength and frequency; this underpins the light‑year calculation.