Astronomers discover the fastest runaway stars in our galaxy racing away at over 5 million mph

• When you gotta go, you gotta go. In the cosmos, this happens in spectacular fashion.
• Astronomers found two renegades, runaway white dwarf stars on an escape route out of our galaxy.

Many of the 100 billion or so stars in our home galaxy, the Milky Way, are locked in succinct, organized orbits of some kind. But there are a handful of runaways.

These runaway stars are on a one-way ticket out of our galaxy. Likely sent on their way by powerful supernovae explosions that kicked them out of their comfortable orbit and on an escape route into the distant cosmos.

The Milky Way has about two supernovae per century, and not every supernova will trigger runaways, making these stellar events relatively rare. But astronomers just reported six new ones in a recent study. And two of them are the fastest ever observed.

Runaway stars racing away at breakneck speeds

In the new study, astronomers using data from the European Space Agency's Gaia survey identified two runaway stars with the fastest radial velocities ever seen.

The first star, J0927, is traveling at 1,420 miles per second (2,285 km/s or 5,112,000 mph).

The second star, J1235, isn't far behind, racing through space at 1,053 mi/s (1,694 km/s or 3,790,800 mph).

These speeds are difficult to fathom from human experience. The fastest human-made spacecraft, for example, is the Parker Solar Probe which reached a record speed of 101 mi/s in 2021 — about ten times slower than these stars.

But if you're going to escape the Milky Way galaxy, it's going to require some serious speed. Just for a rocket to escape the gravitational clutches of the tiny planet Earth is 7 mi/s (11.2 km/s). The escape velocity for an entire galaxy like the Milky Way? A whopping 341 mi/s (550 km/s).

Suffice it to say, the hypervelocity stars J0927 and J1235 have what it takes. They likely gained their tremendous speed from what astronomers call a dynamically driven double-degenerate double-detonation supernova, or D6 for short.

D6 supernovae launch stars on a blazing runaway path

Double detonations typically occur when a white dwarf — a type of star that is much older and cooler than our sun — orbits another white dwarf.

White dwarf stars are peculiar objects because they can exist for billions of years in a relatively quiet state, or they can erupt in a brilliant explosive burst called a supernova.

The key to getting a white dwarf to explode, typically what's called a Type Ia supernova, is just to give the star a little more mass than it can handle. That critical mass usually comes from a companion star orbiting the white dwarf.

But when that companion star is another white dwarf shrouded by a thick layer of helium gas, something slightly more special occurs, a D6 supernova, according to space reporter Michelle Starr for Science Alert.

The D6 is called a double detonation because it involves two different explosions. The first explosion kicks off when one of the white dwarf stars accumulates too much helium gas, which triggers a thermonuclear explosion, reported Starr.

"But the helium detonation goes a step further: its shockwave triggers a second detonation in the core of the white dwarf, producing a colossal kaboom," Starr added.

That second explosion — the colossal kaboom — is powerful enough to ultimately launch the companion white dwarf at breakneck speeds out of our galaxy.

The authors reported in the study that "A significant population of fainter low-mass runaways may still await discovery." So, there may be more runaway stories to come.

The study has been submitted for publication in the Open Journal of Astrophysics.