Courses › The Universe from Scratch › Lesson 9
The galaxy spins too fast · premieres Friday, November 6
The Milky Way is rotating. The Sun, with the entire Solar System, completes one lap around the centre of the galaxy at about two hundred and thirty kilometres per second. Today we'll see how that rotation is measured and why it is one of the strongest hints that something we cannot see exists: dark matter.
In the scene, the shape of the arms, the bar and the Sun's position come from measurements. The stars are one million two hundred thousand random points that follow that shape, not real stars one by one. The dust is missing, which in reality hides the centre. And only the Sun moves; the bar and the arms do not rotate in the animation.
| Figure | Value | Source |
|---|---|---|
| Sun's velocity | 236 ± 7 km/s (circular) | Reid et al. 2019 |
| Bar | Half-length 5.0 ± 0.2 kpc ≈ 16,300 light-years; 27 ± 2° | Bland-Hawthorn and Gerhard 2016 |
| Spiral arms | Fit to ~200 maser parallaxes | Reid et al. 2019, Table 2 |
| Measured rotation curve | 229.0 ± 0.2 km/s at the Sun; slope −1.7 ± 0.1 km/s/kpc from 5 to 25 kpc (≈200 km/s at 25 kpc, calculated) | Eilers et al. 2019, ApJ 871, 120 (Gaia + APOGEE + WISE + 2MASS), verified 12 Oct 2026 |
| Mass model (bulge, disks, gas, dark halo) | Curve computed in the script | McMillan 2017 (Milky Way template v1.1) |
| Points in the scene | ≈1.2 million, at random | Milky Way template (README) |
They are Solaris Core Blender templates, built from measured data. You can download them, change the camera and make your own images.
See templates