Scientists May Be Missing Many Star Explosions
Artist's impression of the central system of the recurrent nova RS Ophiuchi early in its 2006 outburst. Hydrogen-rich material from the red giant falls onto the surface of its companion white dwarf, leading to a runaway thermonuclear explosion. Ejecta travelling at several thousand kilometers per second slams into the wind of the red giant, setting up shocks with temperatures more than 10 times higher than in the sun's core. RS Ophiuchi may end its life as a Type Ia supernova.
Credit: STFC/David Hardy.

Some of the brightest stellar explosions in the galaxy may be flying under astronomers' radar, a new study suggests.

Researchers using observations from a sun-studying satellite detected four novas — exploding stars not quite as bright or dramatic as supernovas. The scientists were able to follow the explosions in intricate detail over time, including before the novas reached maximum brightness.

While other astronomers had discovered all four novas before, two of them escaped detection until after they had reached peak luminosity, the study revealed. This fact suggests that many other stellar explosions — even some that are incredibly bright — may be occurring unnoticed, researchers said. [Illustration of nova]

"So far, this research has shown that some novae become so bright that they could have been easily detected with the naked eye by anyone looking in the right direction at the right time, but are being missed, even in our age of sophisticated professional observatories," study lead author Rebekah Hounsell, a graduate student at Liverpool John Moores University (LJMU) in England, said in a statement.

The new observations are also allowing scientists to study nova explosions in unprecedented detail, according to researchers.

"The data have in fact opened a new chapter in our observations and understanding of novae," Hounsell added.

A sun-monitoring satellite

Hounsell and her colleagues analyzed measurements from an instrument aboard the U.S. Department of Defense's Coriolis satellite. The instrument, called the Solar Mass Ejection Imager (SMEI), was designed to detect disturbances in the solar wind. SMEI maps out the entire sky during its 102-minute orbit around the Earth.

The researchers found that SMEI was also detecting star explosions, or novas. Novas occur when small, extremely dense stars called white dwarfs suck up gas from a nearby companion star, igniting a runaway thermonuclear explosion.

Unlike supernovas, novas do not result in the destruction of their stars. Stars can go nova repeatedly.

SMEI detected four novas, including one confirmed repeater called RS Ophiuchi, which is found about 5,000 light-years away in the constellation Ophiuchus. RS Ophiuchi may ultimately die in a supernova exposion — one of the brightest, most dramatic events in the universe — researchers said.

Ground-based instruments missed the peak flare-up of two of these four novas, according to researchers. That suggests that space-based instruments like SMEI might be needed to pick up many novas, after which their progress can be tracked with telescopes on the ground, researchers said.

"Two of the novae observed by SMEI have confirmed that even the brightest novae may be missed by conventional ground-based observing techniques," said co-author Mike Bode, also of LJMU.

The researchers reported their results in a recent issue of the Astrophysical Journal.

Getting to know novas

The new observations are giving astronomers key insights into novas' earlier days, revealing a great deal about how they start and evolve, researchers said.

"The SMEI's very even cadences and uniformly exposed images allow us to sample the sky every 102 minutes and trace the entire evolution of these explosions as they brighten and dim," said co-author Bernard Jackson of the University of California, San Diego.

The new observations have revealed, for example, that three of the explosions faltered significantly before regaining strength and proceeding. Such a "pre-maximum halt" had been theorized before, but evidence for its existence had been inconclusive, researchers said.

Since SMEI performs a survey of the entire sky every 102 minutes, the instrument could also help astronomers understand a wide variety of transient objects and phenomena, according to the research team.

"[This] work has shown how important all-sky surveys such as SMEI are and how their data sets can potentially hold the key to a better understanding of many variable objects," Bode said.