Extreme & Unusual Objects

Hypervelocity Stars

Hypervelocity stars are stars flung outward at speeds high enough to escape the Milky Way.

The leading explanation involves a close encounter with the supermassive black hole at the galaxy's center.

Observe
A star with an extremely high velocity far from the galactic disk.
Measure
Doppler shifts and precise motion on the sky from Gaia.
Model
Three-body encounters with a massive black hole.
Explain
Gravity can fling stars out of the galaxy.

What is it?

A hypervelocity star travels at hundreds to thousands of kilometers per second, faster than the Milky Way's escape speed of roughly 500 to 600 kilometers per second in the Sun's neighborhood.

In the Hills mechanism, proposed in 1988, a pair of stars orbiting each other passes close to the supermassive black hole at the galactic center. The black hole captures one star and ejects the other. Other processes, such as a companion being flung out when its partner explodes, can also produce fast stars.

How does it happen?

Stage 1
Binary star approaches the central black hole
Stage 2
Gravitational slingshot
One star captured
Stage 3
Other star flung out at extreme speed
Stage 4
It may escape the galaxy

What do we actually observe?

Speed: The fastest known, S5-HVS1, moves at roughly 1,700 kilometers per second and is thought to have been ejected from near the galactic center a few million years ago.
Path: Traced backward, their paths point toward the galactic center.

How do scientists detect it?

Spectroscopic surveys. Measure radial velocities.
Gaia. Provides precise positions and motions.

Why does it matter?

They point to the presence and properties of the central black hole.
Their trajectories can help map the Milky Way's dark matter halo.

The SpaceWeatherTracker connection

A star in flight carries its own stellar wind and magnetic field; we know the physics best for our own Sun, whose wind SpaceWeatherTracker measures.

This is an explainer. SpaceWeatherTracker does not show live data for this phenomenon; see the “Space Phenomena Now” section on the hub for what is currently monitored.

Primary family: Extreme & Unusual Objects