Milky Way

galaxy

The Milky Way — the barred spiral galaxy that contains the Sun and every star visible to the naked eye. Roughly 100,000 light-years across, with 100–400 billion stars wound into spiral arms around a central bar and the supermassive black hole Sagittarius A*. Because we sit inside it, its whole shape is reconstructed from star and gas surveys, never photographed from outside.

Open in 3D viewSee Milky Way in the live, interactive 3D map

Key facts

Type

Barred spiral

Diameter

~100,000 light-years

Stars

100-400 billion

Science

Classification

Type: Barred spiral galaxy (SBbc)
Central object: Sagittarius A* — a 4.15-million-solar-mass black hole
Satellite galaxies: Dozens, incl. the Large & Small Magellanic Clouds

Scale

Diameter: ~100,000 light-years
Sun's distance from centre: ~26,700 light-years
Stars: 100-400 billion · estimated — counted from the inside
Total mass: ~1.5 trillion solar masses · incl. dark matter (estimated)
Age: ~13.6 billion years

Shown here

In the galaxy (real positions): Sgr A* at the centre; 8 globular clusters in the halo
Satellites, OUTSIDE the disc: the Large & Small Magellanic Clouds (~163,000 & 200,000 ly — well beyond the ~100,000 ly edge)
Modelled: the spiral arms, bulge & disc (a reconstruction)
Descend to: the Orion Arm — our spiral arm

How we know

Note: We sit INSIDE the Milky Way, so its whole spiral shape is a reconstruction from star and gas surveys (radio maps of hydrogen; ESA's Gaia charting a billion stars) — never a photograph from outside. Everything here is at its REAL measured position with a real telescope image: Sagittarius A* at the centre, the globular clusters orbiting in the halo, and the Magellanic Clouds — which are separate dwarf galaxies orbiting WELL OUTSIDE the disc (~163,000 & 200,000 light-years, beyond its ~100,000 ly edge), not part of it. Only the spiral arms, bulge and disc are MODELLED / illustrative, dimmer in Realism. The only true photographs of a whole spiral are of OTHER galaxies (one is shown for comparison).

Discovery & history

Method: Naked-eye star
Name: 'Milky Way' translates the Latin Via Lactea, from the Greek galaxías kýklos, 'milky circle' — the pale band the galaxy's combined starlight makes across the sky. The word 'galaxy' shares the same root.

Galileo's telescope resolved the Milky Way's soft band into countless stars in 1610. In 1785 William Herschel mapped its rough shape by counting stars in every direction; in 1918 Harlow Shapley used globular clusters to show the Sun lies far from the centre. The 1920s 'Great Debate' and Edwin Hubble's distance to the Andromeda 'nebula' settled that the Milky Way is one galaxy among billions — and in 2022 the Event Horizon Telescope imaged the shadow of its central black hole, Sagittarius A*.

Images

The Milky Way's band across the sky — our view from INSIDE the Galaxy (a 360° photographic panorama; no photograph shows it from outside) — ESO / S. Brunier (CC BY 4.0) · Wikimedia Commons

Another galaxy (Messier 74), shown for comparison — a real face-on spiral suggesting what ours likely looks like from outside; we cannot photograph our own — NASA, ESA & the Hubble Heritage (STScI/AURA)-ESA/Hubble (public domain) · Wikimedia Commons

Images from open archives · credit shown per image