"Really, when you look at the visible light from the crab, you're looking at the tail on the dog," said Jeff Hester, an Arizona State University astronomer who has studied the Crab at mostly visible wavelengths with Hubble. "That is, the particles that are responsible for really making things happen are particles that are much more energetic than the ones that give you visible light. Instead they're the particles that give you X-rays," he said.
Just 6,000 light years from Earth, the Crab nebula stands out from others because it is one of the brightest objects in the nighttime sky. It radiates energy across the full electromagnetic spectrum, and is so bright at X-ray wavelengths, that it is the standard against which all other X-ray sources are compared.
The nebula is the glowing remnant of a stellar explosion that was seen from Earth almost 950 years ago. In 1054 A.D. Chinese astronomers reported a star that appeared suddenly and remained visible for weeks, even during daylight. That "guest star," as it was called, appeared where the Crab nebula is now seen.
Now some ten light years across, the nebula is a dynamic glowing cloud in which energy is created by intense streams of high energy particles that pour out from the collapsed neutron star at the nebula's center at nearly the speed of light. That star is called a pulsar, or pulsating star, because it is rotating some 33 times per second, flashing at the same rate. The pulsar is more massive than the sun, but is about the size of a small city -- perhaps only 10 to 20 miles across. It produces energy 130,000 faster than the sun does.
The Crab's brilliance has puzzled scientists because it has not been clear how energy from the pulsar at the center of the nebula was being conducted into the outer nebula, explained Wallace Tucker, an astrophysicist at the Harvard Smithsonian Center for Astrophysics, the organization which operates Chandra telescope.
"This (pulsar) is a mighty spinning generator that's just whirling around creating electric fields which are quadrillions of times greater than anything you can get on Earth, and accelerating particles to very high energies. And they just rush out and glow in all these different wavelengths," Tucker said in an interview.
"Everyone has sort of known this must be happening because they knew the pulsar was there, they knew the nebula was there and they deduced from the pulses that somehow it was feeding the nebula with energy. Now they're beginning to see that in some detail with this X-ray picture," Tucker said. "They have direct proof, I think."
That proof appears to be in the bright ring seen in the new X-ray image. It circles the pulsar at the Crab nebula's center.
"We're finally seeing the place where the pulsar and the larger nebula couple themselves together," Hester said. He compared the ring to one that might be created by swinging a paintbrush saturated with wet paint.
"As you fling the paint brush around very rapidly, like the pulsar is doing, you're going to splatter paint in a ring," he said. "And when you look at that ring around the pulsar, that seems to be where the splatter of material coming off the pulsar is running into that larger nebula."
The same forces that are driving the Crab's pulsar and powering the nebula are likely operating in other neutron stars, black holes and at the centers of galaxies throughout the universe, most scientists believe. Hester explained that studying these processes where they can be seen nearby helps scientists understand processes that operate throughout the universe.
"The Crab nebula is one of the most important objects in all of astrophysics. It's important for understanding what happens in the hearts of galactic nuclei. It's important for understanding about how supernovae explode, it's important for understanding where the atoms that you or I are made of come from," Hester said. "It's the only place nearby that you can study particles moving close to the speed of light in this kind of a context."
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