Hubble Spots "Runaway Star" Kicked Out Of A System 540 Years Ago

The three stars and the newly discovered 'source x'. NASA, ESA, K. Luhman (Penn State University), and M. Robberto (STScI)

NASA’s Hubble telescope has been used to track down a missing “runaway star” in the Orion Nebula. The research was published in The Astrophysical Journal Letters.

The nebula, 1,300 light-years from Earth, is thick with dust and gas. But Hubble can peer inside using its Near Infrared Camera and Multi-Object Spectrometer (NICMOS), allowing it to see hidden stars and other objects.

Two stars in a region known as the Kleinmann-Low Nebula had been observed previously, and were believed to have been ejected from a multiple star system. And now Hubble has found a third star dubbed “source x” from that system, which helps explain how the stars got enough energy to escape in the breakup 540 years ago.

"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 a statement.

This third runaway star was found to be moving at about 210,000 kilometers (130,000 miles) per hour. Together these three stars are one of several broken apart multiple-star systems we know of. A multiple star system is one where several stars are orbiting each other, the closest to Earth being Alpha Centauri, which also has three stars.

An example of how star systems can break apart. NASA, ESA, and Z. Levy (STScI)

Source x was spotted by watching how its position had changed relative to other stars over 17 years. By projecting the movement of the star back in time, Luhman realized it must have originated in the same place as the other two stars, known as Becklin-Neugebauer (BN) and “source l”. They are traveling at 97,000 and 54,000 kilometers (60,000 and 22,000 miles) per hour, respectively.

These stars appear to be the youngest ejected stars we’ve ever seen, perhaps only a few hundred thousand years old. They may even be young enough to still have disks of material around them from their formation. In a star’s early life, it is surrounded by a disk of dust and gas that later can form planets and other objects.

In such a system, two of the stars can move close enough to orbit extremely close or form a binary, releasing enough gravitational energy to propel other stars outwards. This can be accompanied by outflows of material, seen by Hubble as “fingers of matter” streaming away from the source l star.

It’s thought that this system may have once had more stars too, although for now we can only see three. This missing third star helps explain how there was enough energy in the system for the stars to escape, as they engaged in a “gravitational tug-of-war”. Or, yes, a star war.

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