M74 as imaged by the Spitzer Space Telescope
Like a doctor trying to understand an elderly patient's sudden demise, astronomers have obtained the most detailed observations ever of an old but otherwise normal massive star just before and after its life ended in a spectacular supernova explosion.

M74 Composite showing the Supernova Star
The star was imaged by the Gemini Observatory and Hubble Space Telescope (HST) less than a year prior to the gigantic explosion, and is located in the nearby galaxy M-74 in the constellation of Pisces. These observations allowed a team of European astronomers led by Dr. Stephen Smartt of the University of Cambridge, England, to verify theoretical models showing how a star like this can meet such a violent fate.

The results were published in the January 23, 2004 issue of the journal Science. This work provides the first confirmation of the long-held theory that some of the most massive (yet normal) old stars in the Universe end their lives in violent supernova explosions.

"It might be argued that a certain amount of luck or serendipity was involved in this finding," said Dr. Smartt. "However, we've been searching for this sort of normal progenitor star on its deathbed for some time. I like to think that finding the superb Gemini and HST data for this star is a vindication of our prediction that one day we had to find one of these stars in the immense data archives that now exist" .

M74 before
Blow-up of region around SN2003gd progenitor star, with star enhanced for visibility. From Gemini North GMOS image. Credit: Gemini Observatory: GMOS Commissioning Team.

During the last few years, Smartt's research team has been using the most powerful telescopes, both in space and on the ground, to image hundreds of galaxies in the hope that one of the millions of stars in these galaxies will some day explode as a supernova.

In this case, the renowned Australian amateur supernova hunter, Reverend Robert Evans, made the initial discovery of the explosion (identified as SN2003gd) while scanning galaxies with a 12-inch (31-cm) backyard telescope from his home in New South Wales, Australia in June 2003.

Following Evans' discovery, Dr. Smartt's team quickly followed up with detailed observations using the Hubble Space Telescope. These observations verified the exact position of the original or "progenitor" star.

Using this positional data, Smartt and his team dug through data archives and discovered that observations by the Gemini Observatory and HST contained the combination of data necessary to reveal the nature of the progenitor.

M74 Supernova just after the Explosion
Image from the Isaac Newton Telescope with the Wide Field Camera taken on 25th August 2003 by Jonathan Irwin (IoA). It shows the position of the fading supernova which by then was some six months old. Credit: Isaac Newton Telescope, Jonathan Irwin.

The Gemini data was obtained during the commissioning of the Gemini Multi-Object Spectrograph (GMOS) on Mauna Kea, Hawaii, in 2001. These data were also used to produce a stunning high-resolution image of the galaxy that clearly shows the red progenitor star.

Armed with the earlier Gemini and HST observations Smartt's team was able to demonstrate that the progenitor star was what astronomers classify as a normal red supergiant.

Prior to exploding, this star appeared to have a mass about 10 times greater, and a diameter about 500 times greater, than that of our Sun. If our sun were the size of the progenitor, it would engulf the entire inner solar system out to about the planet Mars.

Red supergiant stars are quite common in the universe. An excellent example can be easily spotted during January from almost anywhere on the Earth by looking at Betelgeuse, the bright red shoulder star in the constellation of Orion.

Orion Map showing Betelgeuse, also voted most likely to explode, any day!
Like SN2003gd, it is believed that Betelgeuse could meet the same explosive fate at any time from next week to thousands of years from now. Credit: Diagram courtesy of Eleanor Gilchrist, Royal Observatory of Edinburgh UK.

After SN2003gd exploded, the team observed its gradually fading light for several months using the Isaac Newton Group of telescopes on La Palma. These observations demonstrated that this was a normal type II supernova, which means that the ejected material from the explosion is rich in hydrogen.

Computer models developed by astronomers have long predicted that red supergiants with extended, thick atmospheres of hydrogen would produce these type II supernovae but until now have not had the observational evidence to back up their theories.

However, the fantastic resolution and depth of the Gemini and Hubble images allowed the Smartt team to estimate the temperature, luminosity, radius and mass of this progenitor star and reveal that it was a normal large, old star. "The bottom-line is that these observations provide a strong confirmation that the theories for both stellar evolution and the origins of these cosmic explosions are correct," said co-author Seppo Mattila of Stockholm Observatory.

This is only the third time astronomers have actually seen the progenitor of a confirmed supernova explosion. The others were peculiar type II supernovae: SN 1987A, which had a blue supergiant progenitor, and SN 1993J, which emerged from a massive interacting binary star system.

Background Information:

Supernovae are among the most energetic phenomena observed in the entire Universe.

When a star of more than about eight times the mass of our Sun reaches the end of its nuclear fuel reserve, its core is no longer stable from collapsing under its own immense weight.

As the core of the star collapses, the outer layers are ejected in a fast-moving shock wave.

This huge energy release results in a supernova that is about one billion times brighter than our Sun, and is comparable to the brightness of an entire galaxy.

Black Hole Simulation
After destroying itself, the core of the star becomes either a neutron star or a black hole.

The team was composed of Stephen J. Smartt, Justyn R. Maund, Margaret A. Hendry, Christopher A. Tout, and Gerald F. Gilmore (University of Cambridge, UK), Seppo Mattila (Stockholm Observatory, Sweden), and Chris R. Benn (Isaac Newton Group of Telescopes, Spain).