Astronomers using NASA's Hubble Space Telescope have obtained the sharpest view yet of a glowing loop of gas called the Ring Nebula (M57), first cataloged more than 200 years ago by French astronomer Charles Messier.
The Hubble telescope images reveal that the "Ring" is actually a cylinder of gas seen almost end-on. Such elongated shapes are common among other planetary nebulae, because thick disks of gas and dust form a waist around a dying star. This "waist" slows down the expansion of material ejected by the doomed object. The easiest escape route for this cast-off material is above and below the star.
Located in the constellation Lyra, the Ring Nebula is the best-known example of a planetary nebula, the glowing remains of a doomed star.
This new view was obtained October 16, 1998 by the Hubble Heritage Program team at the Space Telescope Science Institute in Baltimore, Md., the Hubble telescope's science operations center. The Heritage team, comprised of astronomers and image-processing specialists, selected this most famous of planetary nebulae as its first new target.
Inaugurated in October 1998, the Heritage Program treats the public to a monthly sampling of stunning celestial views from the Hubble telescope. Until now, the pictures have been selected from the telescope's rich banquet of archival images, originally taken for scientific research. But the Heritage team also will occasionally employ the orbiting observatory to obtain new images of pictorially stunning celestial objects, such as the Ring Nebula, using a small amount of the Institute director's discretionary time.
"We chose the Ring Nebula as our first new target, in part, 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."
The Ring Nebula is about 2,000 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.
In this colorful image, appearances are deceiving. What looks like an elliptical ring is actually believed to be a barrel-shaped structure surrounding the faint central star, the small white dot in the center. The Ring looks nearly round only because we are looking down the barrel.
This new view of the Ring has surprised Bond, who has studied this object for several decades.
"I first saw the Ring through a small telescope in my back yard, when I was a high-school student in the 50's," Bond recalls. "My astronomy books taught me that it was a round sphere of expanding gas."
Two centuries ago, astronomers studying these round-shaped objects through small telescopes called them "planetary nebulae," because their circular disks resemble those of planets.
Astronomers, however, have suspected for some time that the Ring Nebula actually has a cylindrical shape and looks round only because of the viewing angle. Close examination of the Hubble telescope image, taken by the Wide Field and Planetary Camera 2, 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 silhouetted against more distant bright gas. These dense dust clouds are too small to be seen with ground-based telescopes but are easily revealed by the Hubble telescope.
Remarkably, these finger-like clouds are only seen in the outer portions of the Ring Nebula; none are seen in the central region. This proves that they are not distributed in a uniform sphere but are instead located only on the walls of the barrel. Many of the finger-like clouds point away from the central star, like spokes on a wheel, due to the forces of radiation and gas ejected from the dying object.
The Heritage team is already planning another new Hubble observation. A list of possible targets, several spectacular edge-on galaxies far beyond the Milky Way, will be posted on the team's Web site. The team will ask the public to pick their favorite. The winning object will be observed with the Hubble telescope in the spring, and the new images will be released to the public shortly thereafter. Those wishing to vote should visit the Heritage Web site at http://heritage.stsci.edu.
The Heritage team believes that the public will enjoy taking on the role of astronomer.
"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."
Space Telescope Science Institute, Baltimore, MD
Space Telescope Science Institute, Baltimore, MD