In the autumn of 1604, sky-watchers of the world saw something special. Actually, it was more than special: the death of one of our own galaxy's stars, an extremely bright light in the sky that appeared seemingly out of nowhere. It lasted for just three weeks.
In a lecture the following year, Galileo described what he had seen. "On the 10th October 1604 a certain strange light was first observed in the heavens," he said (in Latin). "At first it was quite small, but soon it was visible even by daylight, surpassing in brightness all fixed and wandering stars with the exception of venus."
A sight like it has not been seen since.
With ever more powerful telescopes, scientists have observed supernovae in other distant galaxies. But there have been no supernovae visible to the naked eye in our own galaxy since 1604. (The 1604 event is sometimes called Kepler's Supernova because of Johannes Kepler's study of it.)
Supernovae—the explosions of dying stars—may seem like distant, irrelevant phenomena, but they are like factories, creating many of the elements necessary for the formation of our planet and life on it. To observe such an event nearby would give scientists a unique opportunity for understanding that process.
This week, NASA promoted on its website a tantalizing photo, seen above, of a star, SBW1 that shares some similar characteristics another star which went supernova in 1987. That star, however, was not in the Milky Way but in the Large Magellanic Cloud, some 168,000 light years from Earth. SBW1, by contrast, is just 20,000 light years away. When would SBW1 become a supernova? "If we are very lucky it may happen in our own lifetimes," NASA teased.
Nathan Smith, an astronomer at the University of Arizona who discovered SBW1 and recently published a paper about it, says that NASA is perhaps getting a bit overexcited. The wording in the press release is, he wrote to me, "oversold."
"We don't know when this star is going to explode," he explained. "It could be tomorrow, or it could be in 10,000 years or a million years—but this can be said of most massive stars that are currently under study and not really newsworthy. The probability that it will explode in our lifetime is rather small, but not zero."
According to Smith, the real scientific value in SBW1 is that it helps us to better understand the 1987 supernova in the Large Magellanic Cloud. "[SBW1] looks to be almost a twin of SN1987A's progenitor star, both in the physical properties of the central star and in the shape and structure of the nebula," he wrote to me. "But SBW1 is 50 times closer in our own Milky Way galaxy, so we get a much sharper view, and perhaps most importantly, the star has not yet exploded. The is why these [Hubble] images were proposed: to study the detailed structure in that nebula and to give us a more detailed view of what SN1987A might have looked like before exploding."
In other words: It is what it is. Maybe one day it will become a supernova, but for now, SBW1 is still an object that can help us understand the universe, a star on the brink.