Starry, Starry Night
<p>For many stargazers, the night sky might look like a backdrop of very similar twinkling lights. But actually the billions of stars that make up the universe are varied and full of tantalizing marvels. From stellar fireworks caused by supernova explosions to invisible black holes, astronomers are gradually figuring out how stars work and what makes each variety unique.</p>
<p>Many mysteries remain, however.</p>
Diamonds in the Sky
When a star the mass of our Sun uses up its nuclear fuel, it expels most of its outer layers to leave just a very hot core called a white dwarf. Scientists had speculated that at the bottom of a white dwarf’s 31 mile (50 kilometer)-thick crust was crystallized carbon and oxygen, similar to a diamond. And in 2004, they found that a white dwarf near the constellation Centaurus, BPM 37093, was made of crystallized carbon weighing 5 million trillion trillion pounds. In diamond-speak, that’s 10 billion trillion trillion carats
Magnetars are dense neutron stars—a type of stellar corpse—with magnetic fields billions of times stronger than any magnet on Earth. They release flashes of X-rays about every 10 seconds with an occasional gamma-ray burst. They weren’t classified as a distinct star type until 1998, nearly two decades after their telltale light shows were first spotted: In March 1979, nine spacecraft observed a release of radiation equaling the amount of energy the Sun lets off in a 1,000 years coming from the location of a supernova remnant called N49.
Stellar clusters are composed of many stars that develop at the same time. Some contain several dozen stars, and others many million stars. Some star clusters can be seen with the naked eye, such as the famous Pleiades cluster in the constellation Taurus. Stars form in the same region, but why some stay together forming clusters is a mystery.
A starquake is thought to be the tearing apart of the surface of a neutron star, much like an earthquake here on Earth. In 1999 astronomers identified these bursts as the cause of gamma rays and X-rays coming from neutron stars. Predicting these powerful bursts has remained a mystery. Recently, John Middleditch of Los Alamos National Laboratory and his team found that for a particular type of spinning neutron star called a pulsar, the time to the next quake is proportional to the size of the last quake.
A neutron star is born out of a supernova explosion, which compresses the dying star's core—with a mass greater than the sun's—into a ball with a diameter the size of a small city. One step from becoming black holes, neutron stars are the densest objects in the universe. Just a teaspoon would weigh roughly a billions tons on Earth. In 2005, NASA scientists found the source of gamma-ray bursts that emit as much light as 100,000 trillion suns—and solved a 35-year mystery: When two neutron stars collided at speeds tens of thousands of miles per second, they emit gamma-ray fireworks.
A new class of stars called rotating radio transients (RRATs) can be fickle flashers. They are massively compressed neutron stars that intermittently send out bursts of radio waves that can last for as few as two milliseconds with dark gaps lasting as long as three hours. Not only are these outbursts short-lived, in order to detect RRATs astronomers must distinguish the fleeting radio flashes from terrestrial radio interference. Even so, there could be hundreds of thousands of them in the Milky Way.
Stars may not be loners, as once thought. Now astronomers predict that 85 percent of the stars in the Milky Way galaxy reside in multiple-star systems. More than half of all stars are binary stars, or two stars that are bound by their mutual gravitational attraction, with each star orbiting around the center of mass. When three or more stars huddle together, it’s called a multiple star system. In 2005, astronomers presented evidence for the first planet orbiting a binary system.
The catastrophic explosion of a star sends out shock waves that radiate outward at 22 million mph (35 million kph). The end of life for some stars can be a spectacular event. Called a supernova, when a star that’s more than eight times the mass of our Sun burns out, gravity’s inward push tears apart the star’s innards. The explosion propels jets of high-energy light and matter out into space. Since Johannes Kepler’s supernova was spotted in 1604, astronomers haven’t witnessed one in our own galaxy.
The Sun’s atmosphere, or corona, can reach a bubbling 3.6 million degrees F (2 million degrees C), and can unpredictably fling out streams of high-energy particles at near light-speed. Called solar flares, these bundles of charged particles accelerate along curved magnetic field lines toward Earth, where they can disrupt communications and satellite technology, electronic devices, and even cell phones. The largest solar flares can release millions of hydrogen bombs’ worth of energy, or enough energy to power the United States for 100,000 years if it could be harnessed. Astronomers are just beginning to understand the inner workings of the sun, with the goal of predicting these fiery flares.
Black holes are so dense that nothing can escape from their gravitational clutches. Once past the event horizon, or the boundary beyond which even light cannot escape, there’s no way out. Now astronomers have strong evidence for the existence of stellar black holes, which form from the collapse of massive stars, as well as super-massive black holes that reach jaw-dropping weights of millions of solar masses.