Planetary nebula NGC 40

This is a ten-second exposure taken on the night of September 1st 1994 (UT of observation 02/09/94:06:15). This photograph shows a region 100 arc seconds square. The image has been compressed in brightness (approximately a double logarithm) to show both bright and faint features. Observing conditions were not very good during this phase of commissioning, so that this image has a "seeing" measurement (average FWHM of several stars) of about 1.0 arc seconds. Orientation: N is up, with W to the left. About this object Planetary nebula NGC 40 is a low-excitation nebula in the constellation of Cepheus, about 4000 light-years away from Earth. The central star is fairly bright (about magnitude 11.6), has a mass of around 0.7 solar masses, and is much hotter than would be expected just from the properties of the surrounding nebula. This is because its temperature of around 90000 degrees should be hot enough to excite the nebula to a much higher ionization state than is found. This suggests the presence of shielding material between the star and the glowing nebula. Such higher density material could form in the shock interface between the fast wind (about 1800 km/s) from the central star and the nebular shells themselves. The nebular material covers about 25% of the sky as viewed by the central star, implying rather asymmetric mass-loss from the star in its asymptotic giant branch stage. NGC 40 has an extended halo, not seen here, probably caused by earlier mass ejection. Location: 00 13.0 +72 32 (2000), size: about 0.6 light-years across.