Magnetars: Neutron Stars With a Kick

artist's concept of a magnetar
A magnetar as visualized by an artist. This one lies in a star cluster glittering with hundreds of massive, hot stars. The magnetar has an incredibly strong magnetic field. ESO/L. Calcada. CC BY 4.0

Meet the Most Magnetic Stars in the Cosmos!

Neutron stars are weird, enigmatic objects out there in the galaxy. They've been studied for decades as astronomers get better instruments capable of observing them. Think of a quivering, solid ball of neutrons squished together tightly into a space the size of a city. 

One class of neutron stars in particular is very intriguing; they're called "magnetars".

The name comes from what they are: objects with extremely powerful magnetic fields. While normal neutron stars themselves have incredibly strong magnetic fields (on the order of 1012 Gauss, for those of you who like to keep track of these things), magnetars are many times more powerful. The most powerful ones can be upwards of a TRILLION Gauss!  By comparison, the magnetic field strength of the Sun is about 1 Gauss; the average field strength on Earth is half a Gauss. (A Gauss is the unit of measurement scientists use to describe the strength of a magnetic field.)

Creation of Magnetars

So, how do magnetars form? It starts with a neutron star. These are created when a massive star runs out of hydrogen fuel to burn in its core. Eventually, the star loses its outer envelope and collapses. The result is a tremendous explosion called a supernova.

During the supernova, the core of a supermassive star gets crammed down into a ball only about 40 kilometers (about 25 miles) across.

During the final catastrophic explosion, the core collapses even more, making an incredibly dense ball about 20 km or 12 miles in diameter.

That incredible pressure causes hydrogen nuclei to absorb electrons and release neutrinos. What's left after the core is through collapsing is a mass of neutrons (which are components of an atomic nucleus) with incredibly high gravity and a very strong magnetic field.

 

To get a magnetar, you need slightly different conditions during the stellar core collapse, which create the final core that rotates very slowly, but also has a much stronger magnetic field. 

Where Do We Find Magnetars?

A couple of dozen known magnetars have been observed, and other possible ones are still being studied. Among the closest is one discovered in a star cluster about 16,000 light-years away from us. The cluster is called Westerlund 1, and it contains some of the most massive main-sequence stars in the universe. Some of these giants are so big their atmospheres would reach to Saturn's orbit, and many are as luminous as a million Suns.

The stars in this cluster are quite extraordinary. With all of them being 30 to 40 times the mass of the Sun, it also makes the cluster quite young. (More massive stars age more quickly.) But this also implies that stars that have already left the main sequence contained at least 35 solar masses. This in of itself is not a startling discovery, however the ensuing detection of a magnetar in the midst of Westerlund 1 sent tremors through the world of astronomy.

Conventionally, neutron stars (and therefore magnetars) form when a 10 - 25 solar mass star leaves the main sequence and dies in a massive supernova.

However, with all the stars in Westerlund 1 having formed at nearly the same time (and considering mass is the key factor in the aging rate) the original star must have been greater than 40 solar masses.

It is not clear why this star did not collapse into a black hole. One possibility is that perhaps magnetars form in a completely different manner from normal neutron stars. Maybe there was a companion star interacting with the evolving star, which made it spend much of its energy prematurely. Much of the mass of the object might have escaped, leaving too little behind to fully evolve into a black hole. However, there is no companion detected. Of course, the companion star could have been destroyed during the energetic interactions with the magnetar's progenitor. Clearly astronomers need to study these objects to understand more about them and how they form.

Magnetic Field Strength

However a magnetar is born, its incredibly powerful magnetic field is its most defining characteristic. Even at distances of 600 miles from a magnetar, the field strength would be so great as to literally rip human tissue apart. If the magnetar floated halfway between the Earth and the Moon, its magnetic field would be strong enough to lift metal objects such as pens or paperclips from your pockets, and completely demagnetize all of the credit cards on Earth. That's not all. The radiation environment around them would be incredibly hazardous. These magnetic fields are so powerful that acceleration of particles easily produce x-ray emissions and gamma-ray photons, the highest energy light in the universe.

 

Edited and updated by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "Magnetars: Neutron Stars With a Kick." ThoughtCo, Jul. 4, 2017, thoughtco.com/magnetars-neutron-stars-with-a-kick-3073298. Millis, John P., Ph.D. (2017, July 4). Magnetars: Neutron Stars With a Kick. Retrieved from https://www.thoughtco.com/magnetars-neutron-stars-with-a-kick-3073298 Millis, John P., Ph.D. "Magnetars: Neutron Stars With a Kick." ThoughtCo. https://www.thoughtco.com/magnetars-neutron-stars-with-a-kick-3073298 (accessed May 24, 2018).