Ballistic Stars

So much for Newton's "theory" of fixed-stars and gravity: Hubble Finds Stars That 'Go Ballistic'.

Even some stars go ballistic, racing through interstellar space like bullets and tearing through clouds of gas.

Images from NASA's Hubble Space Telescope reveal 14 young, runaway stars plowing through regions of dense interstellar gas, creating brilliant arrowhead structures and trailing tails of glowing gas. These arrowheads, or bow shocks, form when the stars' powerful stellar winds, streams of matter flowing from the stars, slam into surrounding dense gas. The phenomenon is similar to that seen when a speeding boat pushes through water on a lake.

"We think we have found a new class of bright, high-velocity stellar interlopers," says astronomer Raghvendra Sahai of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and leader of the Hubble study. "Finding these stars is a complete surprise because we were not looking for them. When I first saw the images, I said 'Wow. This is like a bullet speeding through the interstellar medium.' Hubble's sharp 'eye' reveals the structure and shape of these bow shocks."

The astronomers can only estimate the ages, masses, and velocities of these renegade stars. The stars appear to be young— just millions of years old. Their ages are based partly on their strong stellar winds.

Most stars produce powerful winds either when they are very young or very old. Only very massive stars greater than 10 times the Sun's mass have stellar winds throughout their lifetimes. But the objects observed by Hubble are not very massive, because they do not have glowing clouds of ionized gas around them. They are medium-sized stars that are a few to eight times more massive than the Sun. The stars are not old because the shapes of the nebulae around aging, dying stars are very different, and old stars are almost never found near dense interstellar clouds.

Depending on their distance from Earth, the bullet-nosed bow shocks could be 100 billion to a trillion miles wide (the equivalent of 17 to 170 solar system diameters, measured out to Neptune's orbit). The bow shocks indicate that the stars are traveling fast, more than 112,000 miles an hour (more than 180,000 kilometers an hour) with respect to the dense gas they are plowing through, which is roughly five times faster than typical young stars.

"The high-speed stars were likely kicked out of their homes, which were probably massive star clusters," Sahai says.

There are two possible ways this stellar expulsion could have happened. One way is if one star in a binary system exploded as a supernova and the partner got kicked out. Another scenario is a collision between two binary star systems or a binary system and a third star. One or more of these stars could have picked up energy from the interaction and escaped the cluster.

Assuming their youthful phase lasts only a million years and they are moving at roughly 112,000 miles an hour, the stars have traveled about 160 light-years.