The Norma Arm is the innermost of the Milky Way’s main spiral arms, a band of gas and young stars curling out from the end of the central bar.
At a glance
| runs from the centre | 4.4 to 10 kpcmodel |
|---|---|
| width | about 980 light-yearsmodel |
| pitch angle | 9.4°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 Norma Arm
The Norma Arm lies deep inside the Galaxy, closer to the centre than the Sun by a wide margin. It takes its name from the southern constellation Norma, the direction in which our line of sight runs along it. Seen from here it sits behind the Sagittarius–Carina and Scutum–Centaurus arms, so almost everything known about it comes from radio and infrared light, which pass through the intervening dust.
Surveys of the inner Galaxy in infrared starlight suggest that the Norma Arm is a minor arm: it is rich in gas and star-forming regions but holds no strong excess of old stars, unlike the Scutum–Centaurus and Perseus arms. Its position is pinned by masers, small bright sources of microwave emission in star-forming clouds whose distances can be measured directly by parallax.
What you see in the piece
In the piece the arm is part of the galaxy’s density field rather than a painted stripe, so from the Sun’s position it is swallowed by dust and only shows clearly once you rise above the disc and look down.
Measured or modelled
The arm’s track, width and pitch angle are a fit to maser positions by Reid and collaborators, published in their survey of the Galaxy’s structure. Beyond the masers the arm is extrapolated, and some models join the Norma Arm to the Outer Arm as a single arm wrapping round the far side of the Galaxy.
Questions
Where is the Norma Arm?
In the piece’s model it runs from 4.4 to 10 kpc from the Galactic Centre, inside the Sun’s orbit and on the inner side of the Scutum–Centaurus Arm.
What is the pitch angle of a spiral arm?
It is the angle between the arm and a circle drawn round the Galactic Centre: a small angle means a tightly wound arm. For the Norma Arm the piece uses 9.4°.