The Ring Nebula Project
Close-Up of Dust Silhouettes
For centuries, amateur and professional astronomers
alike have been captivated by a faintly glowing
doughnut, named the Ring Nebula, which lies in the
constellation Lyra. The Ring, first cataloged over
200 years ago by French astronomer Charles Messier
and called M57, is the best-known example of a planetary
nebula. It is a cloud of gas ejected from the doomed
star at its center.
Now, astronomers have used NASA's Hubble Space
Telescope to obtain the sharpest view yet of the
Ring Nebula. The Hubble images reveal that the Ring
actually has a cylindrical or hourglass shape. Its
apparent nearly circular appearance is due only
to its being seen almost end-on. Such elonged shapes
are common among other planetary nebulae, because
thick disks of gas and dust around the waist of
the star slow down the expansion in that direction,
leaving the gas free to flow out perpendicular to
The Ring Nebula is about 2000 light years from
Earth, and has a diameter of about one light year.
The faint speck at its center was once a star of
greater mass than our own Sun. Now, near the end
of its life, it has ejected its outer layers into
space, and the remnant is destined to die as a tiny
white dwarf star, about the size of the Earth.
This new view was obtained in October by the Hubble
Heritage Program team at the Space Telescope Science
Institute in Baltimore, Md., the Hubble telescope's
operations center. The Heritage team, comprised
of astronomers and image-processing specialists,
selected this most famous of planetary nebulae as
its first new target for the orbiting observatory.
Each month, the Heritage Program will treat the
public to a new sampling of celestial views from
the Hubble Telescope. Until now, the pictures have
been drawn from the telescope's treasure trove of
archival images, and have been byproducts of the
main purpose of the observations, scientific research.
But the Heritage team will also occasionally employ
the space observatory to obtain new images of pictorially
stunning sky objects, using a small amount of the
Institute Director's discretionary time which he
has made available.
"We made the Ring Nebula our first new target because
it is so well known among amateur astronomers,"
explains Heritage team astronomer Howard Bond, who
was once an amateur himself. "We
knew the Ring photo would be spectacular because
we had already imaged a portion
of the nebula with short Hubble exposures in 1995,
and what we saw was absolutely amazing." Adds Heritage
Project head scientist Keith Noll, "We knew we had
to go back and finish the Hubble picture of the
entire Ring Nebula. In the future, we will involve
members of the public, and astronomers from other
institutions, in the selection of the next celestial
targets for Hubble Heritage observations."
In this colorful image, taken with Hubble on October
16, 1998, appearances are deceiving. What looks
like an elliptical ring is actually believed to
be a cylindrical or barrel-shaped structure, surrounding
the faint central star, which is seen as a small
white dot in the center. The Ring looks nearly round
only because we are looking down the axis of the
Bond recalls that "I first saw the Ring through
a small telescope in my back yard, when I was a
high-school student in the 50's. My astronomy books
taught me that it was a round sphere of expanding
gas." In fact, the round shapes of planetary nebulae
when seen in small telescopes gave rise to the name
two centuries ago: their circular disks resemble
those of planets, and thus they were called nebulae
with a "planetary" shape.
Astronomers have, however, suspected for some
time that the Ring Nebula actually has an elongated
shape, and looks round only because of our viewing
angle. Close examination of the Hubble image strongly
supports this newer opinion. The photo shows numerous
small dark clouds of dust that
have formed in the gas flowing out from the star,
and are silhoutted against more distant bright gas.
These small, dense dust clouds are too small to
be seen with ground-based telescopes, but are easily
revealed by Hubble.
Remarkably, the dark dust clouds are only seen
in the outer portions of the Ring Nebula; none are
seen silhoutted against the central region. This
proves that they are not distributed in a uniform
sphere, but are instead located only on the walls
of the cylinder. Many of the dust clouds are elongated
in directions pointing away from the central star,
due to the forces of radiation and gas outflow from
The color image of the Ring was built up from
three black-and-white photos taken through different color filters with the Hubble Telescope's
Wide Field Planetary Camera 2. Blue isolates emission
from very hot helium, which is located primarily
close to the hot central star. Green represents
ionized oxygen, which is located further from the
star. Red shows ionized nitrogen, which is radiated
from the coolest gas, located furthest from the
star. The gradations of color illustrate how the
gas glows because it is bathed in ultraviolet radiation
from the remnant central star, whose surface temperature
is a white-hot 120,000 C.
The Heritage team is already planning its next
new Hubble observations. A list of candidate targets,
this time several spectacular edge-on galaxies far
beyond the Milky Way, will be posted on the team's
web site, and the public will be allowed to vote
for their favorite. The winner will be observed
with Hubble in the spring of 1999, and the new images
made available shortly thereafter. Those wishing
to vote should set their internet browsers to http://heritage.stsci.edu.
"People have an intense interest in everything
Hubble does," Noll says. "The Heritage Project is
a wonderful way for the public to participate directly
in the process of selecting Hubble targets. The
turnaround will be relatively quick. The process,
from voting to public release, should take about
three to four months. Although the main purpose
of the Hubble Heritage observations is to provide
the spectacular images to the public, our data--including
the new Ring Nebula images--will also be released
to the professional astronomical community at the
same time in digital form, so that detailed scientific
analyses can be conducted."
The Hubble Heritage Project was unveiled in October
1998, with the aim of providing the public with
pictorially striking images of celestial objects
obtained with the Hubble Space Telescope.
The Heritage Project will primarily select, process,
and release the most visually compelling images
available in the HST archive, but will on occasion
obtain new images of objects selected for their
pictorial, rather than purely scientific, impact.
The public will be asked to join in target selection
for these new observations. The digital image data
will be placed immediately into the HST archive,
for the benefit of interested professional astronomers.
For our first new Heritage observation, we selected
the famous Ring Nebula in Lyra (M57 = NGC 6720).
This object is so large (the major axis of the bright
ring is about 90") that it required two WFPC2 telescope
pointings to cover it entirely. We imaged M57 in
three emission-line species, color-coded
as follows in the final rendition of the image:
He II 4686 A (blue), [O III] 5007 A (green), and
[N II] 6584 A (red).
In addition to its pictorial and outreach appeal,
the image reveals a host of subarcsecond dark globules
around the periphery of the nebula. The fact
that no globules are seen projected against the
central region demonstrates that their distribution
is in fact toroidal or cylindrical, rather than
spherical. Thus the Ring Nebula is in fact a non-spherical,
axisymmetric planetary nebula (like many other PNe),
which we happen to view from a direction close to
its axis of symmetry. A ground-based kinematical
study by Bryce, Balick, & Meaburn (1994, MNRAS,
266, 721) indeed supports the probable bipolar geometry
and low viewing angle.