June 5, 2003: Remnants from a star that exploded thousands of years ago created a celestial abstract portrait, as captured in this NASA Hubble Space Telescope image of the Pencil Nebula. Officially known as NGC 2736, the Pencil Nebula is part of the huge Vela supernova remnant, located in the southern constellation Vela. Discovered by Sir John Herschel in the 1840s, the nebula's linear appearance triggered its popular name. The nebula's shape suggests that it is part of the supernova shock wave that recently encountered a region of dense gas. It is this interaction that causes the nebula to glow, appearing like a rippled sheet.See the rest:
The photo shows the edge of the undulating sheet of gas. In this view, astronomers see large, wispy filamentary structures, smaller bright knots of gas, and patches of diffuse gas. The nebula's luminous appearance comes from dense gas regions that have been struck by the supernova shock wave. As the shock wave travels through space [from right to left in the image], it rams into interstellar material. Initially the gas is heated to millions of degrees, but then subsequently cools down, emitting the optical light visible in the image.
The colors show the temperature of the various gases that make up the nebula. The gases are cooling down. Some regions, though, are still very hot. They are dominated by ionized oxygen atoms, which glow blue in the picture. The red regions (dominated by cooler hydrogen atoms) are cooler areas of gas.