Ring Nebula structure. Hubble Space Telescope image of the Ring Nebula (M57, NGC 6720) fades into a 3D visualisation of the nebula's structure. This is a planetary nebula lying around 2000 light years from Earth in the constellation Lyra. Planetary nebulae are glowing shells of gas and plasma ejected by medium and low-mass stars at the end of their lives. Helium (blue) fills the central rugby ball shaped structure of the Ring Nebula. Radiation from the white dwarf star, the white dot in the centre of the ring, causes the helium to glow blue. This gives way to cooler yellow and orange rings of gas, with irregular knots of dense gas embedded along the inner rim. Red hydrogen emission reveals the outer halos which formed when the star began to burn out.