Milky Way star flashes at over 2.3 million km/h

A low-mass star, named CWISE J124909+362116.0 (J1249+36), is hurtling through the Milky Way at a staggering speed of over two million kilometers per hour. Discovered through the efforts of citizen scientists from the Backyard Worlds: Planet 9 project, the star may have been the victim of a supernova explosion or may have passed too close to a black hole. This investigation into the origin of J1249+36 could not only shed light on the mystery surrounding it, but also enrich our understanding of the extreme dynamics in our galaxy.

The star that fled the Milky Way

J1249+36 was first spotted in data collected by the Wide-field Infrared Survey Explorer (WISE) of the NASA. To uncover the secrets of this star, astronomer Adam Burgasser and his team from the University of California, San Diego, observed its infrared spectrum at the WM Keck Observatory in Maunakea, Hawaii (USA). They then discovered that J1249+36 belongs to the class of sub-dwarfs Lvery old and rare stars characterized by a low mass and relatively low temperatures. The analyses also revealed that the star was moving at a speed sufficient to potentially to one day free itself from the “gravitational clutches” of the Milky Way. However, the origin of this speed remains mysterious. Two main scenarios have been proposed to explain this phenomenon: an expulsion due to a supernova of a “vampire star” or an ejection caused by an interaction with black holes.

The Vampire Star and the Supernova Explosion

In the first scenario, J1249+36 would have been the stellar companion of a white dwarfa type of “dead” star that is reborn by sucking up material from a nearby star. This accumulation of material can push the white dwarf past the Chandrasekhar limit (1.4 times the mass of the Sun), causing a cataclysmic explosion called a type Ia supernova. The latter completely destroys the white dwarf, releasing its stellar companion at high orbital speed. Although this scenario is plausible, direct evidence is lacking, as the remains of the explosion probably dissipated millions of years ago.
An illustration shows a white dwarf star beginning to erupt as it feeds on a companion star. Credit: Robert Lea (created with Canva)

Interaction with black holes

The second scenario considers that J1249+36 began its life in a globular cluster which is a compact area containing thousands or even millions of stars. Stars concentrate toward the center of these clusters, where black holes of various masses may also reside. The complex interactions between a star and a binary black hole system can then expel it at a phenomenal speed, to the point of being able to propel it out of its original cluster, although this phenomenon is rare. simulations showed that this type of interaction could indeed place a star on a trajectory similar to that observed with J1249+36. Moreover, its trajectory points to a dense region of space, potentially the location of an unknown globular cluster.
A tightly packed globular cluster of stars that may contain a binary pair of black holes at its core. Credits: NASA, ESA, ESA/Hubble, Roger Cohen (UK) The team of researchers now plans to examine the elemental composition of J1249+36 to clarify its origin, which could reveal clues about its birth, whether from a white dwarf supernova or a globular cluster. Whatever its origin, the discovery of J1249+36 offers a unique opportunity to study so-called “hypervelocity” stars and the mechanisms that propel them through the galaxy.