Structure of the Galaxy
A galaxy is a huge family of stars, gas, dust and dark matter held together by gravity. Ours is the Milky Way, a barred spiral galaxy.
- Disc: flat and thin, about 100,000 light-years across but only about 1,000 light-years thick. It holds young stars, gas and dust.
- Bulge and bar: a thick, bright middle made of older stars, stretched into a bar.
- Halo: a faint, round cloud around everything, with old stars, globular clusters and most of the dark matter.
When we look along the disc from inside, we see many stars in a band. That band is the "milky way" in our night sky.
Spiral arms and the 21 cm hydrogen line
The disc has spiral arms, where gas is squeezed and many new stars form, so arms look bright and blue. Our galaxy has two main arms and several smaller ones; the Sun lies in a smaller one, the Orion Arm.
Dust blocks visible light, so how do we map arms from inside? Cold hydrogen atoms give out radio waves of wavelength 21 cm (frequency about 1420 MHz). These pass through dust. Each gas cloud moves at a different speed, so its 21 cm line is shifted a little (Doppler effect). From these shifts astronomers work out where the clouds are, and the arms appear.
The Sun's place, rotation and dark matter
The Sun is about 26,000 light-years from the centre, roughly halfway out. It moves at about 230 km/s and takes about 230 million years for one trip round, called a galactic year.
Rotation curve
A rotation curve plots orbit speed against distance from the centre. If most mass were in the bright centre, far stars should move slower (like outer planets). Instead, the speed stays almost flat far out. So the galaxy must contain a lot of unseen mass: dark matter, perhaps 5–10 times the visible matter.
Try it
In free play, move the distance slider. Compare the real speed with the speed expected from visible matter alone.
Star clusters, associations, nebulae and the central black hole
- Open clusters: loose groups of tens to thousands of young stars in the disc, like the Pleiades (Krittika).
- Globular clusters: tight balls of hundreds of thousands of very old stars in the halo. About 150 are known around our galaxy.
- Stellar associations: very young, loosely bound groups of hot blue stars that slowly drift apart.
- Nebulae: clouds of gas and dust. Emission nebulae glow (like the Orion Nebula, where stars are forming), reflection nebulae reflect starlight, and dark nebulae block light behind them.
Sagittarius A*
At the very centre is Sgr A*, a supermassive black hole of about 4 million solar masses. We cannot see it directly, but stars close to it orbit in just years at thousands of km/s. Their orbits give its mass. In 2022 the Event Horizon Telescope released an image of its shadow.
Key formulas and definitions
- Galactic year ≈ 2πr ÷ v ≈ 230 million years
- Sun: r ≈ 26,000 ly (8 kpc), v ≈ 230 km/s
- 1 light-year ≈ 9.46 × 10¹² km
- 21 cm line: λ = 21.1 cm, f ≈ 1420 MHz
- Flat rotation curve ⇒ mass grows with radius ⇒ dark matter
- Sgr A* ≈ 4 × 10⁶ solar masses
Worked examples
1. The Sun moves at 230 km/s on a circle of radius 26,000 ly. Estimate the galactic year.
r = 26,000 × 9.46×10¹² km ≈ 2.46×10¹⁷ km. Distance = 2πr ≈ 1.55×10¹⁸ km. Time = 1.55×10¹⁸ ÷ 230 ≈ 6.7×10¹⁵ s ≈ 2.1×10⁸ years, about 210–230 million years.
2. Why do we use radio waves to map the spiral arms?
Dust in the disc blocks visible light. The 21 cm radio waves from hydrogen pass through dust, so we can see gas across the whole galaxy.
3. A cluster has 300,000 old, red-yellow stars packed in a ball far above the disc. What is it?
A globular cluster in the halo.
4. Why is the Milky Way band brightest towards Sagittarius?
That direction points to the centre, where the bulge has the most stars.
Common mistakes
- Thinking the Sun is at the centre of the galaxy. It is about halfway out.
- Mixing open and globular clusters. Open = young, loose, in the disc; globular = old, dense, in the halo.
- Thinking the 21 cm line is light we see. It is a radio wave.
- Believing a flat rotation curve means no mass far out. It means more mass than we see.