Bow-Tie nebula

NGC 40，也称为蝴蝶结星云和科德韦尔2，是威廉·赫歇尔在1788年11月25日发现的一个行星状星云。这颗垂死的恒星已经抛出了它的外壳，并且留下较小、且有炙热气体围绕着的，表面温度高达50,000K；来自恒星的辐射导致外层倍加热至10,000K，并且膨胀至约1光年的直径。 科学家估计，在30,000年后，NGC 40将逐渐暗淡，而只留下一颗如同地球大小的白矮星。

NGC 40, also known as the Bow-Tie Nebula or Caldwell 2, is a planetary nebula located around 5280 light years (1619 parsecs) from Earth in the Constellation of Cepheus. The nebula is about one light-year across. It was discovered by William Herschel on November 25, 1788.

The nebula formed due to a dying star ejecting its outer layers of gas leaving behind a small, hot stellar remnant known as a white dwarf. Radiation from the star causes the shed outer layer to heat to about 10,000 degrees Celsius and become visible as a planetary nebula. 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.

Structure

The main structure of NGC 40 has a barrel-like shape with two moves expanding towards the northern and southern directions. 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. There is a cavity located in NGC 40 that is around 6500 years old. The northern and southern lobes were ejected around 4200 years ago.

Central 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 71,000 K, but the temperature of the source ionizing the nebula is only about 45,000 K. 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.