Discovery of runaway star yields clues to breakup of multiple-star system

As British royal families fought the War of the Roses in the 1400s for control of England's throne, a grouping of stars was waging its own contentious skirmish — a star war far away in the Orion Nebula.

The stars were battling each other in a gravitational tussle, which ended with the system breaking apart and at least three stars being ejected in different directions. The speedy, wayward stars went unnoticed for hundreds of years until, over the past few decades, two of them were spotted in infrared and radio observations, which could penetrate the thick dust in the Orion Nebula.

This dramatic view of the center of the Orion Nebula reveals the home of three speedy, wayward stars that were members of a now-defunct multiple-star system. The stellar grouping broke apart 500 years ago, flinging the three stars out of their birthplace.

The image, taken by NASA's Hubble Space Telescope, combines observations taken in visible light with the Advanced Camera for Surveys and in near-infrared light with the Wide Field Camera 3. A grouping of hefty, young stars, called the Trapezium Cluster, is at the center of the image. Several hundred stars are sprinkled throughout the image. Many of them appear red because their light is being scattered by dust.

The box just above the Trapezium Cluster outlines the location of the three stars. A Hubble close-up view of the stars is shown at top right. The birthplace of the multi-star system is marked "initial position." Two of the stars — labeled BN, for Becklin-Neugebauer, and "I," for source I — were discovered decades ago. Source I is embedded in thick dust and cannot be seen. The third star, "x," for source x, was recently discovered to have moved noticeably between 1998 and 2015, as shown in the inset image at bottom right. Source x is traveling at an unusually high speed of 209 200 km/h (130 000 mph), which is 30 times faster than the velocity of most stars in the nebula. 

Astronomers found the speedy source x by comparing observations taken in 1998 by the Near Infrared Camera and Multi-Object Spectrometer with those taken in 2015 by the Wide Field Camera 3. Hubble's discovery of the high velocity of source x has helped astronomers solve the long-standing mystery of how the stars BN and source I acquired their fast motions. Credit: NASA, ESA, K. Luhman (Penn State University), and M. Robberto (STScI)

The observations showed that the two stars were traveling at high speeds in opposite directions from each other. The stars' origin, however, was a mystery. Astronomers traced both stars back 540 years to the same location and suggested they were part of a now-defunct multiple-star system. But the duo's combined energy, which is propelling them outward, didn't add up. The researchers reasoned there must be at least one other culprit that robbed energy from the stellar toss-up.

Now NASA's Hubble Space Telescope has helped astronomers find the final piece of the puzzle by nabbing a third runaway star. The astronomers followed the path of the newly found star back to the same location where the two previously known stars were located 540 years ago. The trio resides in a small region of young stars called the Kleinmann-Low Nebula, near the center of the vast Orion Nebula complex, located 1 300 light-years away.

"The new Hubble observations provide very strong evidence that the three stars were ejected from a multiple-star system," said lead researcher Kevin Luhman of Penn State University in University Park, Pennsylvania. "Astronomers had previously found a few other examples of fast-moving stars that trace back to multiple-star systems, and therefore were likely ejected. But these three stars are the youngest examples of such ejected stars. They're probably only a few hundred thousand years old. In fact, based on infrared images, the stars are still young enough to have disks of material leftover from their formation."

This three-frame illustration shows how a grouping of stars can break apart, flinging the members into space.

The first panel shows four members of a multiple-star system orbiting each other. In the second panel, two of the stars move closer together in their orbits. In the third panel, the closely orbiting stars eventually either merge or form a tight binary. This event releases enough gravitational energy to propel all of the stars in the system outward, as shown in the third panel. Credit: NASA, ESA, and Z. Levy (STScI)

All three stars are moving extremely fast on their way out of the Kleinmann-Low Nebula, up to almost 30 times the speed of most of the nebula's stellar inhabitants. Based on computer simulations, astronomers predicted that these gravitational tugs-of-war should occur in young clusters, where newborn stars are crowded together. "But we haven't observed many examples, especially in very young clusters," Luhman said. "The Orion Nebula could be surrounded by additional fledging stars that were ejected from it in the past and are now streaming away into space."

This video reveals the motion of a newly discovered runaway star in the Orion Nebula.

The images in the two frames were taken 17 years apart by NASA's Hubble Space Telescope. The first image was taken in 1998 by Hubble's Near Infrared Camera and Multi-Object Spectrometer; the second, in 2015 by the Wide Field Camera 3. The bright object at the bottom right of both frames is a foreground star.

The speedy star, called "source x," is moving at roughly 209 200 km/h (130 000 mph). The star was a member of a multiple-star system that was propelled from its birthplace 500 years ago. The system members engaged in a gravitational tussle that resulted in the breakup of the grouping. Source x and at least two other stars were ejected in different directions.

The team's results will appear in the March 20, 2017 issue of The Astrophysical Journal Letters.

Luhman stumbled across the third speedy star, called "source x," while he was hunting for free-floating planets in the Orion Nebula as a member of an international team led by Massimo Robberto of the Space Telescope Science Institute in Baltimore, Maryland. The team used the near-infrared vision of Hubble's Wide Field Camera 3 to conduct the survey. During the analysis, Luhman was comparing the new infrared images taken in 2015 with infrared observations taken in 1998 by the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). He noticed that source x had changed its position considerably, relative to nearby stars over the 17 years between Hubble images, indicating the star was moving fast, about 209 200 km/h (130 000 mph).

The astronomer then looked at the star's previous locations, projecting its path back in time. He realized that in the 1470s source x had been near the same initial location in the Kleinmann-Low Nebula as two other runaway stars, Becklin-Neugebauer (BN) and "source I."

This video starts with a ground-based image of the night sky, taken by Akira Fujii, zooms on the star formation region of the Orion Nebula — observed by Martin Kornmesser — and ends with a detailed view of the nebula as seen by Hubble. Video courtesy HubbleESA

BN was discovered in infrared images in 1967, but its rapid motion wasn't detected until 1995, when radio observations measured the star's speed at 60,000 miles per hour. Source I is traveling roughly 35 405 km/h (22 000 mph). The star had only been detected in radio observations; because it is so heavily enshrouded in dust, its visible and infrared light is largely blocked.

The three stars were most likely kicked out of their home when they engaged in a game of gravitational billiards, Luhman said. What often happens when a multiple system falls apart is that two of the member stars move close enough to each other that they merge or form a very tight binary. In either case, the event releases enough gravitational energy to propel all of the stars in the system outward. The energetic episode also produces a massive outflow of material, which is seen in the NICMOS images as fingers of matter streaming away from the location of the embedded source I star.

Scale and compass image for Orion Nebula. Credit: NASA, ESA, and STScI

Future telescopes, such as the James Webb Space Telescope, will be able to observe a large swath of the Orion Nebula. By comparing images of the nebula taken by the Webb telescope with those made by Hubble years earlier, astronomers hope to identify more runaway stars from other multiple-star systems that broke apart.

Source: Hubble

Featured image: Orion Nebula. Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team