They are stellar interlopers: Stars that instead of rotating quietly around a galaxy, rip their way across interstellar gas creating bow shocks, like giant proton torpedoes. These are a new kind, according to the JPL:
We think we have found a new class of bright, high-velocity stellar interlopers. 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.
So colour NASA's Jet Propulsion Laboratory astronomer Raghvendra Sahai suprised.
With their tails and arrowhead shapes the may look like crazy comets, but these young runaway stars are that, stars moving quickly through space, with their powerful stellar winds pushing the dense gas around them: They travel at 180,000kph, five times as fast as typical young stars. According to the JPL, they are like speedboats through a lake. The only difference is that the waves they generate are 100 billion to a trillion miles wide (17 to 170 solar systems measured up to Neptune).
Hopefully, none of these will decide to pay a visit anytime soon. [Hubble Site]