Wednesday, December 10, 2008

Echoes of Light

Somewhere in the Universe, perhaps 100 million years ago, a star explodes. Within a few seconds it goes from a nearly burned-out, second-rate stellar attraction to a brilliant supernova, brighter by far than all the other billions of stars in its galaxy put together. Over the next few minutes, the next few hours, even the next few days and weeks, it passes convulsively through a series of transformations, violently spewing forth the billions of tons of complex nuclear matter that have been formed under the heat and compression of its dying years.

The light from that supernova races outward across galactic and inter-galactic space. When it passes Earth, perhaps--as in this example--100 million years later, astronomers have a chance to observe it. But it is a fleeting chance; the light passes Earth as quickly as it was created and continues to rush outward into the void of space.

Until recently it has been very hard to study the light from a supernova closely. There are only one or two per century among the hundred-billion-or-so stars in our Milky Way Galaxy. But as telescopes have gotten bigger and more powerful, hundreds of millions of distant galaxies have been brought into view. And modern telecommunications have been geared up and fine-tuned. Finally, in January, 2008, astronomers happened to have the right telescope pointed in the right direction at the right time. They saw, for the first time, the moment of birth of a supernova. Within seconds the news raced around the world, and within minutes the most powerful radio, optical, infrared, and X-ray telescopes on the ground and in space were trained on the spot, observing and recording the frequencies and intensities of the energy patterns emitted.

But another fascinating technique has also emerged for observing closely the light from the birth of a supernova. As the light explodes outward in all directions, it meets clouds of interstellar dust and it is reflected; perhaps, if the angle of reflection is just right, some of that original light may arrive at the Earth many years after the arrival of the light from the original explosion.

There have been three supernovae reported by observers on Earth within the past millennium (but before the recent advent of powerful telescopes). One was in the year 1006 by Chinese observers; one was described in detail by the famous astronomer Tycho Brahe in 1572; and the third, by another famous astronomer, Johannes Kepler, in 1604. In 2008 astronomers happened to notice some curious arcs of light. When they carefully calculated the geometry of these arcs, they realized they were seeing the light echo of the supernova Kepler had first observed 400 years earlier. They were able to watch the supernova closely and do a precise, timed analysis of the light frequencies emitted as it evolved over several months. Subsequently, having been alerted to the possibility, they were able to find echoes of reflected light from both the supernovae observed by Tycho Brahe and even the one reported by the Chinese astronomers 1,000 years earlier.

It seems like time travel to be able to look back and watch, second by second and minute by minute, events that were seen on Earth hundreds of years ago--in fact, events that happened hundreds of millions of years ago.