12. Galaxies & the Milky Way
High SchoolOur Sun is just one of hundreds of billions of stars in a single galaxy — and that galaxy is just one of billions more. This chapter zooms out to galactic scale.
Types of galaxy and our own address
Galaxies are broadly classified as spiral (a flat rotating disc with arms, like ours), elliptical (smooth, football- to sphere-shaped, often older and gas-poor), or irregular (no defined shape, often disturbed by collisions).
Our own galaxy, the Milky Way, is a barred spiral galaxy containing an estimated 100–400 billion stars, roughly 100,000 light-years across. The Sun sits in a minor spiral arm called the Orion Arm, about 26,000 light-years from the galactic centre, where a supermassive black hole called Sagittarius A* (about 4 million solar masses) resides.
Mapping a galaxy from inside it
We cannot photograph the Milky Way from outside — every image you've seen of "our galaxy" is either an artist's impression or a photo of a different spiral galaxy. Astronomers instead map our galaxy's structure using radio and infrared light, which — unlike visible light — can pass through the thick clouds of dust that block our view of the galactic disc and centre in ordinary starlight.
Worked example — crossing the galaxy at light speed
Light travels at c ≈ 3.00×10⁵ km/s. The Milky Way is about 100,000 light-years across by definition — but let's verify what a "light-year" means and find how long light takes to cross the disc.
- A light-year is the distance light travels in one year: about 9.46×10¹² km.
- By definition, crossing "100,000 light-years" of distance takes light exactly 100,000 years.
- Compare to the Sun's distance from the galactic centre — 26,000 light-years — meaning light (or a radio signal) from the galactic centre left there 26,000 years ago by the time it reaches us.
Answer: it takes light 100,000 years to cross the Milky Way, and 26,000 years to travel from the galactic centre to the Sun — we always see the galaxy's centre as it looked 26 millennia in the past.
Test your understanding
What type of galaxy is the Milky Way?
A barred spiral galaxy — a flat, rotating disc with spiral arms and a central elongated "bar" of stars, containing an estimated 100 to 400 billion stars.
What lies at the very centre of the Milky Way?
A supermassive black hole named Sagittarius A*, with a mass of about 4 million Suns, whose gravity governs the orbits of stars in the galaxy's central region.
Why can't we take a normal photograph of the whole Milky Way from Earth?
Because we are embedded inside the galactic disc itself, and thick interstellar dust blocks our visible-light view of much of the disc and the galactic centre. Every "photo" of the whole Milky Way you've seen is really an illustration or a different galaxy standing in for ours.
What is expected to happen between the Milky Way and the Andromeda Galaxy?
They are approaching each other and are predicted to collide and merge in roughly 4.5 billion years, eventually forming a single, larger galaxy — sometimes nicknamed "Milkomeda."
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1The Milky Way is about 100,000 light-years across. How long would light take to cross it?
- A light-year is the distance light travels in one year
- Light crossing 100,000 light-years takes 100,000 years
- So even at light speed the crossing is very long
Example 2Why is it hard to see the true shape of the Milky Way?
- We sit inside the disc, not outside it
- Dust and stars block our view across the plane
- We must map it indirectly from within
Now you try
Work each one out first, then tap to reveal the worked answer.