Somewhere in the southern sky, a star is moving fast enough to cross the distance from Earth to the Moon in under four minutes. Its name is S5-HVS1, and it is travelling at about 1,700 kilometers per second, more than 6 million km/h. That’s roughly ten times faster than most stars in our galaxy. It is never coming back.
S5-HVS1 is one of the fastest stars ever found, and astronomers can trace its path back to a single violent moment: a close encounter with the supermassive black hole at the center of the Milky Way.
S5-HVS1 at a Glance
| Type | Hypervelocity star (A-type, about 2.3 times the Sun’s mass) |
|---|---|
| Speed | About 1,700 km/s (over 6 million km/h) |
| Distance from Earth | About 29,000 light-years |
| Constellation | Grus (the Crane), southern sky |
| Ejected | About 4.8 million years ago, from the galactic center |
| Discovered | 2019, by the Southern Stellar Stream Spectroscopy Survey (S5) |
What Is a Hypervelocity Star?
Most stars in the Milky Way, including the Sun, orbit the galactic center at a few hundred kilometers per second. They are fast, but they are bound to the galaxy the way planets are bound to the Sun.
A hypervelocity star is moving so fast that the galaxy’s gravity can’t hold on to it. These stars are on a one-way trip out of the Milky Way into intergalactic space. The “HVS” in S5-HVS1 stands for exactly that: hypervelocity star.
Astronomers have found a few dozen hypervelocity stars since the first was spotted in 2005. S5-HVS1 stands out because it is one of the fastest of them and because its path points straight back to the galactic core.
How S5-HVS1 Was Discovered
The star was found in 2019 by an international team led by Sergey Koposov of Carnegie Mellon University. They were using the 3.9-meter Anglo-Australian Telescope in New South Wales, Australia, to study streams of stars as part of the Southern Stellar Stream Spectroscopy Survey, which is where the “S5” comes from.
The telescope’s spectrograph measured how fast the star was moving toward or away from us. ESA’s Gaia space telescope supplied its precise position and motion across the sky. Combining the two gave the star’s full 3D speed and direction, and the numbers were extreme. The team published their findings in the journal Monthly Notices of the Royal Astronomical Society.
How Our Galaxy’s Black Hole Threw a Star
When the team traced S5-HVS1’s path backward in time, it led to the center of the Milky Way, to Sagittarius A*, a black hole with about 4 million times the mass of the Sun.
The most likely explanation is the Hills mechanism, proposed by astronomer Jack Hills in 1988:
- A binary star wanders too close. S5-HVS1 probably started life as one half of a pair of stars orbiting each other.
- The black hole splits the pair. As the binary passed Sagittarius A*, the black hole’s gravity pulled the two stars apart.
- One star is captured. The partner fell into a tight orbit around the black hole, where it may still be today.
- The other is flung away. S5-HVS1 took the energy and was launched outward at about 1,700 km/s, like a stone from a sling.
Based on its speed and distance, this happened about 4.8 million years ago. That makes S5-HVS1 the first star clearly linked to ejection by the Milky Way’s black hole, which is strong evidence that the Hills mechanism really happens.
Where Is S5-HVS1 Going?
S5-HVS1 is currently about 29,000 light-years from Earth in the constellation Grus. It is moving away from the galactic center and will never fall back. In roughly 100 million years it will leave the Milky Way completely and drift into the near-empty space between galaxies.
The star itself is fine. It survived its brush with the black hole intact and is still shining normally. It is about twice the mass of the Sun and noticeably brighter. It will simply live out the rest of its life alone in intergalactic space.
Could the Sun Ever Be Thrown Out Like This?
No. The Sun is about 26,000 light-years from Sagittarius A*, in a stable orbit that takes roughly 230 million years to complete. To be thrown out, a star has to pass extremely close to the black hole, deep in the crowded galactic core. Our solar system never gets anywhere near that region.
Why S5-HVS1 Matters to Astronomers
Hypervelocity stars are useful scientific tools as well as curiosities:
- They prove the black hole is active. S5-HVS1 is direct evidence that Sagittarius A* can disrupt star systems and fling stars away.
- They help weigh the galaxy. The way the Milky Way’s gravity slows these stars as they climb out reveals how much mass, including dark matter, the galaxy holds.
- They reveal the galactic core. Each ejected star carries information about the crowded environment it came from, a region that is hard to observe directly through all the dust.
Keep exploring: Sagittarius A* has done far more than throw a star. Read Fermi Bubbles: The 50,000 Light-Year Scars Our Galaxy Can’t Hide to see what happened when it flared to life.
Frequently Asked Questions
How fast is S5-HVS1 moving?
S5-HVS1 is moving at about 1,700 kilometers per second, more than 6 million km/h. That’s fast enough to escape the Milky Way’s gravity completely.
Why is S5-HVS1 leaving the Milky Way?
About 4.8 million years ago, S5-HVS1 was most likely part of a binary star system that passed too close to Sagittarius A*, the Milky Way’s central black hole. The black hole captured its partner and flung S5-HVS1 outward at enormous speed.
Where is S5-HVS1 located?
S5-HVS1 is about 29,000 light-years from Earth in the southern constellation Grus (the Crane).
Is S5-HVS1 the fastest star ever found?
It is one of the fastest stars known and the fastest confirmed to have been ejected by the Milky Way’s central black hole. A few stars thrown out by exploding white dwarfs move even faster.