Extreme Universe: Magnetic Fields and Magnetars March 12, 2009

Neutron Stars are extreme to begin with, but magnetars add a whole new level of extreme to these exotic objects. Magnetars,  as the name implies, are neutron stars with ultra high magnetic fields. As a matter of fact, the most extreme magnetic fields ever found in the universe!

An artist's rendering of a magnetar, a type of neutron star. (Image Credit: NASA, CXC, M. Weiss)

There are about 15 magnetars known, they are all examples of a class of objects called “soft gamma repeaters” . The most magnetic one, and the most magnetized object in the known universe is SGR 1806-20. The magnetic field of this magnetar is estimated to be about 2 x 10¹¹ Teslas or 2 x 10¹⁵ gauss, one Tesla being equal to 10,000 gauss.

Now, to give you some sense of how big this is, the Earth’s magnetic field is about 1/2 gauss or .00005 Tesla.  The magnet in a hospital’s  MRI is about 3.2 Tesla or 32,000 gauss, and the largest sustained magnetic field created in a lab is about 40 Tesla.

So we’re talking about magnetic fields 1000 trillion times bigger than the Earth’s field. Very weird things can happen with fields this large. One thing that’s interesting  is how much energy is stored in such a field. So let’s break out an equation from physics and use an example I did in my electricity  & magnetism  class last week. If you look it up,  you’ll find  the energy per cubic meter, or energy density, of a magnetic field is given by:

u = B²/2 μ₀

u is the energy density given in Joules per cubic meter. A Joule is the energy you use to lift a kilogram about 10 centimeters off the ground. 

B is the strength of the magnetic field  given in Teslas, and  μo is a constant that has a value of 4π x 10^-7 (it has units , but we’ll ignore them).  Using a field of B = 2 x 10¹¹ Teslas, the most powerful magnetar, we will get a huge number…

1.6 x 10²⁸ Joules/(cubic meter)

or every cubic meter contains this amount of energy. To put this in context, the largest hydrogen bombs have a yield of 20 Megatons of TNT, which is about 10¹⁷ Joules of energy. So in each cubic meter of magnetic field has the stored energy of 160,000,000,000 (160 billion), 20 Megaton bombs.

Since we’re having so much fun, lets think about it this way. Einstein showed mass and energy are equivalent, so how much mass would one cubic meter of this HUGE magnetic field have? Well…

E=mc²

or  m = E/ c² = 1.6 x 10²⁸ Joules/(3 x 10⁸m/s)² = 1.78 x 10¹¹kilograms

Each cubic centimeter of magnetic field would have a mass of 178 metric tons!!! If you multiply this by the number of cubic meters in the Magnetar, about 40 trillion, assuming the whole neutron star is magnetized, you get a lot of magnetic energy stored in Magnetar.

To give you an idea of what a small amount of this energy would do, consider the events of December 27, 2004. On that day the magnetar we’ve been using as a example, SGR 1806-20, under went a “superflare”. The “superflare,” from a magnetar named SGR 1806–20, irradiated Earth with more total energy than a powerful solar flare. Yet this object is an estimated 50,000 light-years away in Sagittarius.  During that flicker of time it outshone the full Moon by a factor of two. The gamma rays struck the ionosphere and created more ionization which briefly expanded the ionosphere. Assuming that the distance estimate is accurate, the magnetar must have let loose as much energy as the Sun generates in 250,000 years.