The Milky Way

Our galaxy: a slowly turning city of a few hundred billion stars, so wide that light needs about 87,000 years to cross it. Almost every star you have ever seen with your own eyes lives inside it — and on a dark night you can see the rest of it edge-on, as a band of haze.

The Milky Way, piece by piece

What it is

A barred spiral galaxy: a flat disc of stars, gas and dust, held together by gravity. The Sun takes about 230 million years to go around it once — the last time we were on this side, dinosaurs were new.

The centre

A crowded golden bar of old stars, and at its very middle Sagittarius A* — a black hole of 4.297 million solar masses. The stars closest to it whip around it the way comets whip around the Sun.

The arms

The bright spiral arms are not separate streams of stars — they are traffic jams, places where stars, gas and dust crowd together as they orbit. That is where new stars are being born, which is why the arms sparkle with young blue stars and pink star-forming clouds like the Orion and Eagle nebulae.

Where we live

The whole solar system — Sun, planets, comets, spacecraft and all — sits in the Orion Arm, a branch between two major arms, 27,000 light-years from the centre. The quiet suburbs: far from the crowded middle, in between the busy arms.

Dust and clouds

Between the stars drift huge clouds of gas and dust. Some, like the Coalsack, are dark and blot out everything behind them; others glow from the inside because new stars are switching on within.

When stars die

The Crab Nebula and the remnants of Tycho’s and Kepler’s supernovae are wreckage of exploded stars. Those explosions scatter the ingredients new planets are built from — including the ones you are made of.

Neighbours

Smaller galaxies circle ours the way moons circle a planet. The biggest of them, the Large Magellanic Cloud, may be on only its first pass, and it is close enough that in 1987 one of its stars was seen exploding with the naked eye.

Why it looks like a band and not a spiral

Every picture of a spiral galaxy is a picture of somebody else’s galaxy, seen from outside. We are inside ours, a little over halfway out from the middle and almost exactly in the plane of the disc. Looking along the disc, you see stars piled behind stars for tens of thousands of light-years — that is the band. Looking up out of the disc, you are through it within a few hundred light-years and the sky goes thin.

From our arm to the halo

From our own arm in to the centre, and out to the halo round it all:

The Orion Armpart of the galaxyBeyond a 150 mm telescopeThe galactic bar and bulgepart of the galaxyNaked eyeSagittarius A*supermassive black holeBeyond a 150 mm telescopeThe stellar halopart of the galaxyBeyond a 150 mm telescopeThe dark matter halopart of the galaxyBeyond a 150 mm telescope

What is in it besides stars

Clouds where new stars are being made, the wreckage of old ones that exploded, tight knots of stars that formed together, and lanes of dust thick enough to blot out everything behind them. Several of these are within reach of binoculars or a small telescope, and some — the Pleiades, the Orion Nebula — are easy.

The Coalsack Nebuladark nebulaNaked eyeThe Helix Nebulaplanetary nebula150 mm telescopeThe Orion Nebulaemission nebulaNaked eyeThe Dumbbell Nebulaplanetary nebulaBinocularsThe Ring Nebulaplanetary nebula150 mm telescopeThe Lagoon Nebulaemission nebulaBinocularsThe Eagle Nebulaemission nebula150 mm telescopeThe Witch Head Nebulareflection nebulaBeyond a 150 mm telescopeThe Pleiades (Seven Sisters)open star clusterNaked eyeThe Beehive Cluster (Praesepe)open star clusterNaked eyeThe Hercules Cluster (M13)globular star clusterBinoculars

Our neighbours

One big neighbour is coming for us — Andromeda, in perhaps several billion years; the odds are about even that it happens within ten billion. Almost no stars will collide, because they are too far apart, and the two galaxies will slowly settle into one.

The Large Magellanic Cloudirregular galaxyNaked eyeThe Sagittarius Dwarf Galaxy and Streamdwarf galaxyBeyond a 150 mm telescopeThe Andromeda Galaxy (M31)spiral galaxyNaked eye

How anyone knows the shape of a thing we are inside

• How far away are the stars? — the Earth’s own orbit as a measuring baseline
• How far away are the galaxies? — stars that pulse at a rate that gives away their true brightness

🌙 Plan an evening outsideEvery object, one by one