Milky Way's youngest supernova found

Remnants of 140-year-old stellar explosion are located close to our galaxy's centre.

Radio observations from 1985 (blue) combined with X-ray data from 2007 (orange) reveal the expanding remains of the exploded star. Credit: X-RAY (NASA/CXC/NCSU/S. REYNOLDS ET AL.); RADIO (NSF/NRAO/VLA/CAMBRIDGE/D. GREEN ET AL.)

Astronomers have released baby pictures of the youngest supernova yet discovered in the Milky Way.

The images, taken in X-ray and radio wavelengths, reveal the expanding remnants of a star that exploded close to the centre of the Galaxy a mere 140 years ago. That makes it, by two centuries, the most recent supernova known in the Milky Way.

Sky-gazers in the nineteenth century probably missed out on seeing the explosion, as it was obscured by gas and dust. It took modern astronomical instruments to detect X-rays and radio waves penetrating the dust, discover the remnant and now determine its age.

"The discovery addresses a puzzling lack of recent supernovae in our Galaxy and also allows us to examine the remnant of a supernova at a stage that’s never been observed before," says study author Stephen Reynolds of North Carolina State University in Raleigh. The find was announced Wednesday at a press briefing held by the NASA Chandra X-ray Observatory in Cambridge, Massachusetts. The results are detailed in two upcoming papers1,2.

Finding such a close and young supernova remnant should yield clues on how stars end their lives, expanding in a rapid shock wave that spews heavier elements such as iron and calcium into the Universe. "This is a stellar death all right, and the corpse is still warm,” says astronomer Robert Kirschner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Invisible remains

The supernova remnant, dubbed G1.9+0.3, was first found by radio astronomers using the Very Large Array in 1985, but its age was unclear. Follow-up X-ray observations last year, taken with the Chandra satellite, showed that the object had enlarged by 16% over the intervening 22 years. This expansion is the fastest yet seen in a remnant, Reynolds says. The speed of the expansion has also allowed the researchers to backtrack to the time of the explosion, estimating the supernova's age at 140 years.

The find fills in a strange gap in the record of supernova explosions. Spiral galaxies such as the Milky Way are supposed to generate roughly three supernovae per century. Astronomers thus expect to see as many as 60 supernova explosions that are younger than 2,000 years old, but fewer than 10 have been found.

The Milky Way may have an anomalously low rate of stellar explosions, says Reynolds. But a search of potential supernovae, of which the latest G1.9+0.3 find is part, might help fill in the gap.

Energetic youth

Already, G1.9+0.3 has yielded some unexpected surprises. In addition to its fast expansion, the radio signals the remnant is emitting seem to be getting brighter.

This is the first time this brightening has been seen in remnants, says Reynolds. Its cause is unknown, but studying it could yield information on how the supernova's shock wave energizes electrons.

Previously, the youngest-known remnant in the Milky Way was Cassiopeia A, which exploded some 330 years ago. Astronomers have caught stellar explosions in the act in other galaxies, but can only follow the light of a supernova for a few years. In between several years and 330 years, "there’s this huge dark ages," says Reynolds. The new find could help illuminate what happens in the first few hundred years after a star explodes.

“Clearly there’s going to be years of research that’s going to be put into this object,” says radio astronomer Chris Salter of the Arecibo Observatory in Puerto Rico. “In a way, you could call this object the missing link that ties together the observations of very young supernova remnants in other galaxies to the remnants in our own galaxies."

The last naked-eye discovery of an exploding star in the Milky Way was in 1604. Sometimes called Kepler's star, the supernova was brighter than Jupiter when first seen.

References

  1. 1

    Reynolds, S. P. et al. Astrophys. J. Lett. 10.1086/589570 (in the press).

  2. 2

    Green, D. A. et al. Mon. Not. R. Astron. Soc. (in the press).

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Courtland, R. Milky Way's youngest supernova found. Nature (2008) doi:10.1038/news.2008.827

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