Astronomers say our galaxy hasn’t seen a supernova in 400 years. Why don’t they count the 1987 supernova?
Judy Pearce
Belton, Texas
The 1987 supernova (SN 1987A) occurred in the Large Magellanic Cloud (LMC), not the Milky Way. The LMC is a smaller satellite of the Milky Way, but astronomers still consider it to be outside our galaxy. The 400-year figure refers to visual observations of supernovae in the Milky Way Galaxy itself.
Astronomers know other supernovae have occurred in the Milky Way since 1604, when Johannes Kepler observed one. But once scientists account for all known remnants and their ages, the Milky Way’s supernova rate still seems lower than that of other spiral galaxies. Astronomers suspect they’ve either misidentified or overlooked some remnants.
Of course, just because we don’t see supernovae doesn’t mean they aren’t occurring. Dust in our galaxy’s disk prevents a lot of light from reaching us. Infrared and radio wavelengths can penetrate this dust and reveal new remnants. Then, astronomers can watch how fast the remnants expand and figure out how old they are.
Supernova remnant Cassiopeia A, the brightest celestial radio source beyond the solar system, is only about 300 years old. The only recorded sighting was by John Flamsteed on Aug. 16, 1680, when he cataloged a star in the remnant’s position.
The Milky Way’s youngest known supernova remnant is a radio source called G1.9+0.3. The remnant, which lies 27,700 light-years away, is only about 140 years old. This means the supernova’s light swept over Earth about 140 years ago, but it took 26,000 years for light from the blast to reach us.
Radio astronomers using the Very Large Array (VLA) in New Mexico found that G1.9+0.3 grew by 16 percent between 1985 and 2008. That corresponds to an expansion velocity of 30 million mph (48 million km/h), or about 5 percent of light-speed. Because it’s so young, G1.9+0.3 is still getting hotter and brightening at radio frequencies.
Supernova remnants usually glow, thanks to interstellar gas heated by the explosion’s expanding shock wave. But astronomers say most of the energy G1.9+0.3 radiates arises from the debris of the exploded star.
Francis Reddy
Senior Science Writer, Astrophysics Science Division at NASA’s Goddard Space Flight Center, Greenbelt, Maryland
This question and answer originally appeared in the December 2008 issue.
