Hubble spies a 'glowing eye' in space

NGC 6751 is about 6,500 light-years distant

By BBC News Online science editor Dr David Whitehouse

Astronomers using the Hubble Space Telescope (HST) have obtained images of a strikingly unusual planetary nebula, called NGC 6751.

Glowing in the constellation of Aquila like a giant eye, it is a luminous cloud of gas ejected from a hot star several thousand years ago.

Planetary nebular is a misnomer. The term has nothing to do with planets; it arose because the object could be mistaken for a planetary disc when viewed under low magnification.

Planetary nebulae are actually shells of gas thrown off by stars like our Sun when they are nearing the end of their lives. The loss of the outer layers of the star into space exposes the hot stellar core, whose strong ultraviolet radiation causes the ejected gas to fluoresce.

Six billion years from now, our Sun is expected to blow off a planetary nebula.

Long streamers

The HST observations were obtained in 1998 with the telescope's Wide Field Planetary Camera 2 by a team of astronomers led by Arsen Hajian of the US Naval Observatory in Washington, DC.

NGC 6751 shows several remarkable and poorly understood features. Blue regions mark the hottest glowing gas. Orange and red show the locations of cooler gas.

The cool gas tends to lie in long streamers pointing away from the central star, and surrounding 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 star at the centre.

Dr Hajian and his team will observe NGC 6751 again in 2001. Due to the expansion of the nebula, at a speed of about 40 km per second (25 miles per second), the high resolution of Hubble's camera will reveal the slight increase in the size of the nebula since 1998.

This will allow the astronomers to calculate an accurate distance to NGC 6751. In the meantime, current estimates are that NGC 6751 is roughly 6,500 light-years from Earth. The nebula's diameter is 0.8 light-years, some 600 times the diameter of our own Solar System.