Approximately 10 million years ago, a nearby cluster of stars erupted as a violent series of supernovae and, according to new observations, the million degree plasma from these powerful detonations surround the solar system today.
Although astronomers have known about this tenuous “Local Bubble” of gas for some time, a suborbital NASA sounding rocket launched by scientists in 2012 has removed any doubt about the origins of this 300 light-year wide feature we are cocooned within.
At a temperature in excess of a million degrees Kelvin, the supernova ‘soup’ is actually very tenuous — around 0.001 atoms per cubic centimeter — and very different from other particles of matter that occupy the interstellar medium.
The idea is that several neighboring massive stars went supernova after reaching the ends of their lives. Although supernovae are very powerful, apparently they weren’t close enough to Earth to sterilize our planet of life. The supernovae occurred during the early stages of human evolution.
Evidence for the Local Bubble has been brewing for several decades and its presence was inferred from X-ray observations of the local galaxy — a background glow of X-ray radiation was detected in all directions. Although the evidence seemed strong for an ancient supernova soup, there was other possible interpretation.
“Within the last decade, some scientists have been challenging the (supernova) interpretation, suggesting that much or all of the soft X-ray diffuse background is instead a result of charge exchange,” said F. Scott Porter of the Goddard Space Flight Center, Greenbelt, Md.
Charge exchange can occur between solar wind ions (charged particles that lack electrons) and neutral gases. When the two gases come into contact within the solar system — between the solar wind and a comet’s coma, for example — electrons can be stripped from the neutral particles, generating X-ray emissions.
Therefore, many astronomers argued that this diffuse X-ray glow observed in all directions may be a phenomenon inside the solar system and not superhot particles from 10 million year-old supernovae outside the solar system.
To put this debate to bed, an international team of scientists constructed an instrument called the Diffuse X-ray emission from the Local Galaxy (or DXL), which could distinguish between the two scenarios. After launching the instrument 160 miles in altitude atop a sounding rocket, well above the Earth’s atmosphere, the instrument specifically detected the amount of charge exchange that is occurring within interplanetary space.
After 5 minutes of observations on Dec. 12, 2012, the DXL was able to determine that only 40 percent of the diffuse soft-X-ray emissions were generated by charge exchange. The rest must be coming from the Local Cloud from outside the solar system — so therefore the Local Bubble is real and we did indeed get rumbled by a series of supernovae 10 million years ago.
“This is a significant discovery,’ said Massimiliano Galeazzi, of the University of Miami in Coral Gables, who led the team. “(It) affects our understanding of the area of the galaxy close to the sun, and can, therefore, be used as a foundation for future models of the galaxy structure.”
This research was published in the journal Nature on July 27.