Wednesday, December 16, 2020

HFG1 planetary nebula

HFG1 is a faint, very old planetary nebula in the constellation of Giraffe with a diameter of more than one light year.

​HFG1 was created by the central star V664 Cas. This is not a single star, but a dense binary star system consisting of a white dwarf and a sun-like star, which are only a few million kilometres apart and orbit every 14 hours.

Because the binary system V664 Cas moves very fast at 29 to 59 kilometres per second and ploughs through the interstellar medium together with the nebula, a bluish arc shock occurs. HFG1 leaves behind a long, red trace of approx. 10,000 year old gas.

The image of HFG1 clearly shows the interesting structure of the planar nebula. HFG1 is defined as a type F planetary nebula, which means that it appears to be evenly filled. The narrow, bluish arch shock surrounds a central region with a gap in between. The arch shock is brightest in the south, indicating that it interacts most strongly with the interstellar medium there. The arch shock is not complete, it is almost invisible in the northwest. The core has three bright lobes to the south and a central opening.

Planetary nebulae have a life span of only about 10,000 years, considering that sun-like stars live to be around 10 billion years old. HFG1 is therefore already very old and will gradually dissolve in space, while the white dwarf will cool down and fade for billions of years. Our own sun is expected to suffer a similar fate, but that will not happen for another 5 billion years.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae originated in the 18th century due to the visual similarity between some circular planetary nebulae and the planets Uranus and Neptune when viewed through small optical telescopes. The name has remained, although modern telescopes make it clear that these objects are not planets.

Planetary nebulae represent the last short phase in the life of a medium star like our sun. The dying star (which collapses from a red giant to a white dwarf) consumes the last part of the fuel in its core and drives a large part of its outer shell into space. This material is then heated and radiated by the radiation of the rest of the stars, resulting in glowing gas clouds that can have complex structures, since the emission of the mass from the star is uneven depending on time and direction.

Image by Marcel Drechsler

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