Stellar Evolution:
Images, spectra, and explanation of the life cycle of stars.
Gallery
of Planetary Nebulae from NOAO
Terzian Planetary Nebula Web Resources
A bright nova was discovered in the constellation Aquila on the evening of December 1st by independent observers in Portugal and Vermont. The nova's brightness was estimated to be magnitude 6 at the time of its discovery, and rose to magnitude 3.7 in the first 24 hours, making it easily visible to the naked eye. That turned out to be its approximate peak brightness. The nova has been given the official designation Nova Aquilae 1999 No. 2 and then V1494 Aquilae. It is the brightest nova since Nova Cygni in September 1975. More information and updates, including finder charts can be found on the AAVSO website at http://www.aavso.org.
Nova eruptions by dying stars were thought to be simple, predictable acts of violence. Astronomers could point a telescope at the most recently exploded novae and see an expanding bubble of gaseous debris around each star. Scientists using NASA's Hubble Space Telescope, however, were surprised to find that some nova outbursts may not produce smooth shells of gas, but thousands of gaseous blobs, each the size of our solar system.
Astronomers acquired this new information by focusing the Hubble telescope's cameras on the recurrent nova T Pyxidis, which erupts about every 20 years. Images from ground-based telescopes show a smooth shell of gas surrounding the nova. But closer inspection by the Hubble telescope reveals that the shell is not smooth at all, but a collection of more than 2,000 gaseous blobs packed into an area that is one light-year across. Resembling shrapnel from a shotgun blast, the blobs may have been produced by the nova explosion, the subsequent expansion of gaseous debris, or collisions between fast- and slow-moving gas from several eruptions.
This new evidence suggests that astronomers may have to rewrite their theory of nova eruptions and accompanying debris.
"Based on these observations, our previously standard view of what nova shells should look like may be fundamentally wrong," says Michael M. Shara, of the Space Telescope Science Institute in Baltimore, Md. "The view is that a nova explosion is the same in all directions, with debris traveling at the same speed, so that a fairly smooth cloud is formed. Instead, we've found this myriad of individual knots [blobs]. This observation suggests that shells of other novae do the same thing, as recently ejected material plows into older, fossil material from previous explosions."
The end of a sun-like star's life was once thought to be simple: the star gracefully casting off a shell of glowing gas and then settling into a long retirement as a burned-out white dwarf.
Now, a dazzling collection of detailed views released today by several teams of astronomers using NASA's Hubble Space Telescope reveals surprisingly intricate glowing patterns spun into space by aging stars: pinwheels, lawn sprinkler style jets, elegant goblet shapes, and even some that look like a rocket engine's exhaust.
These eerie fireworks offer a preview of the final stage of our own Sun's life," says Bruce Balick of the University of Washington in Seattle. More than simply a stellar "light-show", these outbursts provide a way for heavier elements -- predominantly carbon -- cooked in the star's core, to be ejected into interstellar space as raw material for successive generations of stars, planets and, potentially, life.
The astronomers say the incandescent sculptures are forcing a re-thinking of stellar evolution. In particular, the patterns may be woven by an aging star's interaction with unseen companions: planets, brown dwarfs, or smaller stars.
"The first time we looked at the Hubble's breathtaking pictures, we knew that our older and simpler ideas of how these objects are formed had to be overhauled," says Howard Bond of the Space Telescope Science Institute (STScI), Baltimore, MD. "The basic question is: how do these nebulae shape themselves?"
"Hubble's colorful views are a feast for the eyes," says Mario Livio, also of the STScI. "Their beauty is matched only by the mystery."
Surprising new details revealed by the Hubble pictures include:
Unexplained disks and "donuts" of dust girdling a star, which pinch outflowing gas. These may be linked to the presence of invisible companions.
Remarkably sharp, inner bubbles of glowing gas --like a balloon inside a balloon -- blown out by the violently outflowing gasses called a "fast wind" (1,000 miles/sec) ejected during the final stages of a star's death.
Strange, glowing "red blobs" placed along the edge of some nebulae may be chunks of older gas caught in the stellar gale of hot flowing material from the dying star.
Jets of high-speed particles which shoot out in opposite directions from a star, and plow through surrounding gas, like a garden hose stream hitting a sand pile.
Pinwheel patterns formed by symmetrical ejection of material so that intricate structures are mirrored on the opposite side of a star.
"We're still reveling in the quality of the data and the wealth of new details. In the longer term we're going to have to confront these strikingly symmetric structures with some fundamentally revised ideas about the final stages of a star's life," says Balick. "The lovely patterns of gas argue that some highly ordered and powerful process orchestrates the ways stars lose their mass, completely unlike an explosion."
A long-standing puzzle is how these nebulae acquire their complex shapes and symmetries. The red giant stars which preceded their formation should have ejected simple, spherical shells of gas. "Hubble's ability to see very fine structural details -- usually blurred beyond recognition in ground-based images -- enables us to look for clues to this puzzle," says Balick.
Several teams of astronomers will be observing planetary nebulae using new infrared instruments installed on the Hubble telescope last February. This way, astronomers can glimpse the ejection of material at a very early stage long before the expelled nebula starts to become visible optically. Given Hubble's high resolution, astronomers also hope to revisit the same nebula in a few years to actually see how the shell has further expanded into space. Their observations will be compared to predictions and either refine or dismiss current ideas on the mass ejection mechanisms of dying stars.
"These nebulae observed by Hubble give us a preview of our own Sun's fate. Some 5 billion years from now, after the Sun has become a red giant and burned the Earth to a cinder, it will eject its own beautiful nebula and then fade away as a white dwarf star," warns Bond.
M2-9 is a striking example of a "butterfly" or a bipolar planetary nebula. Another more revealing name might be the "Twin Jet Nebula." If the nebula is sliced across the star, each side of it appears much like a pair of exhausts from jet engines. Indeed, because of the nebula's shape and the measured velocity of the gas, in excess of 200 miles per second, astronomers believe that the description as a super-super-sonic jet exhaust is quite apt. Ground-based studies have shown that the nebula's size increases with time, suggesting that the stellar outburst that formed the lobes occurred just 1,200 years ago.
The central star in M2-9 is known to be one of a very close pair which orbit one another at perilously close distances. It is even possible that one star is being engulfed by the other. Astronomers suspect the gravity of one star pulls weakly bound gas from the surface of the other and flings it into a thin, dense disk which surrounds both stars and extends well into space.
The disk can actually be seen in shorter exposure images obtained with the Hubble telescope. It measures approximately 10 times the diameter of Pluto's orbit. Models of the type that are used to design jet engines ("hydrodynamics") show that such a disk can successfully account for the jet-exhaust-like appearance of M2-9. The high-speed wind from one of the stars rams into the surrounding disk, which serves as a nozzle. The wind is deflected in a perpendicular direction and forms the pair of jets that we see in the nebula's image. This is much the same process that takes place in a jet engine: The burning and expanding gases are deflected by the engine walls through a nozzle to form long, collimated jets of hot air at high speeds.
M2-9 is 2,100 light-years away in the constellation Ophiucus. The observation was taken Aug. 2, 1997 by the Hubble telescope's Wide Field and Planetary Camera 2. In this image, neutral oxygen is shown in red, once-ionized nitrogen in green, and twice-ionized oxygen in blue.
