Strange fast-spinning stars called magnetars get their names from the fact that they are the universe's most powerful magnets and unleash massive amounts of radiation. But now scientists have found that some magnetars can release mighty explosions without needing giant magnetic fields as previously thought.

Magnetars are a kind of pulsing neutron star (or pulsar) — a remnant of a star left over after a supernova, when a star's explosion crushed protons together with electrons into neutrons. They are a thousand times more magnetic than an ordinary neutron star and 10 trillion times more so than a refrigerator magnet.

Neutron stars are some of the densest objects known. Although they are usually only about 12 miles (20 km) across, they are often 1.5 to 3 times the mass of the sun. A piece of a neutron star the size of a sugar cube can weigh about 100 million tons, or roughly as much as a mountain.

Now researchers have discovered a magnetic neutron star that may rewrite the book on magnetars — SGR 0418+5729, roughly 7,000 light-years from Earth.

Measuring a magnetic star

Astrophysicist Nanda Rea at the Institute of Space Sciences in Barcelona, Spain, and her colleagues measured how fast SGR 0418+5729 was spinning using data from four different X-ray space observatories.

The more powerful the magnetic field of a neutron star is, the more it tends to slow down the speed of the star's spin. As such, measuring this slowdown allows researchers to estimate the intensity of the neutron stars' surface magnetic fields.

Typical magnetars have magnetic fields measuring up to a million billion gauss in strength — strong enough to rip atoms apart. Only 16 or so magnetars are known to date.

But the researchers said their magnetar appeared to have a magnetic field that is roughly as small as those typically seen with ordinary neutron stars. [The Strangest Things In Space]

This suggests a lot of regular neutron stars "might start behaving as magnetars at anytime, regardless of their low magnetic fields," Rea told

Magnetar's hidden power?

Magnetars are believed to include stars known as soft gamma repeaters (like SGR 0418+5729) and anomalous X-ray pulsars, which are known for explosions of gamma rays and X-rays that can last from days to years. Scientists had thought the magnetars' powerful magnetic fields were the driving force behind these extraordinary bursts.

The researchers suggest the source of their magnetar's power might be hidden deep within its surface. Although its surface magnetic field might not be as strong as typically seen in magnetars, it could still have a huge internal magnetic field.

"It is the very first time this has been observed and the discovery poses the question of where the powering mechanism is in this case," study co-author Silvia Zane, of the University College London, said in a statement. "At this point, we are also interested in how many of the other normal, low field neutron stars that populate the galaxy can at some point wake up and manifest themselves as a flaring source."

SGR 0418+5729 is 24 million years old, making it quite older than other soft gamma repeaters and anomalous X-ray pulsars.

"We might be seeing an old population of magnetar which are dissipating in bursts its last bit of its internal magnetic energy," Rea said.

The research is detailed in the Oct. 15 issue of the journal Science.