Planetary nebula NGC 2371, Hubble Space Telescope (HST) image. A planetary nebula is a series of shells of gas cast off by a dying star. The ejection of the star's outer layers exposes its hot core, whose radiation ionises the shells, causing them to glow. Here, the glow is seen as the green and blue colours. The pink dot at centre is the core of the red giant star that cast off these layers. NGC 2371 is 4,300 light-years from Earth, in the constellation Gemini. This image was obtained with Hubble's Wide Field Planetary Camera 2 (WFPC2) in November 2007.