Why is a strange metal star flying out of the Milky Way?

Why is a strange metal star flying out of the Milky Way?
Why is a strange metal star flying out of the Milky Way?

A star is moving from Earth to the edge of the Milky Way about 2,000 light years away. Known as LP 40–365, this star is one of a unique breed of fast-moving stars - the remnants of massive white dwarfs that were partially preserved from a giant stellar explosion.

"This star is moving so fast that it almost certainly leaves the galaxy … [it] moves at almost 2 million miles per hour," says Jay Jay Hermes, assistant professor of astronomy at the Boston University College of Arts and Sciences. But why is this flying object being carried away from the Milky Way? Because it is a splinter from a past supernova explosion that is still moving forward.

“To go through partial detonation and still survive is very cool and unique, and it was only in the last few years that we began to think that such a star could exist,” says Odelia Putterman, second author of the article.

In their article, Hermes and Putterman reveal new observations about the remaining "stellar shrapnel", which provides insight into other stars with a similar catastrophic past.

Patterman and Hermes analyzed data from NASA's Hubble Space Telescope and the Exoplanet Exploration Satellite (TESS), which explores the sky and collects information about light from nearby and distant stars. By studying different types of light data from both telescopes, the researchers and their collaborators found that LP 40 × 365 not only leaves the galaxy, but, judging by the structure of the brightness in the data, also rotates in the process.

“Basically, the star was fired from a slingshot, and we see it spinning out,” says Putterman.

“We dug a little deeper to find out why this star has repeatedly gotten brighter and dimmer, and the simplest explanation is that we see something on its surface spinning and disappearing from view every nine hours,” says Hermes. All stars rotate - even the Sun rotates slowly on its axis every 27 days. But for a fragment of a star that survived a supernova explosion, nine hours is considered relatively slow.

Supernovae occur when a white dwarf becomes too massive to support itself, eventually causing a cosmic explosion. Determining the speed of rotation of LP 40 × 365 after a supernova may provide clues to the original two-star system from which it originated. In the Universe, stars usually come together in close pairs, including white dwarfs, which are very dense stars that form towards the end of a star's life. If one white dwarf lends too much mass to another, the star it is dropped onto can self-destruct, leading to a supernova. Supernovae are common in the galaxy and can happen in a variety of ways, but they are usually very difficult to see, according to the researchers. Because of this, it is difficult to understand which star exploded and which star dropped too much mass onto its stellar partner.

Based on the relatively low rotational speed of LP 40–365, Hermes and Putterman are more confident that it is a shard of a star that self-destructed after being fed too much mass by a partner as they rotated around each other at high speed. Because the stars were spinning so fast and close, the explosion moved both stars and now we only see LP 40–365.

“This [article] adds another layer of knowledge about what role these stars played when the supernova happened,” and what can happen after the explosion,”says Putterman."By understanding what is happening to this particular star, we can begin to understand what is happening to many other similar stars that have arisen in a similar situation."

LP 40–365 stars are not only some of the fastest stars known to astronomers, but also the richest in metals ever discovered. Stars like the Sun are made of helium and hydrogen, but a supernova star is mostly made of metallic material because "what we see are byproducts of violent nuclear reactions that occur when a star explodes." - says Hermes, which makes this stellar shrapnel especially fascinating to study.

The article was published in Astrophysical Journal Letters.

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