Meet G1.9+0.3, the Milky Way’s most recent supernova that nobody saw. This composite photo was created from three images. The red was taken by the light of X-rays with the Chandra space telescope in 2007; the blue by the Very Large Array (VLA) radio telescope in 1985. The background starfield is from a recent star survey made in infrared (heat) light. These days astronomers map the universe in as many wavelengths of light as possible for a more complete understand of what they’re seeing. Credit: X-ray (NASA/CXC/NCSU/S. Reynolds et al.); Radio (NSF/NRAO/VLA/Cambridge/D. Green et al.); Infrared (2MASS/UMass/IPAC-Caltech/NASA/NSF/CfA/E. Bressert)
Back around 1866 a massive star exploded near the center of the Milky Way. Obscured by light years of intergalactic dust, not a single person on Earth saw it. It cooked away for almost 120 years until astronomers in 1985, using a telescope that "sees" radio waves from the stars, found the brightly-glowing remnant of the blast in Sagittarius the Archer. It wound up in a catalog with the cryptic-sounding name of G1.9+0.3.
Radio waves easily pass through the fine dust left from previous generations of stars that litters the flat plane of our galaxy and can reveal objects that visible light cannot. Fast forward to 2007 when the orbiting Chandra X-ray observatory examined the same remnant in X-rays, another form of radiation that penetrates with ease. It also passes through skin and bone as we’re aware of when the dentist x-rays our teeth. Chandra showed that since 1985, the glowing ball of fast-moving gas from the explosion has been continuously expanding.
If we could see G1.9+0.3, it would lie just above the spout in the "teapot" of Sagittarius, now setting in the southwestern sky at mid-evening. Created with Stellarium.
Supernovas occur when stars much larger than the sun run out of the nuclear fuel they need to keep themselves lit. Stars stay aflame by first burning hydrogen inside their blazing cores to create helium. Later they chew through helium, carbon, oxygen, neon and more all the way to iron at which point the burning stops. You can’t burn iron to create energy as it turns out. With no burning pressure to hold back the force of gravity, the star collapses in upon itself and then rebounds in a titanic finale of fireworks, flinging material into space at 9,000 to 25,000 miles per second. I’ve seen supernovas in galaxies beyond the Milky Way that are so bright, they temporarily outshine their host galaxies. Additional elements are created in the blast, and both old material and new rush into space along the expanding edge of a shock wave. To live within 30 light years of one would be a terrifying experience. The blast would rival the sun in brightness and send enough energy our way to damage the ozone layer of our atmosphere, allowing large doses of harmful ultraviolet radiation from the sun to reach the ground.
The good news is that chances are slim this will anytime soon. There are no candidate stars anywhere near that close to Earth.
Supernovas do good things too like enrich space with heavier elements like carbon, oxygen, magnesium and iron that are excellent materials for building planets and other stars. And that shock wave? As it sails away from the blast, it compresses existing dust and gas in the star’s neighborhood into dense nebulas within which new stars can form. When it comes to the cosmos, creativity and rebirth are often coupled to violence events. Many supernovas leave behind a supernova remnant or cloud of expanding gas and dust. Some sport a special surprise — a hot, superdense object called a neutron star at their centers or even a black hole.
The lovely SN1987A in the Large Magellanic Cloud. The former star, now invisible at center, is encircled by three rings of glowing gas. One of red rings lies in front of the supernova, the other behind it. The bright spots are beams of radiation related to a possible neutron star or black hole at the center of the remnant. Credit: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis
G1.9+0.3 is one a series of supernovae that we’re aware of in our galaxy. The last one that was visible with the naked eye happened in 1987 (top) in one of the Milky Way’s satellite galaxies called the Large Magellanic Cloud, visible from southern latitudes. Before that we have to go back to 1680 to Cassiopeia A which may have been spotted by just one astronomer at the time as a very faint naked eye star. Scientists believe Cas A was faint because it may have cloaked itself in multiple layers of dust before the final blast. To this day, Cas A and numerous other supernova remnants continue to expand like a slo-mo smoke rings.
You can see that all these pretty baubles all have something in common — a roughly spherical shape created by outrushing gases and dust from the center of the original explosion. The photos were taken by the light of X-rays and/or radio waves. All these remnants except the Crab are faint in visible light but emit plenty of X-rays and radio waves as they expand into space.
Further back in time we arrive at Kepler’s supernova which flared into view in the constellation Ophiuchus in the fall of 1604 just a few years before the telescope was invented. This one was widely seen since it grew to be as bright as Jupiter. It was named for astronomer Johannes Kepler, famed for his laws of planetary motion. In 1572 the astronomer Tycho Brahe was the first to report on a "new star" in Cassiopeia that matched Venus in brightness and was visible in the daytime.
RCW 86, the oldest recorded supernova, was seen by Chinese astronomers in 185 A.D. Today a dim remnant glows in the light of X-rays is all that’s left. Credit: handra: NASA/CXC/Univ. of Utrecht/J.Vink et al. XMM-Newton: ESA/Univ. of Utrecht/J.Vink et al.
Before that the supernova of 1054 A.D. in Taurus the Bull grew to be as bright as Venus and was studied closely by the Chinese but inexplicably ignored by the Europeans. Were they blinded to its presence because it violated the widespread belief that stars never changed? When we point our telescopes there today, we see the leftover remains — an eerie, crab-shaped nebula with a neutron star spinning 33 times a second at its center. The supernova in Lupus the Wolf near Scorpius in the year 1006 was the most brilliant on record, rivaling the first quarter moon in brightness while the earliest recorded was in 185 A.D. by the Chinese.
This begs the question of where the supernovas are nowadays. Unless you count Cas A, not a one has been seen since 1604, a gap of over four hundred years. Have they all been hiding behind curtains of galactic dust or will the next one appear tomorrow evening like a mischief maker’s cherry bomb thrown in your front yard?
(Credits for supernova panel above– Cas A:NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.; Kepler: NASA/CXC/NCSU/S.Reynolds et al.; Crab: NASA/CXC/ASU/J.Hester et al.; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz and Lupus: X-ray: NASA/CXC/Rutgers/G.Cassam-Chenaï, J.Hughes et al.; Radio: NRAO/AUI/NSF/GBT/VLA/Dyer, Maddalena & Cornwell; Optical: Middlebury College/F.Winkler, NOAO/AURA/NSF/CTIO Schmidt & DSS)