The Sciences

Sky Lights

Nature's weirdest stars turn magnetism into radiation, unleashing as much energy in the blink of an eye as the sun does in 250,000 years.

By Bob BermanFeb 20, 2006 6:00 AM


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The stars of February are a varied lot. Ruddy Betelgeuse, in Orion's shoulder, is a cool giant. Blue Rigel, in Orion's heel, is a furiously hot powerhouse. Whitish Sirius, located off to the southeast, is relatively small and tame, but it shines brighter than any other star in the heavens because of its close proximity to Earth. Still, Betelgeuse, Rigel, and Sirius have one essential thing in common. Like every bright star, they shine by fusion, a process in which atomic nuclei smash together and combine into heavier ones.

In recent years, however, researchers have discovered a rare group of stars that don't play by the usual rules. These oddballs, known as magnetars, draw their power not from nuclear energy but from erupting magnetic fields. Magnetars literally rip themselves apart, unleashing a blast of invisible but incredibly potent radiation in the process.

The story of the magnetars began on March 5, 1979, when radiation monitors aboard space probes near Venus and Earth shot off the scale, saturated by a gamma-ray flash whose properties did not match those of any known phenomenon. Similar but weaker events kept the mystery alive in the ensuing years. Then came the all-time prizewinner on December 27, 2004, when a flood of X-rays and gamma rays swept through our neighborhood. The radiation was so intense that it ionized atoms in Earth's upper atmosphere for five minutes, disrupting some radio communications.

This time, scientists were ready. An armada of space observatories measured gamma rays from the burst, while radio telescopes on Earth helped pinpoint the source of the activity. It turned out to be a neutron star, just 12 miles wide, located about 50,000 light-years away in the direction of the constellation Sagittarius. Neutron stars were already known to be bizarre—they are the only stars with solid crusts and are so dense that a tablespoon of material from deep inside the star would weigh as much as Mount Everest. Even by neutron-star standards, however, the December 27 burst seemed extreme. In one-fifth of a second it emitted as much energy as the sun does in a quarter of a million years.

Astrophysicists Robert Duncan of the University of Texas at Austin and Christopher Thompson of the University of Toronto had an explanation. A neutron star forms when a massive star explodes as a supernova, blowing off its outer layers while its core collapses. The implosion of the core causes it to rotate rapidly, up to hundreds of times per second. Duncan and Thompson deduced that the superfast spin would cause the remnant star's magnetic field to wrap around itself, growing far more intense. In some cases, they estimated, it could reach a thousand trillion times the strength of Earth's magnetic field. One of these hypermagnetic stars could kill you from 600 miles away by reshaping all your atoms.

According to the model, a magnetar's intense magnetism bends and deforms its crust to produce a series of starquakes. At the same time, the shape of the star's magnetic field changes catastrophically. As the field resettles into a new configuration, it releases a tremendous amount of energy in the form of electrons, positrons (antimatter electrons), and gamma rays. The electrons and positrons soon annihilate each other, creating yet more gamma rays. The December 27 burst also ejected a strange cloud, or jet, of particles.

A magnetar can't maintain that kind of activity for long. After about 10,000 years, the star has cooled off, and its field has decayed so much that it no longer triggers starquakes; at that point, the gamma-ray flares die out. The brief life of a magnetar means that new ones must form at a rate of about one every thousand years in our galaxy. Ten million to 100 million such stars—most of them old and quiescent—may loiter unseen in the dusty lanes of the Milky Way.

Scientists are still trying to figure out the complicated details of a magnetar eruption. So while you are enjoying a look at the ordinary stars, the magnetar sleuths will be busily scanning the sky with gamma-ray goggles, waiting for the next big blast.

The Sky This Month

Every night The Orion nebula, a vast cloud of gas and newborn stars, is prominent in the south a couple of hours after sunset. It looks like a fuzzy star to the unaided eye and resembles a glowing cloud through binoculars.February 6–22 Mercury puts on one of its two good evening appearances this year. It shines low in the west, 40 minutes after sunset.

February 14 Mercury passes extremely close to dim, greenish Uranus. The pairing of the planets is best seen through a telescope.

February 27 The moon makes its closest approach to Earth of 2006; its surface is only 217,000 miles away. The new moon—when it is aligned with the sun—occurs on the same day, ensuring unusually high tides.

All month Saturn is prominent throughout the night. Its rings are seen more nearly edge-on this year than they were last year, so the planet appears noticeably dimmer.

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