NGC 40, Planetary Nebula
The planetary nebula (PN) with the designation "NGC 40" is located in the constellation Cepheus and is about 3,500 light years away from us, although more recent
sources give the distance as 6,900 light years. Due to its high apparent brightness of 12mag, the PN was already discovered in 1788 by Friedrich Wilhelm Herschel.
The nebula has an extension of about one light year and expands every second by another 29 kilometres into the surrounding space. The temperature of the envelope
is currently 10,000 Kelvin. This is due to the ionisation of the gas by the hot central star. In contrast to most other planetary nebulae, the envelope radiates primarily in the
light of the OII line and not in the OIII line as usual. The age of NGC 40 is estimated at around 4,500 years, and it is also assumed that the PN will fade in the next
30,000 years. The inner envelope, which can be seen in the telescope, is surrounded by an extensive halo of filaments that extend into space to a distance of 2.3 light
years. On deep-exposure images in the infrared range, one can also see other very irregularly distributed nebular masses, which suggest an interesting prehistory.
The central star (ZS) of the nebula, with an apparent brightness of 11.6mag, is one of the brightest of its kind and therefore even has its own number in the Henry Draper
catalogue, which was compiled between 1918 and 1924. The ZS belongs to the group of white dwarfs and is still extremely hot with an effective temperature of 71,000
Kelvin. It has a mass of 0.7 solar masses (like most central stars of planetary nebulae, by the way) and loses 1/500,000 of a solar mass of matter per year, which leaves the
star at a speed of about 1,000km/s. Because of its high temperature, the luminosity of the white dwarf is very high. Due to its high temperature, the luminosity of the
white dwarf is 7,000 times that of the sun.
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My 20 inch telescope shows a very beautiful planetary nebula with a pretty bright central star. The nebular envelope appears oval at a magnification of 270x, and two
prominent shells can be seen on the longer sides, which are much brighter than the interior of the nebula. The western side appears more pronounced than the eastern
and contains a nebular knot. To the north and south, the nebula's interior appears somewhat darker.
The colour of the nebula appears whitish at low magnification and not greenish-turquoise as in most other PNs.
