5 Feb 2017

Star predicted to explode in 2022


Prediction always is a tricky business, especially in astronomy.

It seems as if the universe always has a surprise whenever we look with a new telescope or use a new wavelength of light. However, sometimes you get lucky with your predictions.

A good example, announced at the January 2017 meeting of the American Astronomical Society, is the prediction of a new nova.

About half of all stars you see in the night sky actually are binary star systems, where two stars orbit each other. These binary stars appear to the eye as a point of light because they are so distant.

Careful observations can determine the orbital period of binary stars, and in a few special cases, the orbits become faster and faster. This happens if the stars happen to be extremely close, so that their stellar atmospheres almost touch and then slowly start to merge. As the two stars get closer, they spin faster, much like an ice skater who spins faster when she pulls her arms in.

Such was the case for a star system called V1309 Sco in the constellation Scorpius. In 2008, it was seen to orbit faster and faster until it exploded as a luminous red nova. (A nova is a star that suddenly becomes several thousand times brighter.)

Most nova are bluish, following the explosion of a white dwarf star. The colour of the light gives an indication of the temperature of the explosion, which in turn suggests what kind of nuclear reactions are happening inside the explosion. A red nova is rare, and is the result of the merging of two stars.

At the recent society meeting, astronomer Larry Molnar of Calvin College in Michigan predicted that a red nova will occur in 2022, give or take a year.

He said the binary star system called KIC 9832227, close to the constellation Cygnus the swan, is spinning in a similar fashion to V1309 Sco. If Molnar is correct, this would be the first prediction of a star system going nova.

Molnar and colleagues will be observing KIC 9832227 in the next year over the full range of wavelengths: using the Very Large Array, the Infrared Telescope Facility, and the XMM-Newton spacecraft to study the star’s radio, infrared and X-ray emission, respectively.

With this prediction in hand, the next question is what kind of equipment would you need to observe it? Because this binary star system is reasonably close by galactic standards, at only 1,800 light-years, it should be bright enough to see with the naked eye. In contrast, the previous red nova in 2008 was about six to seven times more distant, and consequently dimmer.

In addition to seeing the light from the red nova in the sky, there is the possibility of detecting it in other ways.

The nuclear reactions that power a red nova probably emit a burst of neutrinos. Whether today's neutrino detectors are big enough to see this is unknown because the mechanism that powers a red nova is still speculative.

Another possibility would be to measure gravitational waves from the merger of two stars. Such a gravitational wave would be millions (or billions) of times smaller than that for merging black holes, but if the red nova is close enough, its signal would be big enough to measure.

The problem is that the frequency of the waves is too high for current facilities. However, there is a proposal for a space-based gravitational-wave detector that could measure these higher frequencies.

Whatever the method of detection, the prediction is a first, and 2022 should be an exciting time for amateur and professional astronomers alike.

Kenneth Hicks is a professor of physics and astronomy at Ohio University in Athens.

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