On 9 October 1604 a star appeared in the constellation Ophiuchus, near the foot of the serpent handler, where no star had been the night before. Within days it was brighter than Jupiter. It stayed visible to the naked eye for about a year and then faded out.
Four centuries later, nothing like it has been seen again in our galaxy. That star is the last supernova in the Milky Way that anybody has watched with their own eyes.
Everyone Saw It Before Kepler Did
The first reports came from northern Italy, where Ilario Altobelli noted it on 9 October. Other observers picked it up over the following nights, and a correspondent in Prague, Johannes Brunowski, wrote to tell the imperial mathematician about it.
The imperial mathematician was Johannes Kepler, and he could not see it. Prague was clouded over. He did not get his own first look until 17 October, more than a week after the discovery, which he mentions with some irritation.
Once he could see it he did what he always did, which was measure carefully and for a long time. He tracked the position and the changing brightness through to the following October, and in 1606 he published De Stella Nova in Pede Serpentarii, on the new star in the foot of the serpent handler. The object has carried his name ever since, even though he was not the first to spot it, because his was the account that did the work.
The Timing Was Almost Too Convenient
In December 1603 Jupiter and Saturn had come together in a great conjunction, an event that occurs roughly every twenty years. This one fell in the constellation Sagittarius, beginning a new cycle through the zodiac, which in the astrology of the period was a serious matter. Then in 1604 Mars joined them.
Into this arrangement, a brilliant new star arrived.
The excitement across Europe was enormous and mostly astrological. Pamphlets predicted the end of the Ottoman Empire, the fall of the papacy, the conversion of the Jews, the Second Coming, and in several cases all of them. Kepler, whose salaried duties as imperial mathematician included casting horoscopes for Rudolf II, had to address this material, and his handling of it is characteristic. He wrote the astrological discussion, and he also wrote, in the same book, that if a new star means anything it is a sign to astronomers to look harder, and he devoted the bulk of the volume to measurement.
What the Measurements Established
The decisive question was distance, and the tool was parallax. If an object is close, observers in different places see it against a slightly different background, and its position shifts against the fixed stars as the Earth turns. The Moon shows a large parallax. A comet shows a small one.
The new star showed none at all. Within the accuracy of naked eye astronomy, which by then was very good indeed, it did not move relative to the stars around it. It was not an atmospheric phenomenon, not a comet, and not anything inside the sphere of the Moon. It was out among the fixed stars, in the region that Aristotelian physics held to be perfect, unchanging and incorruptible.
Galileo, then teaching at Padua, gave three public lectures on it to overflowing audiences and argued the same conclusion. The lecture notes survive in fragments. This is five years before he pointed a telescope at anything, and it is one of the earliest occasions on which he publicly used an observation to contradict Aristotle in front of a crowd.
Why the Second One Mattered More Than the First
A new star had appeared once before within living memory. In November 1572 a star had blazed up in Cassiopeia, bright enough to be seen in daylight, and Tycho Brahe had measured it with the accuracy that made his reputation and published De Nova Stella about it. He too had found no parallax.
One anomaly can be absorbed. A single unexplained event in the heavens could be treated as a special case, a miracle, or an error, and the doctrine of the immutable sky could be preserved with some effort.
Two in thirty two years, both measured by competent astronomers, both beyond the Moon, could not. The conclusion was unavoidable: the heavens change. Stars appear. Whatever the sphere of the fixed stars was, it was not perfect and unchanging, and a physics built on the assumption that it was had a serious problem.
Kepler was the right person to press the point, because he had already spent years dismantling another piece of the same inheritance. His work on the orbit of Mars, published in 1609, replaced circles with ellipses and abolished the uniform circular motion that had been required of the heavens since antiquity. The new star of 1604 was evidence from a completely different direction pointing the same way.
What It Actually Was
Kepler had no possibility of knowing. He speculated, in a passage that has aged remarkably well, that new stars might form from material in space rather than being created from nothing.
The modern account: SN 1604 was a supernova about 20,000 light years away, in the constellation Ophiuchus, and the evidence points to a Type Ia, the detonation of a white dwarf that has accumulated too much matter from a companion. At peak it reached roughly magnitude minus 2.5, brighter than Jupiter and second only to Venus among the objects of the night sky.
The remnant was found in 1943 by Walter Baade, who went back to Kepler’s recorded position and looked there. It is catalogued as 3C 358 and is now a shell of expanding gas about four light years across, observed in X rays and radio, still travelling outwards at thousands of kilometres per second. The light we see from it left four hundred years after the explosion had already happened 20,000 years earlier, which is worth pausing on: the star was gone long before there was anybody in Ophiuchus’s direction to be surprised.
Four Hundred Years of Nothing
The statistics suggest a supernova in the Milky Way every fifty years or so. Since 1604 there has not been one visible to the unaided eye.
The explanation is mostly dust. We sit inside the galactic disc, and the plane of the galaxy is full of obscuring material, so most of the supernovae that have gone off since 1604 have been hidden. Cassiopeia A probably exploded around 1680 and produced no reliable naked eye sightings at all. SN 1987A was bright and well studied but went off in the Large Magellanic Cloud, a satellite galaxy, not in our own.
So the entire history of the telescope, the whole of modern astronomy, spectroscopy, photography and space based observation, has taken place without one nearby supernova to point any of it at. Everything we know about them has been assembled from remnants, from distant galaxies, and from theory.
There is a mild irony in that. Galileo built his telescope five years too late. He lectured about the new star of 1604 with no instrument at all, and by the time he had one, the last new star anybody would see for four centuries had already faded.
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