In 1936, scientists witnessed one of the most extreme events ever seen. It involved FU Orionis, a star about the size of the Sun, 1,500 light-years away from Earth in the constellation Orion.
With little warning, it started to binge on material from the surrounding disc of gas and dust, becoming 100 times brighter over just three months in the process, and sending temperatures in the disc soaring to 7,000 Kelvin (6,726.85 °C).
FU Orionis has eaten the equivalent of 18 Jupiters in the past 80 years and is still devouring gas to this day, although not as quickly.
Scientists believe the phenomenon could give new insights into the way planets are formed – a window on the “baby years” of a solar system, as Joel Green of the Space Telescope Science Institute in Baltimore puts it.
He says that massive temperature increase in the surrounding disc has probably changed the chemistry in material destined to turn into planets.
“Our own Sun may have gone through a similar brightening, which would have been a crucial step in the formation of Earth and other planets in our solar system,” says Green.
The star is now fading from its initial 1936 burst, raising new questions about whether FU Orionis is still gorging on the material in the disc and if it will return to its pre-burst brightness.
To measure it, Green and his team compared infrared data obtained in 2016 using the Stratospheric Observatory for Infrared Astronomy, SOFIA, to observations made with NASA’s Spitzer Space Telescope.
By combining data from the two telescopes collected over a 12-year interval, we were able to gain a unique perspective on the star’s behaviour over time,” said Green, who presented the results at the American Astronomical Society meeting in San Diego.
The study shows that total amount of visible and infrared light energy coming out of the FU Orionis system decreased by about 13% over the 12 years since the Spitzer observations.
This was caused by dimming of the star at short infrared wavelengths, but not at longer wavelengths, suggesting up to 13% of the hottest material of the disc has disappeared, leaving colder material intact.
“A decrease in the hottest gas means that the star is eating the innermost part of the disc, but the rest of the disk has essentially not changed in the last 12 years,” Green said.
“This result is consistent with computer models, but for the first time we are able to confirm the theory with observations.”
Astronomers predict, partly based on the new results, that FU Orionis will run out of hot material within the next few hundred years. The star will then return to the state it was in before 1936.
Green and his team believe that if our Sun had a brightening event to the one FU Orionis had in 1936, it could explain why certain elements are more abundant on Mars than on Earth.
Such a sudden brightening would have altered the chemical composition of material close to the star, but not as much farther from it.
Because Mars formed farther from the sun, its component material would not have been heated up as much as Earth’s was.
Green plans hopes that observations with NASA’s James Webb Space Telescope, due for launch in 2018, will give new insights into what has been going on.
