THE GLOWING EYE
OF NGC 6751
Astronomers using NASA's Hubble Space
Telescope have obtained images of the strikingly
unusual planetary nebula, NGC 6751. Glowing in the
constellation Aquila like a giant eye, the nebula
is a cloud of gas ejected several thousand years
ago from the hot star visible in its center.
"Planetary nebulae" are named after
their round shapes as seen visually in small telescopes,
and have nothing else to do with planets. They are
shells of gas thrown off by stars of masses similar
to that of our own Sun, when the stars are nearing
the ends of their lives. The loss of the outer layers
of the star into space exposes the hot stellar core,
whose strong ultraviolet radiation then causes the
ejected gas to fluoresce as the planetary nebula.
Our own Sun is predicted to eject its planetary
nebula some 6 billion years from now.
The Hubble observations were obtained
in 1998 with the Wide Field Planetary Camera 2 (WFPC2)
by a team of astronomers led by Arsen Hajian of
the U.S. Naval Observatory in Washington, DC. The
Hubble Heritage team, working at the Space Telescope
Science Institute in Baltimore, has prepared this
color rendition by combining the Hajian team's WFPC2
images taken through three different color filters
that isolate nebular gases of different temperatures.
The nebula shows several remarkable
and poorly understood features. 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 tends to lie in long streamers pointing away
from the central star, and in a surrounding, 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 star at the center. The
star's surface temperature is estimated at a scorching
140,000 degrees Celsius (250,000 degrees Fahrenheit).
Hajian and his team are scheduled
to reobserve NGC 6751 with Hubble's WFPC2 in 2001.
Due to the expansion of the nebula, at a speed of
about 40 kilometers 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 measurement 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.
Image Credit: NASA and The Hubble
Heritage Team (STScI/AURA)
Acknowledgment: A. Hajian (US Naval Observatory)
andB. Balick (University of Washington)