Planetary Nebula in the constellation of Cepheus
NGC 40 (also known as the Bow-Tie Nebula and Caldwell 2) is a planetary nebula discovered by William Herschel on November 25, 1788, and is composed of hot gas around a dying star. The star has ejected its outer layer which has left behind a small, hot star. Radiation from the star causes the shed outer layer to heat to about 10,000 degrees Celsius and become visible as a planetary nebula. The nebula is about one light-year across. About 30,000 years from now, scientists theorize that NGC 40 will fade away, leaving only a white dwarf star approximately the size of Earth.
Morphologically, the shape of NGC 40 resembles a barrel with the long axis pointing towards the north-northeast. There are two additional pairs of lobes around the poles, which correspond to additional ejections from the star.
The central star of NGC 40 has a Henry Draper Catalogue designation of HD 826. It has a spectral type of [WC8], indicating a spectrum similar to that of a carbon-rich Wolf–Rayet star. The central star has a bolometric luminosity of about 7,000 L☉ and radius of 0.56 R☉. The star appears to have an effective temperature of about , but the temperature of the source ionizing the nebula is only about . One proposed explanation to this contradiction is that the star was previously cooler, but has experienced a late thermal pulse which re-ignited fusion and caused its temperature to increase.
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