Abell 6 & HFG1   (Heckathorn-Fesen-Gull)

Abell 6 is an example of a bubble-shaped planetary nebula. However, it remains quite faint (Mag = 15). It emits more in OIII than in Ha.

The larger structure in the picture, HFG1 (also known as PK 136+05) is a faint, very old planetary nebula of a light-year or more across, located in the northern constellation of Cassiopeia . It was discovered in 1982 by Heckathorn, Fesen and Gull.

Planetary nebulae represent the final brief stage in the life of a medium-sized star like our Sun. While consuming the last of the fuel in its core, the dying star (collapsing from a red giant to a white dwarf) expels a large portion of its outer envelope. This material then becomes heated by the radiation from the stellar remnant and radiates, producing glowing clouds of gas that can show complex structures.  Despite their name, planetary nebulae have nothing to do with planets but were named years ago because of how they appeared in telescopes of that time.

HFG1 was produced by the 14.5 magnitude central star V664 Cas. This is not a single star but a close binary star system of a white dwarf and a Sun-like star, which are only a few million kilometers apart and are orbiting each other every 14 hours. This binary system is moving rapidly through our Milky Way galaxy.

As HFG1 plows through the interstellar medium, a bluish bowshock is produced. A long, red trail of about 10,000 year old gas is left behind by V664 Cas at anywhere between 29 and 59 kilometers per second, depending on its distance from the Sun.

Planetary nebulae last for only about 10,000 years, a very short period in the 10-billion-year lifespan of Sun-like stars. So, HFG1 is already very old, and will gradually disperse into space, while the white dwarf will cool and fade away for billions of years. Our own Sun is expected to undergo a similar fate, but this will not occur until some 5 billion years from now.     (various on-line publications)