Abell 31 (Sharp Catalogue 2 - 290) is a planetary nebula of nearly ten light-years wide, located about 2,000 light-years away from Earth in the constellation of Cancer. Although it is one of the largest planetary nebulae of the sky, Abell 31 is not very bright. It is a very ancient nebula and its gas has already started to disperse into space.
Its central star is a tiny white dwarf (about 4 times bigger than Earth or about 0.04 times the size of the Sun), which is incredibly hot, about 85,000° Celsius (150,000°F). It has about half the mass of the Sun, meaning it probably started out life as a star with about twice the mass of our Sun, and lost the rest as it aged and its stellar winds blew its outer envelope away. Judging from how fast those winds are blowing outward, the star probably started dying about 130,000 years ago, after a billion or more years of normal life.
The nebula is moving through space at a relatively high speed. The direction of motion is toward the southern part of the cloud. This southern portion is sharply defined by a wide bow shock where the nebular gas and dust is compressed by the interaction with the surrounding interstellar medium. Gas heading in the opposite direction is moving downwind, and remains relatively untouched, making the northern boundary of the nebula rather diffuse.
This image shows both red and blue portions to this nebula. The red gas is hydrogen imaged using a hydrogen alpha filter, which blocks all light spectra except for the 6nm range that it occurs. The blue is oxygen, imaged using an Oxygen III filter that blocks all spectra except for the 6nm range of Oxygen. The blue portion sits close to the progenitor star. The oxygen is probably located throughout the entire nebula, but only in the center is it close enough to the central star to get lit up and glow.