Arms and bar

The shape of our galaxy

A bar in the middle, spiral arms coming off it, and a halo of old stars all round.

How this site works. Every fact is in our own words and linked to its source, beside it. Where good sources disagree, we show each version in a “Sources differ” box.

Astronomers agree it is a spiral with a central bar, with arms of stars and gas coming off each end of the bar. Royal Observatory

Our galaxy from above: the bar, the arms and the Sun about halfway out (our drawing; arm shapes are approximate).Drawing: our own, simplified and not to scale

The bar and the bulge

The bar is roughly 30,000 light-years across. Royal Observatory

The bar wasn’t proven until 2005, when a team used NASA’s Spitzer Space Telescope to survey stars at the centre in infrared light, which gets through the dust. Royal Observatory

The central bulge holds about ten billion mainly old, red stars and is shaped like a peanut. ESA, guide

The bar joins the bulge on two opposite sides, like a pencil skewering a peanut lengthways. Royal Observatory

An artist’s impression of a barred spiral galaxy seen from above: a yellow bar of stars in the middle and bluish arms winding out from its ends — An artist’s impression of the Milky Way, based on Spitzer’s infrared survey and other data.Picture: NASA/JPL-Caltech/ESO/R. Hurt, Public domain, via Wikimedia Commons (cropped and resized)

Two arms or four?

How many major arms? Sources differ

In 2008 NASA’s Spitzer team found two major arms, Scutum-Centaurus and Perseus, coming off the ends of the bar; before that four major arms had been assumed. NASA, Milky Way map
Later Gaia star maps gave clear evidence for four arms, an ESA scientist says. ESA, Gaia
The Royal Observatory names four major arms after the constellations they pass through: Perseus, Scutum-Centaurus, Sagittarius-Carina and Norma/Outer. Royal Observatory

Astronomers have argued about this for decades. NASA’s 2008 Spitzer map found two major arms; later Gaia star maps, and the Royal Observatory, say four.

Most of the galaxy’s stars are born in the nebulae of the arms. Royal Observatory

Arms don’t keep the same stars. ESA’s Jos de Bruijne compares them to traffic jams: stars leave at the front while more pile up at the back. ESA, Gaia

Jan Oort mapped the first hints of the spiral structure with radio observations in the 1950s. ESA, Gaia

Side on

Side on: a thin disc, a bulge in the middle and a halo of old star clusters (our drawing, not to scale).Drawing: our own, simplified and not to scale

The disc is only about 1,000 light-years thick at the spiral arms, rising to around 10,000 light-years at the central bulge. Royal Observatory

Around the disc is a stellar halo of loose stars and globular clusters, thought to be over 300,000 light-years across but holding only 1% of the stars’ mass. Royal Observatory

There are around 160 known globular clusters in the Milky Way: tight balls of hundreds of thousands to millions of stars. Royal Observatory

The disc is warped, because a ripple is passing through it. Royal Observatory

Our opinion

Our view: “a peanut skewered by a pencil” is the best description of anything in astronomy, and we will be using it at parties.

Next: where we are →