The Milky Way

The structure of the Galaxy

• The Central Molecular Zone

  The reservoir of dense molecular gas within a few hundred parsecs of the Galactic Centre — a fraction of the Galaxy's volume that holds a large share of its molecular material. Turbulent, magnetised, and forming stars under extreme conditions, it is a nearby laboratory for the hearts of other galaxies.

• The Galactic Bar

  An elongated bar of stars spanning the central few kiloparsecs, which makes the Milky Way a barred spiral galaxy. The bar funnels gas inward, drives the boxy bulge, and sets up resonances that shape the orbits of disc stars far from the centre.

• The Galactic Bulge

  The dense, roughly spheroidal concentration of mostly old stars at the heart of the Milky Way. Boxy and peanut-shaped when seen edge-on, the bulge is intimately tied to the central bar and holds clues to how the inner Galaxy assembled.

• The Galactic Centre

  The crowded, dust-shrouded core of the Milky Way, about 26,000 light-years away, home to a dense star cluster and the four-million-solar-mass black hole Sagittarius A*. Hidden at optical wavelengths, it is studied in radio, infrared, and X-rays, and its stars' orbits weigh the central black hole directly.

• The Galactic Corona

  An enormous halo of tenuous, million-degree gas that surrounds the Milky Way, detectable in X-rays and in absorption against distant sources. This hot corona is a reservoir of baryons and the medium through which fresh gas accretes onto the Galaxy and enriched gas is expelled.

• The Galactic Warp

  The outer disc of the Milky Way is not flat but bent, curving up on one side and down on the other like a vinyl record left in the sun. This warp, traced by gas and by young stars, is likely stirred by the gravitational tug of the Magellanic Clouds and the dark-matter halo.

• The Solar Neighbourhood

  The Sun's local patch of the Galaxy — the stars, gas, and structure within a few hundred light-years. It includes the Local Bubble, a cavity of hot, thin gas blown by ancient supernovae, and it is the vantage point from which we map the entire Milky Way, orbiting the Galactic Centre once every roughly 230 million years.

• The Spiral Arms

  The bright lanes that wind through the disc, marked out by gas, dust, and freshly-formed luminous stars. The arms are thought to be density waves — regions where orbits crowd together and trigger star formation — rather than fixed structures, so stars drift in and out of them over time.

• The Stellar Halo

  A vast, sparse, roughly spherical cloud of the Galaxy's oldest and most metal-poor stars, together with its globular clusters, reaching far beyond the disc. Built largely from the shredded remains of smaller galaxies, the stellar halo is the Milky Way's deep archaeological archive.

• The Thick Disk

  A more extended, puffed-up disc of older, more metal-poor stars that envelops the thin disk. Its stars move on hotter orbits and formed early in the Galaxy's history, making the thick disk a fossil record of a more turbulent youth — perhaps heated by an ancient merger.

• The Thin Disk

  The flattened, rotating layer that holds most of the Milky Way's stars, gas, and dust, and where stars are still being born today. Only a few hundred parsecs thick but tens of thousands across, it contains the spiral arms and the young, metal-rich stars of the Galactic disc — the Sun among them.