August 13, 1997

Colliding Supernova Shells

The insert in the above image shows a gigantic cosmic collision in one of the spiral arms of galaxy NGC 6946 some 17 million light-years from Earth. The gaseous shells of two or possibly more supernovae are colliding, plowing into each other, and emitting large amounts of X-rays and visible radiation. Such a collision had never before been observed.

Supernovae are among the most violent events in today's universe. They are the explosions of stars that are more massive than the Sun and have reached the ends of their lives. In some of these explosions, gaseous matter amounting to a dozen times that of the Sun or even more get ejected at speeds of ten to more than thirty million miles per hour. Intense emission of gamma rays, X-rays, and visible radiation accompanies supernova explosions. For some weeks, a supernova may outshine all the other billions of stars in the host galaxy. In the course of months and years, the brightness fades and the ejected matter slows down.

When astronomers first examined the explosion in galaxy NGC 6946 they were puzzled. The X-rays and visible radiation they observed were very intense, like those from the ejected shell of a recent supernova, but the low speed of expansion and mixture of elements resembled those of the shell of a much older supernova.

How could a supernova shell be so bright and also old?

The Hubble Space Telescope (HST), with its ability to resolve fine spatial detail of distant objects, provided the answer. An image taken with HST's Wide Field and Planetary Camera 2 (WFPC2) suggests that we are not seeing the expanding shell of a single supernova. The bright crescent in the above insert indicates that two (and possibly more than two) supernovae are involved – a young one and an older one. The expanding shell from the young supernova is crashing into the shell of the older supernova. The violent collision is heating the matter and produces the intense display witnessed in X-rays and visible light.

Dr. William Blair of the Johns Hopkins University and his colleagues Dr. Robert A. Fesen of Dartmouth College and Dr. Eric M. Schlegel of the Harvard-Smithsonian Center for Astrophysics, who carried out this research, estimate that the exploding stars that gave rise to the shells are about forty light-years apart. The collision – i.e., the crashing together and intermingling of the matter of the supernova shells, heating, and intense emission of X-rays and visible radiation - may go on for a few hundred years.

We should note that spiral galaxy NGC 6946 has had an unusually prolific recent history of supernova explosions. Six have been observed in this galaxy since 1917. In comparison, the last supernova observed in our galaxy, the Milky Way, dates to 1604 AD.

More Cool Stuff

We obtained the above images and information from a June 10, 1997, press release of the Johns Hopkins University:

http://www.jhu.edu/news_info/news/home97/jun97/ngc.html

For more information on stars and their origin, structure, and evolution, go to the Space Science section of the Observatorium:

http://observe.ivv.nasa.gov/nasa/space/space_index.shtml

There are several other NASA-supported sites on the Web that have excellent material on the space sciences:

"Eyes on the Sky, Feet on the Ground," a set of pages of the Web site "Using Science and the Internet as Everyday Classroom Tools" from the Astrophysical Observatory:

http://hea-www.harvard.edu/ECT/the_book/

The "Public Connection" Web site, an outreach project of Rice University and the Houston Museum of Natural Science:

http://space.rice.edu/hmns/connect.html

The "Science Information Infrastructure" project of the University of California at Berkeley:

http://www.cea.berkeley.edu/~edsci/sii/sii_sii.html

"Windows to the Universe," an innovative and engaging Web site about the Earth and space sciences:

http://www.windows.umich.edu/