Mystery Black Hole Appears to Have Hurled a Star Across the Milky Way

A mysterious black hole—that has yet to be discovered—in one of our galaxy's spiral arms appears to have hurled a star across the Milky Way, an international team of astronomers has said.

The scientists found a so-called "runaway star," dubbed PG 1610+062, located in the outer halo of the Milky Way—a vast region which surrounds the galaxy—far away from its star-forming regions.

"Runaway stars are stars that are currently leaving or already have left their natal star cluster due to some ejection event that forced them to run away from the environment in which they have formed," Andreas Irrgang, from Friedrich-Alexander University in Germany, told Newsweek.

Typically, the halo contains some of the galaxy's oldest stars and researchers had previously thought that PG 1610+062 was one of these with a mass around half that of our sun's. However, observations with the W. M. Keck Observatory revealed that the star is surprisingly young and ten times more massive than the sun.

"PG 1610+062 is a relatively young (82 million years) and massive (4.4
times the mass of the Sun) star that is currently located in the
Galactic halo far above the Galactic plane," Irrgang said. "This is unusual because such stars are typically expected to be found only close to the star-forming
regions in the Milky Way—i.e., the spiral arms inside the Galactic

"Similar objects—i.e., young stars in the halo—have already been found decades ago, and it is generally assumed that they are runaway stars from the Galactic disk, meaning they were born in the disk and later on ejected into the halo," he said.

But how exactly did PG 1610+062 find itself in the halo, far away from the galaxy's star-forming regions? Well, according to a study published in the journal Astronomy & Astrophysics, the Keck observations indicate that the star was ejected from its home cluster at nearly enough speed to escape the galaxy.

Astronomers already know about a handful of objects in the galaxy known as "hyper-velocity stars"—rare objects which do travel at sufficient speeds to escape the galaxy by overcoming its strong gravitational pull.

Milky Way
Panorama of the Milky Way as seen from the Atacama desert, Chile, 2013. Adhemar Duro/Getty Images

In order for this to be possible, these stars need to be accelerated to extremely high velocities by incredibly massive objects. For around three decades, the general consensus among scientists was that a black hole with the mass of four million suns would be capable of such a feat if a binary—or double—star system came too close to it. In this case, the black hole would swallow one of the stars and eject the other at high velocities.

This model became more accepted in the 2000s as scientists started to uncover strong evidence to suggest that such a black hole existed at the center of our galaxy. In light of this, the possibility that PG 1610+062 was ejected from the Galactic Center by the supermassive black hole there seems plausible.

However in the latest study, the team made further observations of the runaway star with the European Space Agency's Gaia spacecraft, finding that it did not originate from the Galactic Center but was ejected from the Sagittarius spiral arm of the galaxy.

Furthermore, data collected on the star's movement indicated that PG1610+062 could only have been hurled across the galaxy by a mid-mass black hole (MMBH,) according to the astronomers.

"The key finding was that the star was ejected from the Galactic disk
with a velocity (around 550 kilometers per second) that exceeds those of almost all other known runaway stars making it one of the most extreme ones," Irrgang said. "In particular, the inferred ejection velocity is so high that typical ejection scenarios
for runaway stars cannot explain it because the corresponding ejection velocities are usually less than 400 kilometers per second."

"Based on the trajectory, we could also exclude the interaction with the supermassive black hole at the Galactic center, which would, in principle, be easily capable of accelerating a star to 550 kilometers per second or even more than 1,000 kilometers per second. Consequently, PG1610+062 severely challenged all of the widely accepted ejection mechanisms for runaway stars," he said.

This result is intriguing because while astronomers have predicted the existence of MMBHs in some regions of our galaxy's spiral arms, no one has yet identified one.

"Because none of the standard ejection mechanisms seems to be able to explain the extreme ejection velocity of PG1610+062, it is quite likely that another powerful ejection mechanism is at work," Irrgang said. "Out of several alternative ejection mechanisms discussed in the literature, the interaction with an intermediate mass
black hole seems to be most likely."

"So far, the 'problem' with intermediate mass black holes is that there is no observational evidence that they indeed exist. However, extreme runaway stars like PG1610+062, for which the standard ejection scenarios fail, could provide a way to find those black holes," he said.

This article was updated to include additional comments from Andreas Irrgang.

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