Ultracold Stars Have Peculiar Orbits Around Milky Way

Ultracold subdwarfs are a newly discovered class of stars that was found revolving around the Milky Way, in orbits that are so peculiar they make almost no sense. These peculiar, faint stars are not very bright, and they may even come from other galaxies, moving across the void that usually separates galactic formations over millions of years. Speaking on June 9th at the meeting of the American Astronomical Society, experts shed some light on their origins, and on how they came to the Milky Way.

These ultracool stars, which are classified at the lower end of stellar magnitude, close to the celestial objects designated as brown dwarfs, are very dim, have an extremely faint light, and are also incredibly rare. Only a few dozens have been observed since 2003, when they were first discovered.

At the time, astronomers noticed that they only shone with an intensity about 10,000 times smaller than that of our Sun, and that they did not incorporate too many chemical elements other than helium and hydrogen, hence the subdwarf designation.

“Most nearby stars travel more or less in tandem with the Sun tracing circular orbits around the center of the Milky Way once every 250 million years,” the lead author of the new study, Massachusetts Institute of Technology Associate Professor of Physics Adam Burgasser, explains. He also underlines the fact that these stellar formations move at speeds of up to 500 kilometers/second, or over a million miles/hour.

“If there are interstellar cops out there, these stars would surely lose their driver's licenses,” he says. “Someone living on a planet around one of these subdwarfs would have an incredible nighttime view of a beautiful spiral galaxy – our Milky Way – spread across the sky.” In the Constellation Virgo, the study shows, experts identified a peculiar ultracold subdwarf that may have extragalactic origins. The star was designated with the catchy name 2MASS 1227‐0447.

“Our calculations show that this subdwarf travels up to 200,000 light years away from the center of the Galaxy, almost 10 times farther than the Sun,” John Bochanski, who is an MIT postdoctoral researcher in Burgasser's group, emphasizes. “Based on the size of its one billion‐year orbit and direction of motion, we speculate that 2MASS 1227‐0447 might have come from another, smaller galaxy that at some point got too close to the Milky Way and was ripped apart by gravitational forces.”

“If we can identify what stream this star is associated with, or which dwarf galaxy it came from, we could learn more about the types of stars that have built up the Milky Way's halo over the past 10 billion years,” Burgasser concludes, quoted by ScienceDaily.