Structure of the Milky Way
On a dark night, the Milky Way appears as a band of light across the sky. It is our view, from inside, of a vast spiral galaxy.
Our home galaxy
The Milky Way is a barred spiral galaxy containing roughly 100-400 billion stars. Its visible disc is about 100,000 light-years across but only about 1,000 light-years thick. The band of light in our sky is the combined glow of countless distant stars in the disc, as Galileo first saw with his telescope.
Finding our place
In the early 1900s, Harlow Shapley measured distances to globular clusters and found they are centred on a point in the constellation Sagittarius, far from the Sun. This showed the Sun is not at the galaxy's centre; we lie about 26,000 light-years from it, in a minor spiral arm called the Orion Arm.
Parts of the galaxy
The Milky Way has a thin disc containing young stars, gas, dust, and spiral arms; a central bulge and bar of older stars; and a roughly spherical halo containing old stars and about 150 globular clusters. Dust in the disc blocks visible light from the centre, which is why infrared and radio observations are essential for studying it.
Stellar populations
Astronomers group stars into populations. Population I stars, like the Sun, are relatively young, found in the disc, and rich in heavy elements made by earlier stars. Population II stars are old, found in the halo and globular clusters, and poor in heavy elements, reflecting the early galaxy's composition.
Galactic rotation
Stars and gas orbit the galactic centre. The Sun moves at about 230 km/s and takes about 230 million years to complete one orbit, a 'galactic year'. Since the Sun formed, it has orbited the galaxy only about 20 times.
Spiral arms
Spiral arms are not fixed structures of stars; they are regions of higher density, like traffic jams, that stars and gas pass through. Gas compressed in the arms forms new stars, and the brightest, hottest young stars that light up the arms die before they can move far from them.
The galactic centre
At the Milky Way's centre lies Sagittarius A*, a supermassive black hole of about 4 million solar masses. Stars there orbit at thousands of kilometres per second. The region is crowded with stars, gas, and magnetic filaments, and is hidden from optical view by dust.
Worked example
Light from the galactic centre, about 26,000 light-years away, left it roughly 26,000 years ago, when humans were living through the last Ice Age. The Sun, orbiting at about 230 km/s, would take about 230 million years to circle the galaxy; the last time it was in its current position, the first dinosaurs were appearing on Earth.
Key takeaways
The Milky Way is a barred spiral of 100-400 billion stars, about 100,000 light-years across. Shapley's globular cluster distances showed the Sun lies about 26,000 light-years from the centre, in the Orion Arm. The galaxy has a disc, a bulge and bar, and a halo. Population I stars are young and metal-rich; Population II old and metal-poor. The Sun orbits at about 230 km/s every 230 million years. Spiral arms are density waves. Sagittarius A* lies at the centre.
Flashcard
Barred spiral galaxy
Check your understanding
How did Harlow Shapley show that the Sun is not at the centre of the Milky Way?