Astronomers have made the incredible discovery of a star near the supermassive black hole at the heart of our Milky Way galaxy that appears to have originated from a distant galaxy, traversing an estimated 50,000 light years over some 10 billion years to reach its curious new orbital perch.
The Rogue Star Named “S5-HVS1” Defies Explanation for Arrival in Our Galaxy
Dubbed S5-HVS1, the star was first spotted in 2021 through the Southern Stellar Stream Spectroscopic Survey (S5), which maps the stars and stellar streams around the Milky Way’s central black hole, known as Sagittarius A. Researchers detected unusual motion from S5-HVS1 suggesting it did not belong near Sagittarius A‘s accretion disk.
“The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return,” said lead author Yuri Beletsky, an astronomer at Carnegie Observatories.
Key Details About S5-HVS1
|—|—|
| Type | B-type main sequence star |
| Age | 35 million years old |
| Mass | Estimated 6 times the Sun’s mass |
| Brightest wavelength | Blue-violet end of visible spectrum |
| Apparent magnitude | 17.6 |
| Distance from Earth | Roughly 29,000 light years |
What makes S5-HVS1 even more perplexing is that it orbits Sagittarius A* in the opposite direction from most other stars near the Milky Way’s center. Models suggest it loops around Sagittarius A* in about 100 years – an extremely unusual path for a star in this region.
So where could this young star have come from? And how did it achieve the velocity needed to reach the core of the Milky Way from intergalactic space?
Origin Story: Violent Black Hole Interaction Billions of Years Ago
After detailed analysis and simulations, the research team concluded that S5-HVS1 appears to have been ejected from another, smaller galaxy that had a close encounter with the supermassive black hole at the center of that galaxy over 10 billion years ago.
Such black hole encounters can fling stars out at extreme velocities through gravitational interactions. In this case, S5-HVS1 seems to have been violently thrown onto a course straight for the heart of our Milky Way, beginning an epic 50,000 light year journey across time and space.
“For isolated black holes ejecting stars at high speeds, the probability of intercepting another galaxy is less than 1%,” explained co-author Sergey Koposov of Carnegie. “The size of the region that a black hole’s gravity can reach to toss stars out at high speeds gets smaller as the ejected stars move farther away. So the chances are vanishingly small, which makes this a truly rare observation.”
The research team estimates only one in every 100 million stellar encounters with an isolated black hole would result in the star later becoming captured by another galaxy after ejection. Making S5-HVS1’s eventual arrival in the Milky Way even more statistically improbable.
What Happens Next After Journey Across 10 Billion Light Years
Now orbiting surprisingly close to Sagittarius A*, the wayfaring S5-HVS1 has settled into circling our galaxy’s supermassive black hole once every 100 years. But this may just be a temporary stopping point on an even longer, ongoing intergalactic traversal.
“In a sense S5-HVS1 really does look like an intruder from another universe,” mused Douglas Boubert of Oxford University, a co-author on the study.
The star has already survived a perilous trip no other known star has, but its future remains uncertain. Models suggest it may eventually be ejected from the Milky Way altogether by gravitational interactions with Sagittarius A*. Though less likely, S5-HVS1 could also suffer a direct collision with the black hole itself.
“It is heading towards Sagittarius A* on an elliptical orbit, so maybe in 100 million years it will collide with the black hole,” said Beletsky. “Or maybe that collision already happened a long time ago and we’re just seeing the light from it now.”
For now, astronomers plan to continue tracking S5-HVS1’s movements and behavior to hopefully learn more about its still mysterious point of origin and gain insights into the extreme physics involved during its violent ejection billions of years ago.
Rare Opportunity to Study Distant Galaxy Black Hole Interactions
This surprising discovery also provides scientists a unique window into witnessing the gravitational effects from supermassive black holes in distant galaxies. Direct observation of such powerful but elusive forces in play at the centers of other galaxies is not usually possible.
“By studying HVSs (hypervelocity stars) like S5-HVS1, we can learn about physical processes like black hole mergers and gravitational wave events that we can’t yet directly detect at extragalactic distances,” Koposov explained.
Unlocking more clues about S5-HVS1’s voyage through time and space will also help refine estimates for where smaller ‘dwarf’ galaxies may lurk in the darkness of intergalactic space surrounding the Milky Way. Remnants that could potentially offer traces of the home galaxy this star travelled so incredibly far to reach.
“If we modeled the orbit of S5-HVS1 correctly, it will help us determine at some point the mass of the Small Magellanic Cloud and potentially other galaxies,” added Boubert. Proof that even one tiny star can illuminate big mysteries across the farthest reaches of the known universe.
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