The Outer Arm is the most distant of the Milky Way’s spiral arms from the centre, a thin, sparse sweep of gas and young stars near the edge of the disc.
At a glance
| runs from the centre | 9 to 21 kpcmodel |
|---|---|
| width | about 1,600 light-yearsmodel |
| pitch angle | 13.8°model |
Read off the piece’s own catalogue, the same values it draws with. Model marks a property of the piece’s galaxy model rather than a measurement.
About Outer Arm
Beyond the Perseus Arm, far out where the disc thins and starlight fades, lies the Outer Arm. In the part of the sky towards Cygnus it is also called the Cygnus Arm. Star formation there is scattered and weak, and the arm is traced mostly by the radio emission of hydrogen and carbon monoxide gas rather than by bright stars.
This far out the Milky Way’s disc is warped, bending above the mid-plane on one side and below it on the other, and it also thickens, or flares. The Outer Arm follows that bent and swollen disc, so it does not lie flat in the plane the inner arms define.
What you see in the piece
Its stars are few and distant, so from the Sun it adds little to the sky. Seen from above the Galaxy it is the faint, wide outermost ring of the spiral.
Measured or modelled
Its track, width and pitch angle are the Reid and collaborators maser fit, constrained by only a handful of distant masers. Some models treat it as the outer continuation of the Norma Arm.
Questions
What is the outermost spiral arm of the Milky Way?
Of the arms the piece draws, the Outer Arm, which in its model runs from 9 to 21 kpc from the Galactic Centre.
How wide is the Outer Arm?
The piece models it as about 1,600 light-years wide, the widest of its arms, with a pitch angle of 13.8°.