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

6.2.2 Stars

Learning Objectives

By the end of this lesson you should be able to state that:

1. Galaxies are each made up of many billions of stars.
2. The Sun is a star in the galaxy known as the Milky Way.
3. Other stars that make up the Milky Way are much farther away from the Earth than the Sun is.
4. Astronomical distances can be measured in light‑years, where one light‑year is the distance travelled in vacuum by light in one year.

Key Concepts

Galaxies and stars

• A galaxy is a massive system of stars, gas, dust and dark matter bound together by gravity.
• Typical galaxies contain \$10^9\$–\$10^{12}\$ stars.
• The Milky Way is a barred spiral galaxy that contains our Solar System.

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 \$8.2\ \text{kpc}\$ (≈ \$2.7\times10^4\$ light‑years).

Relative distances of stars

• The average distance from the Earth to the Sun is \$1\ \text{AU}=1.496\times10^{11}\ \text{m}\$.
• The nearest star system, Alpha Centauri, is about \$4.37\$ light‑years away – roughly \$275{,}000\$ times farther than the Sun.
• Most visible stars in the night sky lie tens to thousands of light‑years away.

Measuring astronomical distances

Because the numbers are so large, astronomers use the unit light‑year (ly):

• Light travels at \$c = 3.00\times10^8\ \text{m s}^{-1}\$ in vacuum.
• One year = \$365.25\$ days \$= 3.156\times10^7\ \text{s}\$.

Therefore:

\$\$

1\ \text{light‑year}=c\times(1\ \text{year})=3.00\times10^8\ \text{m s}^{-1}\times3.156\times10^7\ \text{s}

\approx 9.46\times10^{15}\ \text{m}.

\$\$

Comparison Table

Summary

Galaxies are vast collections of billions of stars. Our Sun is one such star within the Milky Way, which also contains many other stars that are vastly farther away than the Sun. Astronomical distances are conveniently expressed in light‑years, the distance light travels in one year, equal to about \$9.46\times10^{15}\ \text{m}\$.