The Milky Way Galaxy
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flagWhat you'll discover
- arrow_forwardDescribe the structure of a spiral galaxy: bulge, disk, arms, halo
- arrow_forwardLocate the Sun’s position in the Milky Way
- arrow_forwardUse light-years to express galactic distances
- arrow_forwardGrasp the scale: 100–400 billion stars
Our city of stars
The Milky Way is our home galaxy: a vast rotating disk of stars, gas and dust about 100,000 light-years across. Astronomers estimate it contains between 100 and 400 billion stars — counting them at one per second would take more than 3,000 years.
From a dark Himalayan sky, the Milky Way appears as a glowing band across the night: you are looking along the plane of the disk, through millions of stars too faint and far to separate by eye.
Bulge, arms and halo
The galaxy has architecture. At the centre sits the bulge, a dense, golden crowd of mostly old stars wrapped around a supermassive black hole called Sagittarius A* — four million times the Sun’s mass.
Spread around it is the thin disk, where bright young blue stars trace sweeping spiral arms — the galaxy’s star factories, rich in gas and glowing nebulae. Surrounding everything is a sparse spherical halo of ancient stars and globular clusters, plus a vast envelope of invisible dark matter that outweighs all the stars combined.
You are here
Find the marker in the simulation: our Sun lives about 26,000 light-years from the galactic centre, on a minor arm called the Orion Spur — comfortably suburban, far from the crowded, radiation-soaked core.
The whole solar system orbits the galaxy at about 230 km/s, completing one revolution — a "galactic year" — every 225 to 250 million years. The last time we were on this side of the galaxy, dinosaurs had not yet evolved. Earth has made roughly 20 galactic laps in its entire history.
The light-year ruler
Galactic distances make kilometres useless, so astronomers use the light-year: the distance light travels in one year, about 9.46 trillion km. Even light, the fastest thing in the universe, needs 100,000 years to cross our galaxy.
A light-year ruler also makes telescopes time machines. The core’s light left 26,000 years ago, when humans were painting cave walls. Look at the Andromeda galaxy and you see light that left 2.5 million years before you were born — you are literally looking into the past.