Astronomers Can Now Predict When a Neutron Star Will Collapse Into a Black Hole

April 12, 2016 | Joanne Kennell

Artist's impression of a neutron star
Photo credit: Kevin Gill/flickr (CC BY-SA 2.0)

“This result has always been in front of our eyes.”

Neutron stars are one of the most awe-inspiring objects known to exist in our universe. They are the smallest of all the stars, with a mass up to twice that of the sun, but a radius of only a dozen miles — meaning they are extremely dense. In fact, they are thousands of billions of times more dense than the densest elements found here on Earth.

Another important and often unknown property of neutron stars, which distinguishes them from normal stars, is that their mass can’t grow without bound. For example, if a non-rotating star increases its mass, its density will also increase.

Normally, this leads to a new balance, and the star can live stably in this state for thousands of years. However, this process cannot repeat itself indefinitely, and eventually the accreting star will reach the “maximum mass” that represents an upper limit to the mass that a non-rotating neutron star can be.

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So what happens once this maximum mass is reached? There are two possibilities. First, it can collapse into a black hole. Second, it can start rotating. A rotating star can actually support a larger mass than a non-rotating star because the additional centrifugal (outward) force can help balance the gravitational force.

However, the star cannot become unlimitedly massive because an increase in mass is also accompanied by an increase in rotation, and there is a limit to how fast a star can rotate before it gets ripped apart. So, there must be an absolute maximum mass — and this should be given by the largest mass of the fastest-spinning neutron star.

Determining this value is really quite difficult because it depends on the state of the matter composing the star, which is still essentially unknown to astronomers. Because of this, determining the maximum mass of a rotating neutron star has been an unsolved problem for decades.

But this recently changed. The new study, published in the Monthly Notices of the Royal Astronomical Society, states that it is possible to predict the maximum mass of a rapidly rotating neutron star by just considering what its maximum mass would be if it were non-rotating.

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“It is quite remarkable that a system as complex as a rotating neutron star can be described by such a simple relation,” said Professor Luciano Rezzolla, one of the authors of the publication and Chair of Theoretical Astrophysics at the Goethe University in Frankfurt, in the press release. “Surprisingly, we now know that even the fastest rotation can at most increase the maximum mass of 20% at most.”

This quite simple but important result opens the door for more universal relationships to be found in rotating stars. “This result has always been in front of our eyes, but we needed to look at it from the right perspective to actually see it,” Cosima Breu, a master’s student at the University of Frankfurt, who analyzed the data during her bachelor’s thesis, said in the release.

This universal behavior found for the maximum mass is actually part of a larger class of universal relationships recently discovered for neutron stars. Using the same idea, Breu and Rezzolla have proposed an improved way to express the moment of inertia (angular mass) of these rotating stars in terms of their compactness, which would allow astronomers to measure the radius of stars with a precision of 10 percent or less.

Rezzolla concluded in the release, “We hope to find more equally exciting results when studying the largely unexplored grounds of differentially rotating neutron stars.”

And we can’t wait to hear about them.

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