Astronomy › Section S · The Milky Way › Topic S.3
The mass of the galaxy and dark matter
You can weigh a galaxy the way you weigh the Sun: by timing the orbits around it. When astronomers did this for the Milky Way, they found far more mass than all its stars and gas, most of it in a form nobody has yet identified.
In this topic
- The Sun's orbit around the galaxy
- Weighing with an orbit
- Worked example: the mass inside the Sun's orbit
- What the rotation curve should look like
- What it actually looks like
- Worked example: a flat curve means growing mass
- The dark matter halo
- What the dark matter is not
- What it might be
- Common mistakes to avoid
- Key takeaways
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