In April 2023, astronomers announced the discovery of DESI-HVS1, a remarkable hypervelocity star that appears to have been ejected from the very heart of our Milky Way galaxy. Using data from the Dark Energy Spectroscopic Instrument (DESI) and the Gaia satellite, researchers identified this ancient, metal-poor star racing through space at extraordinary speeds. Below, we answer key questions about this stellar runaway and what it reveals about the galaxy's violent history.

What exactly is DESI-HVS1 and why does it matter?

DESI-HVS1 is a hypervelocity star—a star moving so fast that it will eventually escape the Milky Way's gravitational pull. What makes it special is its age and composition: it is old and metal-poor, meaning it formed early in the universe's history when heavy elements were scarce. Its trajectory points back to the galactic center, suggesting it was kicked out by interactions near the supermassive black hole, Sagittarius A*. This discovery matters because it provides direct evidence that the galactic center can eject ancient stars, offering clues about the environment and dynamics near the black hole billions of years ago.

How was DESI-HVS1 discovered?

The star was first identified by analyzing data from the Dark Energy Spectroscopic Instrument (DESI), which measures the velocities of millions of galaxies and stars. Astronomers spotted an object with an unusually high radial velocity—over 1,000 km/s relative to the Sun. They then cross-matched this with precise positions and motions from the Gaia satellite to confirm its trajectory. The discovery was published in a research paper on April 23, 2023, on the arXiv pre-print server. The combination of DESI's spectroscopic power and Gaia's astrometry allowed scientists to pinpoint DESI-HVS1 as a likely galactic center escapee.

What makes a star a hypervelocity star?

A hypervelocity star is defined by its speed: it must exceed the escape velocity of the Milky Way, which is roughly 600–800 km/s depending on location. DESI-HVS1 clocks in at over 1,000 km/s, meaning it is on a one-way trip out of the galaxy. These stars are rare—only a few dozen are known. They are thought to be accelerated by close encounters with the supermassive black hole at the galactic center, either through the Hills mechanism (where a binary star is torn apart, flinging one component away) or through interactions with a massive, dense cluster of stars.

What are the key characteristics of DESI-HVS1?

DESI-HVS1 is an old, metal-poor star, indicating it formed early in cosmic history, possibly when the universe was less than a few billion years old. Its low metallicity (few heavy elements) suggests it originated from a pristine cloud of gas. The star is likely a main-sequence or subgiant with a mass slightly less than that of the Sun. Its trajectory traces back to the galactic center with high probability, reinforcing the idea that it was ejected from the region around Sagittarius A*. The combination of age, low metallicity, and origin makes DESI-HVS1 a unique probe of the early Milky Way's central environment.

Which instruments were used to study DESI-HVS1?

Two key instruments were essential. First, the Dark Energy Spectroscopic Instrument (DESI), mounted on the Mayall Telescope at Kitt Peak National Observatory, provided high-precision spectroscopy to measure the star's radial velocity and chemical composition. DESI can observe thousands of objects simultaneously. Second, the Gaia satellite from the European Space Agency supplied astrometric data—precise positions, proper motions, and parallaxes—which allowed astronomers to trace the star's three-dimensional path through the galaxy. Together, DESI's spectroscopic data and Gaia's astrometry confirmed that DESI-HVS1 is moving fast enough to escape the Milky Way and that its origin point is the galactic center.

What does this discovery tell us about the Milky Way's past?

The existence of an old, metal-poor hypervelocity star like DESI-HVS1 suggests that the galactic center has been capable of ejecting stars for billions of years. This implies that the supermassive black hole or the dense star cluster at the center has been active since early cosmic times. The star's low metallicity hints that the material near the galactic center was already chemically enriched only to a small extent, consistent with models of rapid early star formation and feedback. Moreover, DESI-HVS1 is a direct tracer of the gravitational slingshot processes that operate near black holes. By studying such stars, astronomers can better understand how the Milky Way's nucleus evolved and how stars get flung out into the galaxy's halo.

For more details, see the original research paper on arXiv (23 April 2023).