State that: (a) galaxies are each made up of many billions of stars (b) the Sun is a star in the galaxy known as the Milky Way (c) other stars that make up the Milky Way are much further away from the Earth than the Sun is from the Earth (d) astronom

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – Topic 6.2.2 Stars

6.2.2 Stars

Learning Objectives

  • State that galaxies are each made up of many billions of stars.

  • Identify the Sun as a star in the galaxy known as the Milky Way.

  • Explain that other stars in the Milky Way are much farther from Earth than the Sun.

  • Understand the concept of a light‑year and how it is used to measure astronomical distances.

1. Galaxies – Collections of Stars

A galaxy is a massive system that contains billions to trillions of stars, together with gas, dust and dark matter, all bound together by gravity.

The Milky Way, our own galaxy, is a typical spiral galaxy. It is estimated to contain roughly \$10^{11}\$ (one hundred billion) stars.

2. The Sun – Our Local Star

The Sun is a G‑type main‑sequence star located in one of the Milky Way’s spiral arms, called the Orion‑Cygnus Arm. It is the star closest to Earth and provides the light and heat necessary for life.

3. Relative Distances of Stars

While the Sun is only about \$1.5 \times 10^{8}\,\text{km}\$ (1 astronomical unit, AU) from Earth, the next nearest star system, Alpha Centauri, lies about \$4.37\$ light‑years away – more than \$270{,}000\$ times farther than the Sun.

Most stars that we see in the night sky are at distances of tens to thousands of light‑years, far beyond the reach of ordinary measuring tools.

4. Measuring Astronomical Distances – The Light‑Year

Light travels at a constant speed in vacuum:

\$c = 3.00 \times 10^{8}\ \text{m s}^{-1}\$

The distance light travels in one year is called a light‑year (ly). It can be calculated as:

\$\text{1 ly} = c \times (\text{1 year}) = 3.00 \times 10^{8}\ \text{m s}^{-1} \times 3.156 \times 10^{7}\ \text{s} \approx 9.46 \times 10^{15}\ \text{m}\$

UnitEquivalent Distance
1 AU (Astronomical Unit)\$1.5 \times 10^{8}\ \text{km}\$
1 Light‑Year\$9.46 \times 10^{12}\ \text{km}\$ (≈ \$63{,}240\$ AU)
1 Parsec\$3.26\$ light‑years

Using light‑years allows astronomers to express the vast distances between stars and galaxies in a convenient way.

Suggested diagram: A schematic showing the Sun, Earth, and a distant star with distances labelled in AU and light‑years.

Summary Checklist

  1. Galaxies contain billions of stars.

  2. The Sun is a star within the Milky Way galaxy.

  3. Other Milky Way stars are vastly farther from Earth than the Sun.

  4. A light‑year is the distance light travels in one year (\$\approx 9.46 \times 10^{15}\ \text{m}\$).