NGC 40 (also known as theBow-Tie Nebula andCaldwell 2) is aplanetary nebula discovered byWilliam 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.[4] 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.[4] 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.[4]
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.[5]
The central star of NGC 40 has aHenry Draper Catalogue designation of HD 826.[6] It has aspectral type of [WC8], indicating a spectrum similar to that of acarbon-richWolf–Rayet star.[7] The central star has abolometric luminosity of about 7,000 L☉ and radius of 0.56 R☉. The star appears to have aneffective temperature of about71,000 K, but the temperature of the source ionizing the nebula is only about45,000 K. One proposed explanation to this contradiction is that the star was previously cooler, but has experienced alate thermal pulse which re-ignited fusion and caused its temperature to increase.[6]
Media related toNGC 40 at Wikimedia Commons
