What's the strongest magnet known in the Universe?

The answer is the magnetar, a rare type of neutron star. You wouldn't want one of these hanging on your refrigerator. Magnetars are so magnetic that they could strip the information off of your gift card or credit card at a distance of 100,000 miles, or halfway to the Moon. Get much closer, and the magnetic force will rearrange the atoms in your body.

There are many kinds of magnets, but essentially these are objects that produce a force called a magnetic field -- a force created by electrical currents, or a flow of electrons. This force is measured in a unit called Gauss.

Refrigerator magnets are actually extraordinary devices, with a force stronger than the Earth's magnetic field. The Earth's magnetic field is about 0.5 Gauss. The Sun's magnetic field is about 1 to 5 Gauss. A refrigerator magnet is about 100 Gauss. How about the magnetar? Keep going.

On Earth, scientists can produce magnets as strong as 40,000 Gauss. That's rather powerful but puny compared to the magnetic field of a typical neutron star, over 100 trillion Gauss (that's 100,000,000,000). A neutron star is the core remains of a star at least eight times more massive than the Sun that exploded in a supernova event. Neutron stars are highly compact, fast-spinning objects with about a Sun's worth of mass compressed into a sphere roughly ten miles in diameter.

The neutron star has a solid crust about a half-mile thick and a center made of a "superfluid" of neutrons. The crust and fluid spin at different speeds, and this, scientists say, likely creates the powerful magnet. Magnetars, for reasons not understood, are up to a thousand times more magnetic than a neutron star. Maybe this is because they start out with more mass than a typical neutron star, or are spinning faster, or are "kicked" in the supernova explosion in a certain way. Only about 10 magnetars have been discovered so far.

Dr. Alaa Ibrahim, a scientist at NASA Goddard Space Flight Center, measured the magnetic field of a magnetar named SGR 1806-20. The field was a whopping 1,000,000,000,000,000 Gauss. That's a million billion, or a quadrillion, or 10^15... or call it what you will. Anyway you measure it, that's the most magnetic object known. Luckily, SGR 1806-20 is parked safely 40,000 light years away from us.

At an astronomy meeting in January 2004, two teams of scientists (with Dr. Ibrahim on one of the teams) found evidence that magnetars might be far more common than previously thought. They're just hard to find. This is because when they are first born, in a star explosion, they aren't very bright. Eventually, the strong magnetic field causes the spinning magnetar to slow down. This breaking releases energy -- lots of energy, in the form of X-ray light, which satellites such as NASA's Rossi X-ray Timing Explorer can detect.

Magnetars only flare periodically, though. Suddenly they are bright for a few days or weeks, then they are dim again for years and years. Why? That's another mystery. But what is becoming clear is that the flaring might be just a short phase in the life of a neutron star, and most of the time they are dim. Thus there could be thousands in our Milky Way galaxy.

NASA will launch a new satellite called Swift this year. Swift's primary mission is to find gamma-ray bursts, the most powerful explosions known. Swift is good at "swiftly" detecting explosions and sudden flaring. So Swift will likely find many more magnetars during their periods of outbursts.

Here's one more fact about magnets: Some folks say that magnets can heal sore muscles and bones, so they strap "therapeutic magnets" around their wrists or backs or whatever is ailing them. The claim is that the magnets are manipulating the blood flow in the body because of its iron content. There are two things wrong with this claim, though. First, those magnets are too weak to penetrate their casing and then our skin. And even if they could, the iron in our blood is not attracted to magnets. If it were, anyone who ever had an MRI scan (generating a 15,000 Gauss field) would blow up! If an MRI doesn't affect the blood, there's no way a 100 Gauss therapeutic magnet can.