The scene witnessed by Johannes Kepler after sunset on Oct. 17, 1604. While he wasn’t the first to see the supernova, Kepler studied it like no one else. To recognize his detailed observations, we now call it Kepler’s Supernova. Created with Stellarium
Hard to believe it’s been over 400 years since anyone’s seen a supernova in the Milky Way galaxy. Amateur and professional astronomers spot them all the time in galaxies external to our own. In 2012 alone 1,048 supernovas were discovered thanks to numerous, mostly-automated searches that photograph hundreds of galaxies each night looking for signs of exploding stars. Given such a large sample, it should come as no surprise that dedicated supernovae searches turn up these “new stars” routinely.
Johannes Kepler in 1610
The last person to see one with his own eyes was astronomer Johannes Kepler, famous as the first person to explain the Laws of Planetary Motion. The new star blazed forth on October 9, 1604 in southern Ophiuchus, a summertime constellation visible in the southwestern sky during early fall. Observing from Prague, overcast skies prevented his from seeing it until the 17th.
When the clouds parted, Kepler got an eyeful. The supernova, which soon waxed as brilliant as Jupiter (mag. -2.5), shone in the company of three bright planets – Mars, Saturn and Jupiter. He must have been beside himself with amazement at the gathering of so many shiny objects together in one corner of the sky.
Kepler relentlessly observed the new star every clear night until at dusk and in the winter months at dawn until it finally faded from view in March 1606. Not long after, he penned a book with his observations titled De Stella Nova. In it Kepler compared the new star with the Star of the Magi and speculated that it might lead to the conversion of the Indians in America. You can flip through a digital version of the tome HERE.
Only 3 years later Galileo would turn one of his early telescopes to the sky. Had the telescope been invented just a few years earlier, Kepler could have continued his supernova observations much longer.
The expanding supernova remnant Cassiopeia A is located near the place John Flamsteed recorded a star in 1680. Working back from the currently observed expansion point to an explosion indicates the supernova would have been visible in the sky around 1667. Photo made with the Hubble Space Telescope. Credit: NASA / ESA
In 1680 English astronomer John Flamsteed recorded a faint 6th magnitude star in the constellation Cassiopeia during his compilation of a new star catalog. Labeled 3 Cassiopeiae, the star was omitted from later catalogs since it couldn’t be found again. 300 years later astronomers discovered a faintly-glowing husk of light called a supernova remnant – the leftover, expanding clouds of gas and debris near the position of Flamsteed’s star. It’s possible he may have seen it during its explosive phase or simply made a mistake in a star position.
Composite of photos showing an all-sky view of the Milky Way. The dark blobs are cosmic dust within the galaxy that blocks the light of stars behind it. Copyright: Axel Mellinger
Previous to Kepler’s supernova was the great daylight supernova of 1572 studied by Kepler’s mentor Tycho Brahe. That’s two in 32 years and nothing since. What gives? Astronomers estimate a star goes boom in the Milky Way galaxy about once every 50 years. Shouldn’t we have seen another in 400 years? Blame it on cosmic dust.
Stardust shed by supernovae and through gentler means permeates space and concentrates in the plane of the galaxy where most of the Milky Way’s approximately 400 billion stars reside. To stare across a few light years of space, you’d never notice it. But over hundreds of light years the dust adds up, effectively screening billions of stars from view and greatly reducing chances of seeing a supernova anytime soon.
Supergiant stars like Betelgeuse in Orion and Antares in Scorpius are both extremely large and extremely rare.
There’s another factor, too. One class of supernovas, called Type II, happen when a supergiant star runs out of fuel to burn. With no heat to hold back the inward pull of gravity, it implodes and then explodes.
These monsters are rare. 80% of stars are tiny, long-lived red dwarfs; 3.5% are stars like our sun and supergiants account for measly 0.001% of known stars. With so few to pick from and dust an issue, it’s yet another way the odds are stacked against seeing a local supernova.
Radio images made by the VLA of supernova remnant G1.9+0.3 show it expanding about 15% over a period of 23 years. Credit: NRAO / VLA / D. A. Green
Not that the galaxy’s been slacking off. The most recent Milky Way supernova was announced in May 2008 by a team of astronomers using the dust-busting Very Large Array (VLA) radio telescope in New Mexico, and confirmed in X-ray images from the Chandra X-ray Observatory in orbit. Both telescopes study the sky at wavelengths of light than can penetrate dust to see what lurks beyond.
G1.9+0.3 is another supernova remnant like Cas A – the expanding remains of the exploded star. It’s located close to the center of the galaxy and utterly obscured from visual view by thick clouds of interstellar dust. Working backward from now to the point of explosion, astronomers estimate the star went supernova about 140 years ago. Were it not for dust, citizens of the Victorian era would have marveled at their new star.
This is the remnant of Kepler’s Supernova. The red, green and blue colors show low, intermediate and high energy X-rays observed with NASA’s Chandra X-ray Observatory, and the star field is from the Digitized Sky Survey. The remnant resulted for Type Ia explosion, where a tiny, dense white dwarf star pulls material from a close companion star onto its surface, becomes unstable and explodes. Other supernovas happen when a supergiant star runs out of fuel at the end of its life and implodes-explodes. Click photo for more info. Credit: NASA/CXC/NCSU/M.Burkey
Somewhere out there a Milky Way star has self-destructed in a supernova explosion. Light from the cataclysm is streaming our way at this very minute. Will it pass unscathed and surprise us tonight or be foiled by dust once again?