Milky Way — spiral structure
The Milky Way's disc of 100-400 billion stars, wound into spiral arms around a central bar and bulge. Its overall shape is a reconstruction from surveys of stars and gas — radio maps of hydrogen and, since 2013, ESA's Gaia charting a billion stars — not a photograph, because we observe it from inside. The arms shown here are illustrative, modelled at this scale.
Science
What it is
- Structure
- A central bar + bulge, four major spiral arms, a thin disc
- Our arm
- The minor Orion Arm, ~26,700 ly from the centre
- Major arms
- Perseus, Sagittarius, Scutum-Centaurus, Norma
How we know
- Note
- The spiral is genuinely present, but its detailed shape is MODELLED — we observe the Galaxy from inside and can't photograph it from outside. Radio maps of hydrogen gas and ESA's Gaia stellar census give its rough form; the arms drawn here are illustrative, shown in both view modes and dimmer in Realism, never a photograph.
Discovery & history
- Name
- The Milky Way is a barred spiral galaxy (Hubble type SBbc); its 'spiral arms' are named for the constellations they lie toward (Perseus, Sagittarius, Scutum-Centaurus, Norma) plus the Sun's own minor Orion Arm.
William Herschel's 1785 star-counts gave the first crude map. Radio surveys of hydrogen gas (from the 1950s) traced spiral arms through the dust; ESA's Gaia mission (2013-) measuring the 3-D positions of over a billion stars has refined the picture — a barred spiral, reconstructed from the inside.
Images
No official imagery available — this object has never been visited and no officially-licensed depiction exists.