Know that the Milky Way is one of many billions of galaxies making up the Universe and that the diameter of the Milky Way is approximately 100000 light-years

6.2.3 The Universe

Learning Objective

Students should be able to state that the Milky Way is one of many billions of galaxies that make up the Universe and that the diameter of the Milky Way is approximately \$100\,000\$ light‑years.

Key Facts

• The observable Universe contains an estimated \$2\$–\$3\$ trillion (≈ \$2\times10^{12}\$) galaxies.
• Our home galaxy, the Milky Way, is a barred spiral galaxy.
• Diameter of the Milky Way ≈ \$100\,000\$ light‑years (ly).
• One light‑year is the distance light travels in one year:

  \$1\text{ ly} \approx 9.461\times10^{12}\,\text{km}\$

• Thus the Milky Way’s diameter in kilometres is roughly:

  \$\$100\,000\ \text{ly} \times 9.461\times10^{12}\,\text{km/ly}

  \approx 9.5\times10^{17}\,\text{km}\$\$

Comparative Size Table

Understanding Scale

1. Imagine the Earth’s diameter as a 1 cm line. The Milky Way’s diameter would be about 7 km on the same scale.
2. Even on a cosmic scale, the Milky Way is only one of billions of galaxies, each with its own vast collection of stars.
3. When we look up at the night sky, the faint smudge we see as the Milky Way is the combined light of billions of stars spread across that \$100\,000\$‑ly span.

Quick Revision Questions

• Approximately how many galaxies are thought to exist in the observable Universe?
• What is the approximate diameter of the Milky Way in light‑years?
• Convert the Milky Way’s diameter to kilometres using the given conversion for a light‑year.
• Why does the Milky Way appear as a faint band in the night sky?