Hubble Pinpoints Doomed Star that Explodes as Supernova

Amidst the glitter of billions of stars in the majestic spiral galaxy called the Whirlpool (M51), a massive star abruptly ends its life in a brilliant flash of light. NASA's Hubble Space Telescope snapped images of the exploding star, called Supernova (SN) 2005cs, 12 days after its discovery. Astronomers then compared those photos with Hubble images of the same region before the supernova blast to pinpoint the progenitor star (the star that exploded).

The color image at left shows a section of M51 taken in January 2005 with the Advanced Camera for Surveys. The small green square marks the region where the progenitor star resides. The lower-right image shows a picture of SN 2005cs (the central bright object), taken July 11, 2005, by Hubble. By comparing the lower-right image with the color image at left, astronomers identified the supernova's progenitor star [marked by the arrow in the (pre-explosion) upper-right image]. The star was found to be a red supergiant whose mass is about seven to 10 times that of the Sun.

Every second, a star somewhere in the universe explodes as a supernova. Astronomers cannot see every supernova. Of the supernovas astronomers have seen, only six progenitor stars have been identified. Since Hubble can easily resolve stars in nearby galaxies, such as the Whirlpool, it allowed astronomers to track down the exploding star's identity in archival pictures.

SN 2005cs belongs to a class of exploding stars called "Type II-plateau." A supernova of this type results from the collapse and subsequent explosion of a massive star whose light remains at a constant brightness (a "plateau") for a period of time.

This finding is consistent with the idea that the progenitors of supernova explosions are red supergiant stars with masses eight to 15 times the Sun's mass. The progenitor star was found to be at the low end of the mass range for supernova explosions. Stars with masses lower than eight solar masses do not explode as supernovae at all, but rather contract to white dwarfs and blow off their outer atmospheres to become planetary nebulae.

Identification of the progenitor star was first reported in IAU Circulars 8556 and 8565 on July 3 and July 12, respectively, by Drs. Weidong Li and Alex Filippenko (University of California, Berkeley) and Schuyler Van Dyk (Spitzer Science Center, Caltech). The team submitted a paper describing their research to The Astrophysical Journal on July 18. A European team composed of Drs. Justyn Maund (Institute of Astronomy, University of Cambridge), Stephen Smartt (Queen's University, Belfast) and John Danziger (Trieste Observatory) reported similar results in a letter submitted on July 21 to the Monthly Notices of the Royal Astronomical Society.