PSR J1719-1438 is probably one of the most fantastical cosmic objects that you'll ever encounter. First, 1438 is a neutron star. A neutron star is a stellar object that is amazingly massive and amazingly small. Although pulsars are at least 1.4 times the mass of our sun, they are generally only about 20km [12mi] across. Essentially, they are stars that are the size of small cities, and a large mass + a small size = extreme density. Since neutron stars are so dense, on Earth, one teaspoon of this stellar material would weigh about a billion tons.
Second, 1438 is a pulsar. Simply put, pulsars are neutron stars that emit beams of radiation that sweep through Earth's line of sight. This occurs because neutron stars have intense magnetic fields (they are about a trillion times stronger than Earth's own field). However, the axis of the magnetic field is not aligned with the neutron star's rotational axis. As a result, the star wobbles, and the pulsar's beam sweep across our gaze at a regular interval, allowing us to determine just how fast the object is spinning. As it turns out, pulsars spin amazingly fast (most pulsars have rotational periods of mere seconds).
Third, 1438 is a millisecond pulsar. As noted, pulsars have an amazingly fast spin--but a millisecond pulsar makes an ordinary pulsar look like it's not even moving. The fastest ones pulse at a mind-boggling 1.6 milliseconds, meaning that the star is rotating 640 times each second. However, 1438 isn't quite that fast; it has a spin period of 5.4 ms (that's still about 10,000 rotations a minute.).
The fourth and (quite possibly) most impressive feature of 1438 is its companion: PSR J1719-1438b. 1438b is a planet that has a mass that is nearly the same as Jupiter's, but it is only 40% the size of the planet. How did it get to be so small and so massive? Scientists believe that 1438b is the remnant of a star whose outer layers were siphoned off by the more massive pulsar. Although the outer-layers of the planet were stripped away by the pulsar, they left behind a remnant that is composed primarily of carbon. Ultimately, this means that 1438b is an alien planet that is, for all intents and purposes, a diamond... a diamond that is about 5 times larger than the Earth. Oddly enough, this also means that this planet is larger than its sun. With a diameter of about 37,300 miles (60,000 km), 1438b is 3,000 times larger than the millisecond pulsar it orbits. What's more, the planet orbits the pulsar at a very close distance. So close, in fact, that the entire system would fit within the diameter of our sun.
To recap: 1438b was once a Star. 1438 transformed it into a planet. During this process, 1438 also transformed 1438b into a massive diamond. I'm not sure about you, but I think that's a pretty impressive accomplishment.