Yale Astronomers Study Superflares on Stars Just Like Our Sun
In the Greek myth of Phaethon, the chariot of the Sun inadvertently drives too close to the Earth, creating the Sahara desert with its scorching heat. Whether that myth is based in some small part on observations of a “superflare” emanating from the Sun and scorching the Earth in past millennia is a question that intrigues Yale University astrophysicists Bradley E. Schaefer and Eric P. Rubenstein.
At a news conference today during the annual meeting of the American Astronomical Society in Austin, Tex., Schaefer and his colleagues reported that nine stars on which superflares have been observed during the past century are disturbingly similar to our Sun in size, age, luminosity and rotation speed. (Other collaborators on the research were Jeremy R. King, Space Telescope Science Institute, Baltimore, Md.; and Constantine P. Deliyannis, Indiana University, Bloomington).
“It’s only natural to ask what would happen on Earth if such a superflare were to suddenly occur on our Sun, or to speculate why such flares apparently have not happened here,” Schaefer said. He noted that a superflare – a flare 100 to 10 million times larger than the largest flare ever seen on our Sun – would severely disrupt radio communication, burn out all orbiting satellites, black out power grids worldwide, and create spectacular auroras visible from the poles to the equator.
“Large superflares could warm a cold winter day into a hot summer day,” he said. “But the primary damage would come from high energy radiation, which would react in the Earth’s upper atmosphere to destroy the protective ozone layer for several years, thereby exposing the Earth’s surface to harmful ultraviolet radiation with subsequent collapse of the food chain.”
Fortunately, such grim possibilities appear to be unlikely, the researchers agreed. Any superflares on our Sun during the last 150 years of scientific monitoring would certainly have been noted, while any superflare within the last two millenia would likely have appeared in the historical record as a sudden heat wave or global aurora, Schaefer said. Furthermore, a large superflare probably would have melted the icy surfaces of moons around Jupiter and Saturn, forming vast flood plains. The absence of smooth frozen surfaces on these moons means that large superflares have not occurred in the last billion years or so. “Despite the myth of Phaethon, our Sun apparently has only rare superflares, if any,” Schaefer concluded.
Stars like our Sun have superflares an average of about once a century, the researchers calculated, and it is just this type of star around which planets recently were discovered, opening the door to the exciting possibility of organic life flourishing elsewhere. It is unknown whether recurring superflares would encourage evolution by providing an energy source for prebiotic chemical reactions or would prevent new lifeforms from gaining a foothold.
Next, Yale scientists hope to find more examples of superflares by monitoring a very large number of stars – a feat that is possible using a Yale camera mounted on a telescope in Venezuela, which nightly scans more than a million solar-type stars in search of mysterious distant objects called quasars as part of the QUEST project. In addition, theoretical work to understand the energy-release mechanism of stars might help answer the question of which stars are prone to superflares, Schaefer said.
One theory currently being studied by Rubenstein is that the stars on which superflares have been observed have relatively strong magnetic fields that interact with a nearby large planet about the size of Jupiter, causing the build-up and periodic release of vast amounts of energy.
Rubenstein believes that these outbursts are similar to energetic eruptions observed from some stars in binary systems, in which a pair of stars are gravitationally bound together and orbit around each other. A category of binary stars called RS CVn binaries routinely have eruptions that release as much energy as superflares, he said.
While astronomers still don’t know all of the details of what causes RS CVn binaries to flare, most accept the theory that energy is released from the intertwined magnetic fields between the two stars. At some point, the twisted fields suddenly reorganize into a simpler geometry via a process called magnetic reconnection, Rubenstein said. When this event occurs, the magnetic field emerging from one star becomes connected temporarily to the other star, and vice versa. Stored energy is released in the form of light and X-rays as the magnetic reconnection occurs. “A similar process would be a physical system composed of, say, rubber bands twisted together,” Rubenstein said. “When the elastic bands are released, they suddenly snap and fly off. The energy is released and channeled into propelling the rubber bands instead of producing light.”
All nine of the superflares identified by Schaefer are either from single stars or stars with companions too distant to interact magnetically with the flaring star. However, a nearby planet could cause the same reaction, even though the planet would be difficult to detect, Rubenstein noted.
“Until two years ago, no planets outside of our solar system had been detected. Now, more than a dozen planetary systems have been found, most of which have planets with masses comparable to Jupiter that orbit close to the parent star. In some cases, these planets are closer to their star than Mercury is to our Sun,” Rubenstein said.
If the Jupiter-sized planets around other stars also have strong magnetic fields like Jupiter, the combination of proximity and magnetic field strength could lead to magnetic interactions similar to those observed in RS CVn binary systems. That interaction would then lead to energy being stored and subsequently released in the form of a superflare. “Fortunately for us, there is no danger of a magnetic reconnection being triggered by Jupiter, which is too far from the Sun, or triggered by the inner four planets, which have much smaller magnetic field strengths,” Rubenstein said.
Rubenstein’s hypothesis can be tested by searching for “stars that have strong magnetic fields and large, close-by planets,” Schaefer said. “Such stars would be more prone to superflares. In the meantime, we should not fear Phaethon’s chariot.”