Abell 39 - Planetary Nebula
The planetary nebula Abell 39 is located 6,800 light-years away from us in the constellation Hercules. It also lies about 4,600 light-years
above the galactic plane, which is why it is not attenuated or reddened by some dust in front of it.
The PN has a spherical geometry, i.e. it is spherical in shape. It is even assumed that Abell 39 is the largest known sphere with a diameter
of 1.3 light years. In the sky it appears at an angle of 155 arcseconds, while the very faint halo is even nearly 200" in diameter. Photographs
show a bright outer rim 10" wide, which at this distance corresponds to a thickness of 0.34 light-years. Although the nebula itself is very
symmetrical, the eastern side still appears 50% brighter than the western side. The nebular envelope has been expanding at a rate of 35
kilometers per second for about 22,000 years and has a temperature of 15,500 Kelvin.
The central star is visually only 15.8 magnitudes bright and with an effective temperature of 160,000 Kelvin it is one of the hottest white
dwarfs known. Its mass is estimated to be 0.58 solar masses and its luminosity ~1,800 suns. The central star is not exactly in the center
of Abell 39, but is shifted 2" to the west. The reason for this could be that the ejection of the nebular envelope was asymmetric and therefore
the star experienced an acceleration.
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In my 20" telescope Abell 39 is best observed at low magnification because of its apparent size. Here it was 120x in combination with a suitable
nebula filter. Despite the large telescope aperture, the PN appears just as a pale, round area with a very low surface brightness. So Abell 39
gets easily lost in the sky background at higher magnifications. The center appeared slightly darker, but unfortunately nothing could be
seen of the narrow outer ring. So basically Abell 39 appears completely featureless.
The search for the central star was done at a magnification of 270x. At the searched position an extremely faint star flashed again and again
for very short moments. But the observation was a bit uncertain.
