In 1572, the astronomer Tycho Brahe noticed a "new" star in the sky that grew in brightness until it was visible in the daytime, and then slowly faded away. This star challenged ideas that the heavens were unchanging, and led Tycho to start studying the heavens. Thirty years later, in 1604, Tycho's protege Johannes Kepler noticed a new star in the constellation Ophiuchus; this "star" also became the brightest star in the sky for some time before fading away.
Today, we know these two events were the explosions of stars, known as supernovae. Using optical and X-ray telescopes, we can study the nebulae left from these explosions. Therefore, Kepler and Tycho's supernovae are important bits of data for studying exploding stars. Unfortunately, no supernovae have been observed in our galaxy since 1604. We would very much like to observe a nearby supernova with all of our modern astronomical instruments. Alas, the closest supernova we've gotten was in 1987, when a star exploded in the Milky Way's companion galaxy the Large Magellanic Cloud. And while we see supernovae in more distant galaxies fairly often, it is often impossible to study these events more than a year later.
About 5 years ago, astronomer Armin Rest and his collaborators discovered filaments of light in the Large Magellanic Cloud that appeared to be moving away from three former supernovae. Further analysis found that this light was an echo, or reflection, of light released in supernovae explosions hundreds of years ago. The light started off moving away from the explosion in random directions, and then it hit a cloud of dust, which reflected the light toward the Earth. The light ended up having to travel a few hundred light-years further than the original light from the supernova, so it is just now arriving at the Earth. What this means is that we can see the original light from those explosions and study it as if the supernova were happening now. The picture at the top of this post is from that study.
A couple of days ago, Dr. Rest presented some newer work on light echos. He and his collaborators have now found light echos from both Tycho's supernova and Kepler's supernova. Using telescopes, Rest has managed to use modern instruments to study light from the same explosions that Kepler and Tycho saw. He's been able to confirm what kind of supernova Kepler and Tycho saw -- Tycho saw a Type Ia supernova (an exploding white dwarf), and he saw a very normal Type Ia. Kepler saw a Type IIL supernova (a rare type of exploding massive star).
One of the exciting things about this research is that not only can we study the explosions themselves 400 years later, but we also do not have to wait to look at the final supernova remnant -- we already see those! We are just now starting to see the new nebula that Supernova 1987A is forming; for Kepler and Tycho, 400 years have already passed, so the nebula is already well-formed and has been well-studied by all kinds of telescopes.
One other great thing about light echoes is that they allow us to see the explosion from different directions, like mirrors held hundreds of light years on either side of the explosion. Tycho looks the same from different directions, but Supernova 1987A looks slightly different. With a little luck and a lot of hard work, we may be able to piece together an even better picture of these explosions.
These light echoes are therefore letting us study the past and present of famous supernovae. We can use modern telescopes to study these famous supernovae, and get information almost as good as if Tycho had today's observatories at his disposal. And we don't have to wait decades or centuries for the next nearby supernova explosion.
The topic of light echoes also reminded me of the famous Diana Ross song Reflections. "Reflections of...the way light used to be." (Or something like that)