April 6, 2000: The Hubble telescope has spied a giant celestial "eye," known as planetary nebula NGC 6751. The Hubble Heritage Project is releasing this picture to commemorate the Hubble telescope's tenth anniversary. Glowing in the constellation Aquila, the nebula is a cloud of gas ejected several thousand years ago from the hot star visible in its center. Planetary nebulae have nothing to do with planets. They are shells of gas thrown off by Sun-like stars nearing the ends of their lives. The star's loss of its outer, gaseous layers exposes the hot stellar core, whose strong ultraviolet radiation then causes the ejected gas to fluoresce as the planetary nebula.
The nebula's shape including the circular arrangement of long streamers of gas moving away from the bright, central star is one of the reasons for its eye-catching appearance. The nebula's vivid colors also contribute to the eye-popping picture. Astronomers used the Wide Field and Planetary Camera 2 to snap images through three color filters. Each filter pinpoints gases at certain temperatures. Blue regions mark the hottest glowing gas, which forms a roughly circular ring around the central stellar remnant. Orange and red show the locations of cooler gas. The cool gas lies in long streamers pointing away from the central star and in a tattered-looking ring at the outer edge of the nebula. The origin of these cooler clouds within the nebula is still uncertain, but the streamers are clear evidence that their shapes are affected by radiation and stellar winds from the hot, central star. The star's surface temperature is estimated at a scorching 250,000 degrees Fahrenheit (140,000 degrees Celsius).