The Perseus Arm is the major spiral arm just outside the Sun’s orbit, one of the Milky Way’s two great stellar arms, running through Perseus and Cassiopeia.
At a glance
| runs from the centre | 5.5 to 16 kpcmodel |
|---|---|
| width | about 1,400 light-yearsmodel |
| pitch angle | 10.3°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 Perseus Arm
Looking away from the Galactic Centre, the first full arm beyond the Local Spur is the Perseus Arm. Like the Scutum–Centaurus Arm it stands out in counts of old stars, which marks it as one of the Galaxy’s two major arms. In our sky it lies along the Milky Way through Perseus, Cassiopeia and Cepheus.
It is home to the Double Cluster in Perseus, the young cluster pair known as h and χ Persei, to the star-forming complex W3 and to the supernova remnant Cassiopeia A. Parallaxes of masers in W3 were among the first such measurements across the Galaxy, and they showed the arm to be nearer than distances inferred from the Galaxy’s rotation had suggested.
What you see in the piece
From the Sun it is seen looking outward, where there is less dust in the way. Pull back above the disc and it is the broad arm wrapping round just beyond the Local Spur.
Measured or modelled
Its placement, width and pitch angle are the maser fit of Reid and collaborators; close to the Sun it is well measured, and its course on the far side of the Galaxy is extrapolated.
Questions
How far is the Perseus Arm from the Galactic Centre?
In the piece’s model the Perseus Arm runs from 5.5 to 16 kpc from the centre, outside the Sun’s orbit.
What can you see in the Perseus Arm?
The Double Cluster, the star-forming region W3 and Cassiopeia A all lie in it. The piece gives the arm a width of about 1,400 light-years and a pitch angle of 10.3°.