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. 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 ionized oxygen gas, which forms a roughly circular ring around the central stellar remnant. Orange shows the locations of cooler hydrogen gas, and the red is indicative of ionized sulphur. The cooler 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.
NGC 6751, estimated to be around 0.8 light-years in diameter or about 600 times the size of our Solar System, is a planetary nebula (named as such due to their round shapes) in the northern constellation Aquila (the Eagle). It is about 6,500 light-years away from Earth, and it is expanding at a speed of 40 kilometers per second.
The central star, with a surface temperature of approximately 140,000 degrees Celsius, was formed when it collapsed and threw off its outer layer of gas several thousand years ago. Winds and radiation from the intensely hot central star have apparently created the nebula’s streamer-like features. It has a luminosity of 9200 Suns, revealing the star to be in its heating phase, following the ejection of its giant-star envelope. The star has its own broad emission lines that tell of a stiff outbound wind of the sort often seen in stars still in the heating phase of their planetary-nebula lives, as mass is still being lost before they turn into real white dwarfs. The chemical composition of the nebula is un-enhanced, showing no obvious effects of nuclear-processed materials in the predecessor star, which is now returning more or less pristine matter back into interstellar space from which it came.
Credits: Hubble, Space Images, Stars Astro; Wikipedia.