The stars are 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,” said Sahai. “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 as 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 180,000 kilometers an hour (more than 112,000 miles an hour) with respect to the dense gas they are plowing through, which is roughly five times faster than typical young stars. Assuming their youthful phase lasts only a million years and they are moving at roughly 180,000 kilometers an hour (about 112,000 mph), the stars have traveled about 160 light-years.
