Stellar Metamorphosis:

Hubble Space Telescope pictures of

Butterfly-like nebulae emerging from stellar cocoons

Sun Kwok and Kate Su, University of Calgary, Canada

Bruce J. Hrivnak, Valparasio University, U.S.A.

Stars do not live forever. The Sun, for example, was born approximately four and half billion years ago, and is expected to have another 5 billion years left in its life. What is the eventual fate of the Sun, and stars like it? It is a popular myth that stars explode when they die, and violent events such as supernova explosions were thought to be the norm. Dr. Sun Kwok of the University of Calgary in Canada has promoted instead the idea that most stars do not explode, but undergo a quiescent death. The majority of stars, in fact over 95%, will go quietly - in other words "not with a bang but a whimper". This theory has been confirmed by observations from space, including observations from the Hubble Space Telescope.

During the last ten thousand years of a star's life, it goes through a glorious stage called the "planetary nebula" phase, during which the star gives off a magnificent display of light (see recent releases of HST images of planetary nebulae). In order to better understand the cause of death of stars, one needs to look back a few hundred years before the planetary nebulae phase, and examine the state of stars before they undergo this transformation. Over the past decade, Dr. Sun Kwok and Dr. Bruce Hrivnak have been searching for such stars, the very old stars which are just beginning to get "sick". Now two such "patients" have been imaged with the HST. The high quality HST pictures have provided much needed information for the diagnosis of the cause of the death of stars.

The accompanying pictures are HST WFPC2 images of two "proto-planetary nebulae": the "Cotton Candy Nebula" (IRAS 17150-3224) and the "Silkworm Nebula" (IRAS 17441-2411) . In both cases, we can see a series of concentric rings, representing the "puffs" of meterial given off by the stars in the last few thousands years of their lives. These "puffs" provide evidence that the star has been "ill" for some time. After a number of these "puffs", the star wraps itself inside a cocoon. In these two HST images, we are seeing the first indication that the nebulae are emerging from their cocoons, like butterflies undergoing metamorphosis.

These observations were made with the HST WFPC2 camera in cycle 6 under GO program 6565 (Kwok et al.: Imaging of Proto-Planetary Nebulae". We have also made use of data in the public archive obtained under the snapshot program 6364 (Bobrowsky et al.: Snapshot survey of protoplanetary nebulae and AGB stars). For a popular account of the discovery of proto-planetary nebulae, please see October 1998 issue of Sky and Telescope magazine

Dr. Sun Kwok has published extensively in the field of planetary nebulae and is best known for his theory of planetary nebulae formation. He is the current chairman of the International Astronomical Union Working Group on Planetary Nebulae.

Link to Space Astronomy Laboratory at University of Calgary.

Link to Infrared and Submm Astronomy at University of Calgary.

Link to Dr. Bruce Hrivnak's page at Valparaiso University.