- Date: December 2016
- Location: Mondsee, Austria
- Camera: Moravian 16200
- Optics: Sky-Watcher Esprit 100/550
- Mount: Sky-Watcher EQ8
- Filters: Astronomik Deepsky HaOIIILRGB
- Exposure: Ha:20x900s + OIII 15x1200s + RGB 8x600s each
PuWe1 (Purgathofer-Weinberger 1, PNG 158.9 + 17.8) is a large, circular planetary nebula in the constellation of Lynx that has an apparent diameter of 20′. It is incredibly faint and to capture it requires many hours of exposure with high contrast narrowband filters.
Planetary nebula are expanding shells of luminous gas expelled by dying stars. They have a relatively round compact appearance rather than the chaotic patchy shapes of other nebulae—hence their name, which was given because of their resemblance to planetary disks when viewed with the instruments of the late 1700s, when the first planetary nebulae were discovered.
There are believed to be about 20,000 planetary nebulae in the Milky Way Galaxy, each representing gas expelled relatively recently from a central star very late in its evolution. Because of the obscuration of dust in the Galaxy, only about 3,600 planetary nebulae have been cataloged. Planetary nebulae are important sources of the gas in the interstellar medium.
Most planetaries show a central star, called the nucleus, which provides the ultraviolet radiation required for ionizing the gas in the ring or shell surrounding it. Those stars are among the hottest known and are in a state of comparatively rapid evolution.
The spectra of planetary nebulae reveal an interesting fact: they are expanding from the central star at 24–56 km (15–35 miles) per second. The gravitational pull of the star is quite small at the distance of the shell from the star, so the shell will continue its expansion until it finally merges with the interstellar gas around it.
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