Spacetime and the cosmos

1970

Galaxies spin too fast

Vera Rubin and Kent Ford

Why don’t the outer parts of a galaxy fly away?

Rubin and Ford measured how fast stars orbit at different distances in the Andromeda galaxy. Far from the centre, where the visible stars thin out, they should have slowed down, like outer planets. They didn’t. Most of a galaxy’s mass must be invisible: what we now call dark matter.

First, make a guess

Rubin measured stars far from a galaxy’s centre. Compared with stars nearer in, they were orbiting…

How they figured it out

Vera Rubin wanted to study astronomy at Princeton, which did not accept women into its graduate astronomy programme at the time. She went elsewhere. In 1965 she became the first woman officially allowed to observe at the Palomar Observatory, where there was no women’s toilet; she taped a paper skirt onto the sign on the door.

With Kent Ford’s sensitive new instrument, she measured how fast stars orbit in the Andromeda galaxy. In 1933 Fritz Zwicky had already suspected “dark matter” in a cluster of galaxies, but few took it seriously. Rubin’s flat rotation curves, galaxy after galaxy, made it impossible to ignore.

The equation

v(r)=G M(r)rv(r) = \sqrt{\frac{G\,M(r)}{r}}

An orbit’s speed depends only on the mass inside it; a flat speed curve means mass keeps growing outward.

Everyday echoLike a merry-go-round where the outer horses keep pace with the inner ones.

See it in real life

The roundabout that shouldn’t work

In the Solar System, outer planets crawl: Neptune moves at about 5 km/s, Earth at 30. In galaxies, stars far out move as fast as stars near the middle, as if the galaxy sat inside a huge invisible ball of mass holding them in.

Where engineers use it

  • Weighing by motion

    Engineers weigh what they cannot see from how things move around it, from satellites to rotating machinery.

Where it came from

What it led to