Huge Quake Cracks Star
Artist's impression of cracks on a neutron star's surface.
Credit: NASA

Astronomers have found the first evidence of cracks in a neutron star's crust. The star cracked when it was rocked by the strongest "starquake" ever recorded, researchers said last week.

Last December, astronomers worldwide monitored the explosion that caused this starquake. The eruption was huge – in the first 200 milliseconds of the event the star released energy equivalent to what our Sun produces in 250,000 years. The explosion was the brightest event ever detected outside of our Solar System.

Now scientists have used a collection of data from various satellites to provide the first observational evidence that the blast caused the star to form cracks several miles long. Scientists hope these cracks will provide a window into the mysterious interiors of neutron stars.

There are millions of neutron stars in the Milky Way galaxy alone, and some of these have magnetic fields trillions of times stronger than Earth's, the strongest of which are called magnetars.

This particular magnetar – SGR 1806-20 – is surrounded by the strongest magnetic field known in the universe. This could explain why the starquake – caused when the magnetar's crust could no longer contain the magnetic stress building in the star's interior – was so intense.

A magnetar's interior is a dense, liquid-like mix of neutrons, protons, and electrons – making it a terrific conductor of electricity. Because it has the characteristics of a fluid, it moves around a lot. The magnetar's magnetic field loops around the star, and all this movement in the interior messes with the field's shape, winding it up like you might do with a rubber band.

But the magnetar's exterior crust is not so pliable. The crust is made mostly of iron. The magnetic field passes through it in places, which isn't a problem for normal neutron stars. But in magnetars, the field interacts with the core and shifts around erratically, causing crustal stress. Eventually, the stress reaches the point where the crust cracks.

"Imagine threading a rubber band between two cards, and then twisting the middle," study leader Steve Schwartz of the Imperial College of London told "All those twisting stresses accumulate at the points where the rubber band threads through the card to the outside. Keep twisting long enough and you will rip the card."

The first crack to form was three miles (five kilometers long) – significant since this magnetar is only six miles (10 kilometers) in diameter. Radiation spewed from this crack, causing a steep initial increase in detectable radiation.

But that was just the beginning. Radiation continued to spill out of the star, but at a much slower rate than the initial burst. This suggests that cracks continued to form.

"Whether this is a set of long, [three mile] cracks, or a multitude of much smaller ones isn't obvious to me," Schwartz said. "My hunch is therefore: one big one, followed by lots and lots of ongoing smaller ones."

What this means for SGR 1806-20 isn't clear, but it seems that cracks form more to relieve pressure than as a sign that the star is blowing apart.

"The result of the cracking is to relax the interior and exterior field to a less twisted state," Schwartz said. "This has very little impact on the star itself, other than the fact that it will take time to twist up the field again."

SGR 1806-20 is 50,000 light-years away, but the blast was so huge it temporarily blinded some satellites and briefly altered Earth's upper atmosphere. A similar blast occurring within 10 light-years of our planet would fry Earth's ozone layer. But don't worry – the closest magnetar is 13,000 light-years away.

Two satellites designed to study the Earth's magnetosphere – the European Space Agency's Cluster and Double Star satellites – didn't go offline and recorded the entire event. Data from these two satellites was combined with observations from around the world to uncover the cracks.

So far, nine magnetars have been firmly identified, and four of these repeatedly emit bursts of X-rays and gamma rays. SGR 1806-20, which has a magnetic field more powerful than any other object in the universe, is one of these so-called soft gamma repeaters.

Researchers still don't know why SGR 1806-20's burst was so incredible, but they hope that a look into its cracks will help solve the mystery.