The Milky Way seems like a nice galaxy. Why would anyone – on Earth or any other planet — want to leave the Milky Way permanently? That’s just one of the many questions to ponder as Chinese astronomers announced the discovery of 591 high-velocity stars and 43 of them appear to be heading out for parts unknown. Was it something we said in one of those SETI messages?
“Though rare in the Milky Way, high-velocity stars, with unique kinematics, can provide deep insight into a wide range of Galactic science, from the central supermassive black hole to distant Galactic halo.”
In a press release announcing the paper on this discovery published in The Astrophysical Journal Supplement Series, co-author Luo Youjun from the National Astronomical Observatories of Chinese Academy of Sciences (NAOC) explains the rarity of high-velocity stars – so-called because they’re moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun’s neighborhood. First discovered in 2005, only 550 had been discovered in total until this new batch more than doubled it. Credit goes to data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) in China and the European Space Agency’s Gaia space observatory.
“From the kinematics and chemistries, we found that the 591 high-velocity stars were inner halo stars.”
NAOC Prof. Luo Ali explains that these stars are in the inner portion of the galactic halo — the spherical outer edge beyond the visible part of a galaxy. There are four types: hypervelocity stars, runaway stars, hyper-runaway stars, and fast halo stars. Of this group, around 80 are metal-rich halo stars and at least 43 are runaways with escape probabilities larger than 50%. They can be ‘running’ away for one of three reasons (or a combination of them): a collision between stars or galaxies; a supernova explosion in a multistar system; or gravitational interactions between stars in a multistar system. Since the Milky Way’s halo is thought to contain the remnants of galactic collisions, that might be the primary cause of these particular runaways.
“The discovery of these high-velocity stars tells us that the combination of multiple large surveys in the future will help us to discover more high-velocity stars and other rare stars, which will be used to study the unsolved mystery about our Galaxy.”
Senior author Professor Gang Zhao from NAOC thinks the high-velocity runaways are key to solving the mystery of the formation of the Milky Way, especially how the halo was formed by galactic accretion (absorbing other galaxies) and tidal disruption of dwarf galaxies (when its stars approach the supermassive black hole in the center and are pulled apart).
Are those sufficient reasons for a star to run away from its galaxy? Astronomers are so unromantic.