Thanks to a fortuitous observation with NASA's Swift satellite, Prof. Albert Kong of the Institute of Astronomy is one of the first astronomers to catch a star in the act of exploding. Astronomers have previously observed thousands of stellar explosions, known as supernovae, but they have always seen them after the fireworks were well underway. This discovery was published in Nature on May 22, 2008.

A typical supernova occurs when the core of a massive star runs out of nuclear fuel and collapses under its own gravity to form an ultradense core. The infalling materials then rebound, triggering a shock wave that plows through the star's gaseous outer layers and blows the star to smithereens. Astronomers thought for nearly four decades that this shock "break-out" will produce bright X-ray emission lasting a few minutes. But until this discovery, astronomers have never observed this signal. Instead, they have observed supernovae brightening days or weeks later, when the expanding shell of debris is energized by the decay of radioactive elements forged in the explosion.

Under the support of the National Science Council, Prof. Kong is leading a team to perform research on high-energy astrophysics by using space-based and ground-based telescopes. In the morning of January 10, 2008, Prof. Kong analyzed data from NASA's Swift satellite which were taken on January 9. In the spiral galaxy NGC 2770, located 90 million light-years from Earth in the constellation Lynx, Prof. Kong found an extremely bright 5-minute X-ray outburst that happened at 9:33pm Taiwan time. At the same time, Dr. Alicia Soderberg and her team from Princeton University discovered the same X-ray outburst independently. Prof. Kong and Dr. Soderberg immediately informed other astronomers to observe this rare astronomical event.

Due to the significance of the X-ray outburst, Dr. Soderberg immediately mounted an international observing campaign to study this event and invited Prof. Kong to join her team. Observations were made with major telescopes such as the Hubble Space Telescope, the Chandra X-ray Observatory, the Very Large Array in New Mexico, the Gemini North telescope in Hawaii, the Keck I telescope in Hawaii, the 200-inch and 60-inch telescopes at the Palomar Observatory in California, and the 3.5-meter telescope at the Apache Point Observatory in New Mexico.

In a paper published in Nature, Dr. Soderberg, Prof. Kong, and their colleagues show that the energy and pattern of the X-ray outburst is consistent with a shock wave bursting through the surface of the progenitor star. This marks the birth of the supernova now known as SN 2008D. The significance of this discovery is that we were able to see the star blow up in real time which gives us unprecedented insight into the explosion process.

Although astronomers were lucky that Swift was observing NGC 2770 just at the moment when SN 2008D's shock wave was blowing up the star, Swift is well equipped to study such an event because of its multiple instruments observing in gamma rays, X-rays, and ultraviolet light. "It was a gift of nature for Swift to be observing that patch of sky when the supernova exploded. But thanks to Swift's flexibility, we have been able to trace its evolution in detail every day since," says Swift lead scientist Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The combined observations helped the team pin down the energy of the initial X-ray outburst, which will help theorists better understand supernovae. The observations also show that SN 2008D is an ordinary Type Ibc supernova, which occurs when a massive, compact star explodes. Significantly, radio and X-ray observations found no evidence that a jet played a role in the explosion, ruling out a rare type of stellar explosion known as a gamma-ray burst.

"For years we have dreamed of seeing a star just as it was exploding, but actually finding one is a once in a lifetime event," says team leader Dr. Alicia Soderberg, a Hubble and Carnegie-Princeton Fellow at Princeton University. "This newly born supernova is going to be the Rosetta stone of supernova studies for years to come."

"When I first looked at the X-ray image of SN 2008D, I immediately realized that this must be an important discovery because typical X-ray sources cannot be that luminous and change their brightness so rapidly," says Prof. Kong. "The first thing in my mind is to inform other astronomers around the world to observe this event as soon as possible."