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Decoding the 2017 space catastrophe

Unique characteristics define the white dwarf star named LP 40-365, leaving astronomers astonished.

The white dwarf that zoomed through the Milky Way may have survived a supernova explosion.

In 2017, astronomers observed a star outside the Milky Way galaxy traveling at speeds of nearly 3.2 million kilometers per hour, four times faster than the orbit of our Sun, and moving opposite to the direction of most stars around the galactic center.

What sets this star apart is its composition, mainly consisting of heavy “metal” elements, as opposed to the lighter elements typically found in stars. This peculiar star is named LP 40-365.

JJ Hermes, the lead author of the study and an astronomer at Boston University, explained: “It’s peculiar in a few ways.”



Did a star fragment explode?

The star’s rapid motion, departing from our galaxy, led astronomers to believe it was propelled here by a cosmic disaster, possibly the explosion of a super supernova. However, the exact mechanism behind this star’s acceleration remains unknown. Is LP 40-365 a fragment of the star that exploded, or is it a companion star blown away by shockwaves associated with starbursts?

A recent analysis of the data reveals that this star, known as a white dwarf, rotates with a “leisurely” pace, suggesting it is a fragment from a white dwarf star that attempted to survive in one of the most violent and mysterious galactic events.

Astronomer Hermes noted: “We can now connect this star with a piece of a white dwarf that exploded with much more confidence.”



Witnessing a cosmic catastrophe

Shortly after its discovery, astronomers realized that the unique characteristics of the white dwarf star meant it had likely witnessed a super supernova. This event occurred rapidly, and its entire structural makeup seemed to have changed.

Most stars are created almost entirely from hydrogen and helium. However, LP 40-365 only shows heavy elements like oxygen, neon, and magnesium, atoms formed in a supernova furnace. A super supernova has transferred all these “metals” to it. (Unlike chemists, astronomers consider any element heavier than helium as a metal.)

Astronomers concluded that this star was once part of a pair of white dwarf stars, a typical fate for small stars like our Sun, and exploded after drawing enough mass from its companion. These explosions are powerful enough to synthesize new elements in their nuclear cores.



Two scenarios may have occurred. In the first scenario, a star exploded cleanly, propelling its partner, along with the heavy metals it had created, into space. In the second scenario, some fragments from the exploding star escaped the super supernova, loaded with newly formed metals.

Understanding the white dwarf star

To better comprehend this peculiar star, Hermes and his colleagues sifted through data collected by NASA’s Transiting Exoplanet Survey Satellite (TESS), searching for distant planets passing in front and dimming their host stars.

While LP 40-365 has no planets, it flickers and brightens every 8.9 hours, as revealed by TESS data. The research team identified the same heartbeat in the data obtained from the Hubble Space Telescope, which they believe indicates the star’s rotation, obscuring and revealing dark spots in its view. The heartbeat might also suggest pulsations, but stars’ pulsations tend to be irregular.



Knowing how rapidly LP 40-365 rotates is highly beneficial because it helps researchers understand what the white dwarf star may have gone through approximately five million years ago during the super supernova phase.

The team initially estimated the pair of stars revolved around each other every 30 to 60 minutes. If LP 40-365 were a companion star ejected clearly, it would still rotate quite rapidly.

However, its relatively relaxed rotation suggests that it underwent a more significant transformation. When a white dwarf star explodes, it puffs up, slowing down its rotation as it expands. The extended rotation period of more than a week indicates that the white dwarf star is a piece of a star that has exploded.