Ejected Star: How fast is fast?

30 years ago, astronomer Jack Hills demonstrated the math behind what has become known as the “Hills Mechanism”. Until this week, it described a celestial event that had never been observed.* But astronomers have always agreed that the physics and math should make what he described possible outcome, if suitable conditions are in play…

Hills explained that under the right conditions, a star might be accelerated to incredible speeds — and might even be flung out of its galaxy. The conditions:

  • A binary star passes close to a black hole, like the one at the center of our galaxy
  • The orbiting stars are caught in the gravity of a black hole, but because of their speed and distance, they are not sucked in.

If conditions are right, one star ends up orbiting the back hole while the other is jettisoned at incredible speed, yet holding onto its mass and shape. All that energy comes from the gravity of the black hole and former momentum of the captured star. [video animation, 20 seconds]

This week, astronomers found clear evidence of this amazing event and traced it back to our galactic center: Five million years ago — as our ancestors learned to walk upright — a star that passed close to the black hole at The Milky Way center was flung away at a stunning 6 million kph. It is traveling so fast, that it is no longer bound to our galaxy or our galactic cluster. It is headed out into the vast intergalactic void.

How fast is fast?

  • Rifle Bullet: Can exceed Mach 3 (2,300 mph)
  • Apollo Rocket: Reached 25,000 mph; Earth escape velocity.
  • Juno Probe: 165,000 mph, a record prior to 2019. (It used Jupiter’s gravity to accelerate)
  • Parker Probe: 213,000 mph (Nov 2019), but will soon reach 430,000 mph

* Prior to this week, astronomers have observed a few stars traveling inexplicably at incredible speeds. But this is the first time, that they have traced the trajectory back to a black hole and the conditions described by Jack Hills.