Stars are gravitationally fastened to their galaxies and move in concert with their surroundings. But sometimes, something breaks the bond. If a star gets too close to a supermassive black hole, for example, the black hole can expel it out into space as a rogue star.
What would happen to Earth if one of these stellar interlopers got too close?
A rogue star is one that has escaped the gravitational pull of its home galaxy. These stars drift through intergalactic space, and so are sometimes called intergalactic stars. Sometimes, when a rogue star is ejected from its galaxy, it drags its binary pair along for the ride.
Hypervelocity stars have been observed traversing the Galaxy at extreme velocities (700 km/s), but the mechanisms that give rise to such phenomena are still debated. Astronomer Thomas M. Tauris argues that lopsided supernova explosions can eject lower-mass Solar stars from the Galaxy at speeds up to 1280 km/s. “[This mechanism] can account for the majority (if not all) of the detected G/K-dwarf hypervelocity candidates,” he said.
Several mechanisms have been proposed as the source for hypervelocity stars, and the hypotheses can vary as a function of stellar type. A simplified summary of the hypothesis Tauris favors begins with a higher-mass star in a tight binary system, which finally undergoes a core-collapse supernova explosion. The close proximity of the stars in the system partly ensures that the orbital velocities are exceedingly large. The binary system is disrupted by the supernova explosion, which is lopsided (asymmetric) and imparts a significant kick to the emerging neutron star. The remnants of supernovae with massive progenitors are neutron stars or potentially a more exotic object (i.e., black hole).
Conversely, Tauris noted that the aforementioned binary origin cannot easily explain the observed velocities of all higher-mass hypervelocity stars, namely the B-stars, which are often linked to an ejection mechanism from a binary interaction with the supermassive black hole at the Milky Way’s center. Others have proposed that interactions between multiple stars near the centers of star clusters can give rise to certain hypervelocity candidates.
There are several potential compact objects (neutron stars) which feature extreme velocities, such as B2011+38, B2224+65, IGR J11014-6103, and B1508+55, with the latter possibly exhibiting a velocity of 1100 km/s. However, Tauris ends by noting that, “a firm identification of a hypervelocity star being ejected from a binary via a supernova is still missing, although a candidate exists (HD 271791) that’s being debated.”
It’s not quite like being kicked off a reality TV show, but some stars can get kicked out from their home galaxy. These stars – called various names like rogue, runaway or hypervelocity stars – were predicted to exist for quite some time, and finally just in the past couple of years a few of them have actually been discovered. But now a group of nearly 700 rogue stars have been found on the outskirts of the Milky Way. The astronomers who found them argue they are hypervelocity stars that have been ejected from the center of the galaxy.
“These stars really stand out. They are red giant stars with high metallicity which gives them an unusual color,” said Kelly Holley-Bockelmann from Vanderbilt University, who conducted the study with graduate student Lauren Palladino.
“We figured that these rogue stars must be there, outside the galaxy, but no one had ever looked for them. So we decided to give it a try,” said Holley-Bockelmann, who is studying the behavior of the black hole at the center of the Milky Way galaxy.
Runaway stars are kicked into motion either through a supernova explosion of a companion star, through gravitational interactions with other stars in a cluster, or through encounters with a black hole. One scenario could involve a binary pair of stars that get caught in the black hole’s grip, and as one of the stars spirals in towards the black hole, its companion is flung outward at a tremendous velocity.
This is the scenario the researchers focused on, and so they looked for red giant stars just outside the Milky Way.
Red giant stars are the end stage in the evolution of small, yellow stars like the Sun. So, the stars in this new red giant rogues category should have been small stars like the Sun when they got ejected. As they traveled outward, they continued to age until they reached the red giant stage. Even traveling at hypervelocities, it would take a star about 10 million years to travel from the central hub to the spiral’s edge, 50,000 light years away.
“Studying these rogue stars can provide us with new insights into the history and evolution of our home galaxy,” said Holley-Bockelmann
There are a few methods for discovering runaway rogue stars. The first is to examine stars individually and analyze their motion in the plane of the sky (proper motion) along with their motion towards or away from us (radial velocity) to determine if a given star has sufficient velocity to escape. The second is to look at the effects some stars have on the local environment. Since young clusters contain large amounts of gas and dust, stars plowing through it will create bow shocks, similar to those a boat makes in the ocean.
The team from Vanderbilt selected these stars based on their location in intergalactic space between the Milky Way and the nearby Andromeda galaxy and by their peculiar red coloration.
The researchers’ next step is to determine if any of their candidates are unusually red brown dwarfs instead of red giants. Because brown dwarfs produce a lot less light than red giants, they would have to be much closer to appear equally bright.