Mystery of fastest white dwarfs solved?

September 7, 2025

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Artist’s concept of a white dwarf remnant being given a high-speed shove from its system. Such an event can happen after 2 white dwarfs merge, causing a supernova explosion. A new study suggests this is how the fastest white dwarfs in the Milky Way achieve their incredible speeds. Image via Technion Israel Institute of Technology.

White dwarfs are the remaining small, dense cores of dead stars. Some white dwarfs move through the Milky Way at incredibly high speeds.
A new study reveals their origins. The study suggests that, when two rare types of white dwarfs collide, the resulting supernova explosion slingshots a remnant of the least dense white dwarf out into deep space.
The white dwarfs’ unusual temperatures and brightness can also be explained in this scenario.

Speedy white dwarfs

All of our Milky Way’s stars orbit our galaxy’s center. And some stars move really fast! On August 31, 2025, astronomers in Israel and Germany said they’ve figured out why some of the fastest stars in our galaxy are so speedy. Led by Hila Glanz of the Technion Israel Institute of Technology, the researchers clocked hypervelocity white dwarfs traveling at more than 1,240 miles per second (44,640,002 mph or 2,000 km/s). And they said they can explain this speed with a scenario in which two rare types of white dwarfs merge. When that happens, the heavier one explodes in a supernova and slings the remnant of its less massive companion out into deep space.

It seems this slingshot effect is powerful enough to send the lesser white dwarf outward, at speeds fast enough to escape the Milky Way entirely!

White dwarfs are the remaining cores of dead stars. They come into being after a star runs out of its internal fuel supply. Without this internal fuel, the star first expands into a red giant, many times its original size. Stellar winds then strip away the gases and the red giant evaporates, leaving behind only the white dwarf.

The researchers published their peer-reviewed results in Nature Astronomy on August 19, 2025.

The fastest white dwarfs in the galaxy

The researchers call these white dwarfs hypervelocity white dwarfs. They are some of the fastest stars astronomers have ever seen. In fact, they can travel at more than 1,240 miles per second (44,640,002 mph or 2,000 km/s).

Indeed, that’s speedy enough that the white dwarf could actually escape the Milky Way and end up in intergalactic space.

So, how does this happen?

Hypervelocity white dwarfs, traveling over 2,000 km/s, are now linked to supernova explosions following mergers of rare hybrid white dwarfs, offering insight into stellar runaways and faint supernovae. doi.org/g9z9sj

— Science X / Phys.org (@sciencex.bsky.social) 2025-09-02T14:28:19-04:00

White dwarf supernovas act as slingshots

The scenario involves the collision and merger of two rare types of white dwarfs. The research team used state-of-the-art 3-dimensional hydrodynamic simulations of a merger between two rare hybrid helium–carbon–oxygen white dwarfs.

One of the white dwarfs is heavier and more massive than the other. The heavier white dwarf explodes in a supernova not once but twice. This is strong enough to slingshot the remnants of the less massive white dwarf out into deeper space. Glanz said:

This is the first time we’ve seen a clean pathway where the remnants of a white dwarf merger can be launched at hypervelocity, with properties matching the hot, faint white dwarfs we observe in the halo. This solves the mystery about the origin of these stellar runaways, and also opens up a new channel for faint and peculiar Type Ia supernovae.

Smiling woman with long dark hair wearing a black blouse. — Hila Glanz at Technion Israel Institute of Technology is the lead author of the new study about hypervelocity white dwarf stars. Image via Technion Israel Institute of Technology.

Closeup of 2 bright white sunlike stars, attached together by a streamer of glowing gas. — Artist’s illustration of 2 nearby white dwarf stars that are on course to collide. The collision would create a supernova explosion in about 23 billion years. Image via Mark Garlick/ University of Warwick.

Solving mysteries of hypervelocity white dwarfs

The findings help solve some mysteries about hypervelocity white dwarfs. For example, it unlocks questions about their extreme speeds and unusual temperatures and brightness. In addition, the findings also provide clues about a phenomenon called underluminous thermonuclear explosions. Astronomers use them to measure how fast the universe is expanding and determine how elements form inside galaxies. Co-author Hagai Perets, also at Technion Israel Institute of Technology, said:

This discovery doesn’t just help us understand hypervelocity stars; it gives us a window into new kinds of stellar explosions.

In addition, the researchers also say that upcoming transient surveys and Gaia data releases might find more of these hypervelocity white dwarfs as they exit our Milky Way. Could there be some that are even faster?

Bottom line: Astronomers have identified why the fastest white dwarfs in the Milky Way are so speedy. They are ejected by supernova explosions after two white dwarfs collide.

Source: The origin of hypervelocity white dwarfs in the merger disruption of He–C–O white dwarfs

Via Technion Israel Institute of Technology

Paul Scott Anderson

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About the Author:

Paul Scott Anderson has had a passion for space exploration that began when he was a child when he watched Carl Sagan’s Cosmos. He studied English, writing, art and computer/publication design in high school and college. He later started his blog The Meridiani Journal in 2005, which was later renamed Planetaria. He also later started the blog Fermi Paradoxica, about the search for life elsewhere in the universe.

While interested in all aspects of space exploration, his primary passion is planetary science and SETI. In 2011, he started writing about space on a freelance basis with Universe Today. He has also written for SpaceFlight Insider and AmericaSpace and has also been published in The Mars Quarterly. He also did some supplementary writing for the iOS app Exoplanet.

He has been writing for EarthSky since 2018, and also assists with proofing and social media.

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