A tale of two tails
In April 2002 a camera on board XMM-Newton took a stunning picture of the neutron star Geminga, which is 522 light-years away. (a) The star is travelling through the interstellar medium at 120 km s-1, which is faster than the local speed of sound. As a result, two distinct tails are produced, which warp the star's magnetic field and cause high-energy electrons to emit X-rays via synchrotron radiation. Each tail is about 3 x 1015 m long, which is 20,000 times greater than the distance between the Earth and the Sun. (b) Astronomers compared the X-ray data from XMM-Newton with 3D models of a supersonic "bow shock". They found the best fit was a bow shock at an inclination of less than 30°. Geminga's Mach number is estimated to be between 7 and 20.